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![Peter Hurley](/assets/img/avatar_default.png)
The user-settable knob, low_latency, has been the source of several BUG reports which stem from flush_to_ldisc() running in interrupt context. Since 3.12, which added several sleeping locks (termios_rwsem and buf->lock) to the input processing path, the frequency of these BUG reports has increased. Note that changes in 3.12 did not introduce this regression; sleeping locks were first added to the input processing path with the removal of the BKL from N_TTY in commita88a69c912
, 'n_tty: Fix loss of echoed characters and remove bkl from n_tty' and later in commit38db89799b
, 'tty: throttling race fix'. Since those changes, executing flush_to_ldisc() in interrupt_context (ie, low_latency set), is unsafe. However, since most devices do not validate if the low_latency setting is appropriate for the context (process or interrupt) in which they receive data, some reports are due to misconfiguration. Further, serial dma devices for which dma fails, resort to interrupt receiving as a backup without resetting low_latency. Historically, low_latency was used to force wake-up the reading process rather than wait for the next scheduler tick. The effect was to trim multiple milliseconds of latency from when the process would receive new data. Recent tests [1] have shown that the reading process now receives data with only 10's of microseconds latency without low_latency set. Remove the low_latency rx steering from tty_flip_buffer_push(); however, leave the knob as an optional hint to drivers that can tune their rx fifos and such like. Cleanup stale code comments regarding low_latency. [1] https://lkml.org/lkml/2014/2/20/434 "Yay.. thats an annoying historical pain in the butt gone." -- Alan Cox Reported-by: Beat Bolli <bbolli@ewanet.ch> Reported-by: Pavel Roskin <proski@gnu.org> Acked-by: David Sterba <dsterba@suse.cz> Cc: Grant Edwards <grant.b.edwards@gmail.com> Cc: Stanislaw Gruszka <sgruszka@redhat.com> Cc: Hal Murray <murray+fedora@ip-64-139-1-69.sjc.megapath.net> Cc: <stable@vger.kernel.org> # 3.12.x+ Signed-off-by: Peter Hurley <peter@hurleysoftware.com> Signed-off-by: Alan Cox <alan@linux.intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
540 lines
14 KiB
C
540 lines
14 KiB
C
/*
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* Tty buffer allocation management
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*/
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#include <linux/types.h>
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#include <linux/errno.h>
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#include <linux/tty.h>
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#include <linux/tty_driver.h>
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#include <linux/tty_flip.h>
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#include <linux/timer.h>
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#include <linux/string.h>
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#include <linux/slab.h>
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#include <linux/sched.h>
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#include <linux/wait.h>
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#include <linux/bitops.h>
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#include <linux/delay.h>
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#include <linux/module.h>
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#include <linux/ratelimit.h>
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#define MIN_TTYB_SIZE 256
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#define TTYB_ALIGN_MASK 255
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/*
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* Byte threshold to limit memory consumption for flip buffers.
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* The actual memory limit is > 2x this amount.
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*/
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#define TTYB_DEFAULT_MEM_LIMIT 65536
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/*
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* We default to dicing tty buffer allocations to this many characters
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* in order to avoid multiple page allocations. We know the size of
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* tty_buffer itself but it must also be taken into account that the
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* the buffer is 256 byte aligned. See tty_buffer_find for the allocation
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* logic this must match
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*/
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#define TTY_BUFFER_PAGE (((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)
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/**
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* tty_buffer_lock_exclusive - gain exclusive access to buffer
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* tty_buffer_unlock_exclusive - release exclusive access
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*
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* @port - tty_port owning the flip buffer
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*
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* Guarantees safe use of the line discipline's receive_buf() method by
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* excluding the buffer work and any pending flush from using the flip
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* buffer. Data can continue to be added concurrently to the flip buffer
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* from the driver side.
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*
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* On release, the buffer work is restarted if there is data in the
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* flip buffer
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*/
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void tty_buffer_lock_exclusive(struct tty_port *port)
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{
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struct tty_bufhead *buf = &port->buf;
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atomic_inc(&buf->priority);
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mutex_lock(&buf->lock);
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}
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void tty_buffer_unlock_exclusive(struct tty_port *port)
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{
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struct tty_bufhead *buf = &port->buf;
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int restart;
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restart = buf->head->commit != buf->head->read;
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atomic_dec(&buf->priority);
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mutex_unlock(&buf->lock);
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if (restart)
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queue_work(system_unbound_wq, &buf->work);
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}
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/**
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* tty_buffer_space_avail - return unused buffer space
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* @port - tty_port owning the flip buffer
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*
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* Returns the # of bytes which can be written by the driver without
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* reaching the buffer limit.
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*
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* Note: this does not guarantee that memory is available to write
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* the returned # of bytes (use tty_prepare_flip_string_xxx() to
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* pre-allocate if memory guarantee is required).
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*/
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int tty_buffer_space_avail(struct tty_port *port)
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{
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int space = port->buf.mem_limit - atomic_read(&port->buf.mem_used);
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return max(space, 0);
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}
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EXPORT_SYMBOL_GPL(tty_buffer_space_avail);
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static void tty_buffer_reset(struct tty_buffer *p, size_t size)
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{
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p->used = 0;
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p->size = size;
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p->next = NULL;
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p->commit = 0;
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p->read = 0;
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p->flags = 0;
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}
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/**
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* tty_buffer_free_all - free buffers used by a tty
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* @tty: tty to free from
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*
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* Remove all the buffers pending on a tty whether queued with data
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* or in the free ring. Must be called when the tty is no longer in use
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*/
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void tty_buffer_free_all(struct tty_port *port)
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{
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struct tty_bufhead *buf = &port->buf;
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struct tty_buffer *p, *next;
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struct llist_node *llist;
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while ((p = buf->head) != NULL) {
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buf->head = p->next;
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if (p->size > 0)
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kfree(p);
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}
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llist = llist_del_all(&buf->free);
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llist_for_each_entry_safe(p, next, llist, free)
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kfree(p);
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tty_buffer_reset(&buf->sentinel, 0);
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buf->head = &buf->sentinel;
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buf->tail = &buf->sentinel;
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atomic_set(&buf->mem_used, 0);
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}
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/**
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* tty_buffer_alloc - allocate a tty buffer
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* @tty: tty device
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* @size: desired size (characters)
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*
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* Allocate a new tty buffer to hold the desired number of characters.
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* We round our buffers off in 256 character chunks to get better
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* allocation behaviour.
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* Return NULL if out of memory or the allocation would exceed the
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* per device queue
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*/
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static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
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{
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struct llist_node *free;
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struct tty_buffer *p;
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/* Round the buffer size out */
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size = __ALIGN_MASK(size, TTYB_ALIGN_MASK);
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if (size <= MIN_TTYB_SIZE) {
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free = llist_del_first(&port->buf.free);
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if (free) {
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p = llist_entry(free, struct tty_buffer, free);
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goto found;
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}
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}
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/* Should possibly check if this fails for the largest buffer we
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have queued and recycle that ? */
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if (atomic_read(&port->buf.mem_used) > port->buf.mem_limit)
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return NULL;
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p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
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if (p == NULL)
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return NULL;
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found:
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tty_buffer_reset(p, size);
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atomic_add(size, &port->buf.mem_used);
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return p;
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}
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/**
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* tty_buffer_free - free a tty buffer
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* @tty: tty owning the buffer
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* @b: the buffer to free
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*
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* Free a tty buffer, or add it to the free list according to our
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* internal strategy
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*/
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static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
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{
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struct tty_bufhead *buf = &port->buf;
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/* Dumb strategy for now - should keep some stats */
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WARN_ON(atomic_sub_return(b->size, &buf->mem_used) < 0);
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if (b->size > MIN_TTYB_SIZE)
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kfree(b);
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else if (b->size > 0)
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llist_add(&b->free, &buf->free);
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}
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/**
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* tty_buffer_flush - flush full tty buffers
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* @tty: tty to flush
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*
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* flush all the buffers containing receive data.
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*
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* Locking: takes buffer lock to ensure single-threaded flip buffer
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* 'consumer'
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*/
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void tty_buffer_flush(struct tty_struct *tty)
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{
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struct tty_port *port = tty->port;
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struct tty_bufhead *buf = &port->buf;
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struct tty_buffer *next;
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atomic_inc(&buf->priority);
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mutex_lock(&buf->lock);
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while ((next = buf->head->next) != NULL) {
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tty_buffer_free(port, buf->head);
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buf->head = next;
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}
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buf->head->read = buf->head->commit;
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atomic_dec(&buf->priority);
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mutex_unlock(&buf->lock);
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}
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/**
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* tty_buffer_request_room - grow tty buffer if needed
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* @tty: tty structure
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* @size: size desired
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* @flags: buffer flags if new buffer allocated (default = 0)
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*
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* Make at least size bytes of linear space available for the tty
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* buffer. If we fail return the size we managed to find.
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*
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* Will change over to a new buffer if the current buffer is encoded as
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* TTY_NORMAL (so has no flags buffer) and the new buffer requires
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* a flags buffer.
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*/
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static int __tty_buffer_request_room(struct tty_port *port, size_t size,
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int flags)
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{
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struct tty_bufhead *buf = &port->buf;
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struct tty_buffer *b, *n;
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int left, change;
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b = buf->tail;
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if (b->flags & TTYB_NORMAL)
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left = 2 * b->size - b->used;
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else
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left = b->size - b->used;
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change = (b->flags & TTYB_NORMAL) && (~flags & TTYB_NORMAL);
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if (change || left < size) {
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/* This is the slow path - looking for new buffers to use */
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if ((n = tty_buffer_alloc(port, size)) != NULL) {
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n->flags = flags;
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buf->tail = n;
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b->commit = b->used;
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smp_mb();
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b->next = n;
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} else if (change)
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size = 0;
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else
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size = left;
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}
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return size;
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}
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int tty_buffer_request_room(struct tty_port *port, size_t size)
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{
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return __tty_buffer_request_room(port, size, 0);
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}
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EXPORT_SYMBOL_GPL(tty_buffer_request_room);
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/**
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* tty_insert_flip_string_fixed_flag - Add characters to the tty buffer
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* @port: tty port
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* @chars: characters
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* @flag: flag value for each character
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* @size: size
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*
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* Queue a series of bytes to the tty buffering. All the characters
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* passed are marked with the supplied flag. Returns the number added.
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*/
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int tty_insert_flip_string_fixed_flag(struct tty_port *port,
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const unsigned char *chars, char flag, size_t size)
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{
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int copied = 0;
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do {
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int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
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int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0;
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int space = __tty_buffer_request_room(port, goal, flags);
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struct tty_buffer *tb = port->buf.tail;
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if (unlikely(space == 0))
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break;
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memcpy(char_buf_ptr(tb, tb->used), chars, space);
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if (~tb->flags & TTYB_NORMAL)
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memset(flag_buf_ptr(tb, tb->used), flag, space);
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tb->used += space;
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copied += space;
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chars += space;
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/* There is a small chance that we need to split the data over
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several buffers. If this is the case we must loop */
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} while (unlikely(size > copied));
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return copied;
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}
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EXPORT_SYMBOL(tty_insert_flip_string_fixed_flag);
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/**
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* tty_insert_flip_string_flags - Add characters to the tty buffer
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* @port: tty port
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* @chars: characters
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* @flags: flag bytes
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* @size: size
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*
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* Queue a series of bytes to the tty buffering. For each character
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* the flags array indicates the status of the character. Returns the
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* number added.
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*/
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int tty_insert_flip_string_flags(struct tty_port *port,
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const unsigned char *chars, const char *flags, size_t size)
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{
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int copied = 0;
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do {
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int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
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int space = tty_buffer_request_room(port, goal);
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struct tty_buffer *tb = port->buf.tail;
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if (unlikely(space == 0))
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break;
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memcpy(char_buf_ptr(tb, tb->used), chars, space);
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memcpy(flag_buf_ptr(tb, tb->used), flags, space);
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tb->used += space;
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copied += space;
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chars += space;
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flags += space;
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/* There is a small chance that we need to split the data over
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several buffers. If this is the case we must loop */
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} while (unlikely(size > copied));
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return copied;
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}
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EXPORT_SYMBOL(tty_insert_flip_string_flags);
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/**
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* tty_schedule_flip - push characters to ldisc
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* @port: tty port to push from
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*
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* Takes any pending buffers and transfers their ownership to the
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* ldisc side of the queue. It then schedules those characters for
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* processing by the line discipline.
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*/
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void tty_schedule_flip(struct tty_port *port)
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{
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struct tty_bufhead *buf = &port->buf;
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buf->tail->commit = buf->tail->used;
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schedule_work(&buf->work);
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}
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EXPORT_SYMBOL(tty_schedule_flip);
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/**
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* tty_prepare_flip_string - make room for characters
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* @port: tty port
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* @chars: return pointer for character write area
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* @size: desired size
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*
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* Prepare a block of space in the buffer for data. Returns the length
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* available and buffer pointer to the space which is now allocated and
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* accounted for as ready for normal characters. This is used for drivers
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* that need their own block copy routines into the buffer. There is no
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* guarantee the buffer is a DMA target!
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*/
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int tty_prepare_flip_string(struct tty_port *port, unsigned char **chars,
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size_t size)
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{
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int space = __tty_buffer_request_room(port, size, TTYB_NORMAL);
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if (likely(space)) {
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struct tty_buffer *tb = port->buf.tail;
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*chars = char_buf_ptr(tb, tb->used);
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if (~tb->flags & TTYB_NORMAL)
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memset(flag_buf_ptr(tb, tb->used), TTY_NORMAL, space);
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tb->used += space;
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}
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return space;
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}
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EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
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static int
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receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
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{
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struct tty_ldisc *disc = tty->ldisc;
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unsigned char *p = char_buf_ptr(head, head->read);
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char *f = NULL;
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if (~head->flags & TTYB_NORMAL)
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f = flag_buf_ptr(head, head->read);
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if (disc->ops->receive_buf2)
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count = disc->ops->receive_buf2(tty, p, f, count);
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else {
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count = min_t(int, count, tty->receive_room);
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if (count)
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disc->ops->receive_buf(tty, p, f, count);
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}
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head->read += count;
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return count;
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}
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/**
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* flush_to_ldisc
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* @work: tty structure passed from work queue.
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*
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* This routine is called out of the software interrupt to flush data
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* from the buffer chain to the line discipline.
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*
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* The receive_buf method is single threaded for each tty instance.
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*
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* Locking: takes buffer lock to ensure single-threaded flip buffer
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* 'consumer'
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*/
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static void flush_to_ldisc(struct work_struct *work)
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{
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struct tty_port *port = container_of(work, struct tty_port, buf.work);
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struct tty_bufhead *buf = &port->buf;
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struct tty_struct *tty;
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struct tty_ldisc *disc;
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tty = port->itty;
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if (tty == NULL)
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return;
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disc = tty_ldisc_ref(tty);
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if (disc == NULL)
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return;
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mutex_lock(&buf->lock);
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while (1) {
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struct tty_buffer *head = buf->head;
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int count;
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/* Ldisc or user is trying to gain exclusive access */
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if (atomic_read(&buf->priority))
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break;
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count = head->commit - head->read;
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if (!count) {
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if (head->next == NULL)
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break;
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buf->head = head->next;
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tty_buffer_free(port, head);
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continue;
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}
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count = receive_buf(tty, head, count);
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if (!count)
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break;
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}
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mutex_unlock(&buf->lock);
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tty_ldisc_deref(disc);
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}
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/**
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* tty_flush_to_ldisc
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* @tty: tty to push
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*
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* Push the terminal flip buffers to the line discipline.
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*
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* Must not be called from IRQ context.
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*/
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void tty_flush_to_ldisc(struct tty_struct *tty)
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{
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flush_work(&tty->port->buf.work);
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}
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/**
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* tty_flip_buffer_push - terminal
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* @port: tty port to push
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*
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* Queue a push of the terminal flip buffers to the line discipline.
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* Can be called from IRQ/atomic context.
|
|
*
|
|
* In the event of the queue being busy for flipping the work will be
|
|
* held off and retried later.
|
|
*/
|
|
|
|
void tty_flip_buffer_push(struct tty_port *port)
|
|
{
|
|
tty_schedule_flip(port);
|
|
}
|
|
EXPORT_SYMBOL(tty_flip_buffer_push);
|
|
|
|
/**
|
|
* tty_buffer_init - prepare a tty buffer structure
|
|
* @tty: tty to initialise
|
|
*
|
|
* Set up the initial state of the buffer management for a tty device.
|
|
* Must be called before the other tty buffer functions are used.
|
|
*/
|
|
|
|
void tty_buffer_init(struct tty_port *port)
|
|
{
|
|
struct tty_bufhead *buf = &port->buf;
|
|
|
|
mutex_init(&buf->lock);
|
|
tty_buffer_reset(&buf->sentinel, 0);
|
|
buf->head = &buf->sentinel;
|
|
buf->tail = &buf->sentinel;
|
|
init_llist_head(&buf->free);
|
|
atomic_set(&buf->mem_used, 0);
|
|
atomic_set(&buf->priority, 0);
|
|
INIT_WORK(&buf->work, flush_to_ldisc);
|
|
buf->mem_limit = TTYB_DEFAULT_MEM_LIMIT;
|
|
}
|
|
|
|
/**
|
|
* tty_buffer_set_limit - change the tty buffer memory limit
|
|
* @port: tty port to change
|
|
*
|
|
* Change the tty buffer memory limit.
|
|
* Must be called before the other tty buffer functions are used.
|
|
*/
|
|
|
|
int tty_buffer_set_limit(struct tty_port *port, int limit)
|
|
{
|
|
if (limit < MIN_TTYB_SIZE)
|
|
return -EINVAL;
|
|
port->buf.mem_limit = limit;
|
|
return 0;
|
|
}
|
|
EXPORT_SYMBOL_GPL(tty_buffer_set_limit);
|