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![Peter Zijlstra](/assets/img/avatar_default.png)
Relay avoids calling wake_up_interruptible() for doing the wakeup of
readers/consumers, waiting for the generation of new data, from the
context of a process which produced the data. This is apparently done to
prevent the possibility of a deadlock in case Scheduler itself is is
generating data for the relay, after acquiring rq->lock.
The following patch used a timer (to be scheduled at next jiffy), for
delegating the wakeup to another context.
commit 7c9cb38302
Author: Tom Zanussi <zanussi@comcast.net>
Date: Wed May 9 02:34:01 2007 -0700
relay: use plain timer instead of delayed work
relay doesn't need to use schedule_delayed_work() for waking readers
when a simple timer will do.
Scheduling a plain timer, at next jiffies boundary, to do the wakeup
causes a significant wakeup latency for the Userspace client, which makes
relay less suitable for the high-frequency low-payload use cases where the
data gets generated at a very high rate, like multiple sub buffers getting
filled within a milli second. Moreover the timer is re-scheduled on every
newly produced sub buffer so the timer keeps getting pushed out if sub
buffers are filled in a very quick succession (less than a jiffy gap
between filling of 2 sub buffers). As a result relay runs out of sub
buffers to store the new data.
By using irq_work it is ensured that wakeup of userspace client, blocked
in the poll call, is done at earliest (through self IPI or next timer
tick) enabling it to always consume the data in time. Also this makes
relay consistent with printk & ring buffers (trace), as they too use
irq_work for deferred wake up of readers.
[arnd@arndb.de: select CONFIG_IRQ_WORK]
Link: http://lkml.kernel.org/r/20160912154035.3222156-1-arnd@arndb.de
[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/1472906487-1559-1-git-send-email-akash.goel@intel.com
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
299 lines
8.8 KiB
C
299 lines
8.8 KiB
C
/*
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* linux/include/linux/relay.h
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*
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* Copyright (C) 2002, 2003 - Tom Zanussi (zanussi@us.ibm.com), IBM Corp
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* Copyright (C) 1999, 2000, 2001, 2002 - Karim Yaghmour (karim@opersys.com)
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*
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* CONFIG_RELAY definitions and declarations
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*/
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#ifndef _LINUX_RELAY_H
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#define _LINUX_RELAY_H
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#include <linux/types.h>
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#include <linux/sched.h>
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#include <linux/timer.h>
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#include <linux/wait.h>
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#include <linux/list.h>
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#include <linux/irq_work.h>
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#include <linux/bug.h>
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#include <linux/fs.h>
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#include <linux/poll.h>
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#include <linux/kref.h>
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#include <linux/percpu.h>
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/*
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* Tracks changes to rchan/rchan_buf structs
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*/
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#define RELAYFS_CHANNEL_VERSION 7
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/*
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* Per-cpu relay channel buffer
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*/
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struct rchan_buf
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{
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void *start; /* start of channel buffer */
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void *data; /* start of current sub-buffer */
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size_t offset; /* current offset into sub-buffer */
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size_t subbufs_produced; /* count of sub-buffers produced */
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size_t subbufs_consumed; /* count of sub-buffers consumed */
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struct rchan *chan; /* associated channel */
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wait_queue_head_t read_wait; /* reader wait queue */
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struct irq_work wakeup_work; /* reader wakeup */
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struct dentry *dentry; /* channel file dentry */
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struct kref kref; /* channel buffer refcount */
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struct page **page_array; /* array of current buffer pages */
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unsigned int page_count; /* number of current buffer pages */
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unsigned int finalized; /* buffer has been finalized */
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size_t *padding; /* padding counts per sub-buffer */
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size_t prev_padding; /* temporary variable */
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size_t bytes_consumed; /* bytes consumed in cur read subbuf */
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size_t early_bytes; /* bytes consumed before VFS inited */
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unsigned int cpu; /* this buf's cpu */
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} ____cacheline_aligned;
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/*
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* Relay channel data structure
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*/
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struct rchan
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{
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u32 version; /* the version of this struct */
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size_t subbuf_size; /* sub-buffer size */
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size_t n_subbufs; /* number of sub-buffers per buffer */
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size_t alloc_size; /* total buffer size allocated */
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struct rchan_callbacks *cb; /* client callbacks */
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struct kref kref; /* channel refcount */
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void *private_data; /* for user-defined data */
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size_t last_toobig; /* tried to log event > subbuf size */
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struct rchan_buf ** __percpu buf; /* per-cpu channel buffers */
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int is_global; /* One global buffer ? */
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struct list_head list; /* for channel list */
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struct dentry *parent; /* parent dentry passed to open */
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int has_base_filename; /* has a filename associated? */
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char base_filename[NAME_MAX]; /* saved base filename */
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};
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/*
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* Relay channel client callbacks
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*/
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struct rchan_callbacks
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{
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/*
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* subbuf_start - called on buffer-switch to a new sub-buffer
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* @buf: the channel buffer containing the new sub-buffer
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* @subbuf: the start of the new sub-buffer
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* @prev_subbuf: the start of the previous sub-buffer
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* @prev_padding: unused space at the end of previous sub-buffer
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*
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* The client should return 1 to continue logging, 0 to stop
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* logging.
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*
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* NOTE: subbuf_start will also be invoked when the buffer is
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* created, so that the first sub-buffer can be initialized
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* if necessary. In this case, prev_subbuf will be NULL.
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*
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* NOTE: the client can reserve bytes at the beginning of the new
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* sub-buffer by calling subbuf_start_reserve() in this callback.
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*/
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int (*subbuf_start) (struct rchan_buf *buf,
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void *subbuf,
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void *prev_subbuf,
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size_t prev_padding);
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/*
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* buf_mapped - relay buffer mmap notification
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* @buf: the channel buffer
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* @filp: relay file pointer
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*
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* Called when a relay file is successfully mmapped
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*/
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void (*buf_mapped)(struct rchan_buf *buf,
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struct file *filp);
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/*
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* buf_unmapped - relay buffer unmap notification
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* @buf: the channel buffer
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* @filp: relay file pointer
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*
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* Called when a relay file is successfully unmapped
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*/
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void (*buf_unmapped)(struct rchan_buf *buf,
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struct file *filp);
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/*
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* create_buf_file - create file to represent a relay channel buffer
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* @filename: the name of the file to create
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* @parent: the parent of the file to create
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* @mode: the mode of the file to create
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* @buf: the channel buffer
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* @is_global: outparam - set non-zero if the buffer should be global
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*
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* Called during relay_open(), once for each per-cpu buffer,
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* to allow the client to create a file to be used to
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* represent the corresponding channel buffer. If the file is
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* created outside of relay, the parent must also exist in
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* that filesystem.
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*
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* The callback should return the dentry of the file created
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* to represent the relay buffer.
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*
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* Setting the is_global outparam to a non-zero value will
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* cause relay_open() to create a single global buffer rather
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* than the default set of per-cpu buffers.
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*
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* See Documentation/filesystems/relay.txt for more info.
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*/
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struct dentry *(*create_buf_file)(const char *filename,
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struct dentry *parent,
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umode_t mode,
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struct rchan_buf *buf,
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int *is_global);
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/*
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* remove_buf_file - remove file representing a relay channel buffer
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* @dentry: the dentry of the file to remove
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*
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* Called during relay_close(), once for each per-cpu buffer,
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* to allow the client to remove a file used to represent a
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* channel buffer.
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*
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* The callback should return 0 if successful, negative if not.
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*/
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int (*remove_buf_file)(struct dentry *dentry);
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};
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/*
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* CONFIG_RELAY kernel API, kernel/relay.c
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*/
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struct rchan *relay_open(const char *base_filename,
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struct dentry *parent,
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size_t subbuf_size,
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size_t n_subbufs,
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struct rchan_callbacks *cb,
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void *private_data);
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extern int relay_late_setup_files(struct rchan *chan,
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const char *base_filename,
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struct dentry *parent);
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extern void relay_close(struct rchan *chan);
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extern void relay_flush(struct rchan *chan);
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extern void relay_subbufs_consumed(struct rchan *chan,
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unsigned int cpu,
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size_t consumed);
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extern void relay_reset(struct rchan *chan);
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extern int relay_buf_full(struct rchan_buf *buf);
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extern size_t relay_switch_subbuf(struct rchan_buf *buf,
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size_t length);
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/**
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* relay_write - write data into the channel
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* @chan: relay channel
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* @data: data to be written
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* @length: number of bytes to write
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*
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* Writes data into the current cpu's channel buffer.
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*
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* Protects the buffer by disabling interrupts. Use this
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* if you might be logging from interrupt context. Try
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* __relay_write() if you know you won't be logging from
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* interrupt context.
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*/
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static inline void relay_write(struct rchan *chan,
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const void *data,
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size_t length)
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{
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unsigned long flags;
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struct rchan_buf *buf;
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local_irq_save(flags);
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buf = *this_cpu_ptr(chan->buf);
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if (unlikely(buf->offset + length > chan->subbuf_size))
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length = relay_switch_subbuf(buf, length);
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memcpy(buf->data + buf->offset, data, length);
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buf->offset += length;
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local_irq_restore(flags);
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}
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/**
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* __relay_write - write data into the channel
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* @chan: relay channel
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* @data: data to be written
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* @length: number of bytes to write
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*
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* Writes data into the current cpu's channel buffer.
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*
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* Protects the buffer by disabling preemption. Use
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* relay_write() if you might be logging from interrupt
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* context.
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*/
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static inline void __relay_write(struct rchan *chan,
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const void *data,
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size_t length)
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{
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struct rchan_buf *buf;
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buf = *get_cpu_ptr(chan->buf);
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if (unlikely(buf->offset + length > buf->chan->subbuf_size))
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length = relay_switch_subbuf(buf, length);
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memcpy(buf->data + buf->offset, data, length);
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buf->offset += length;
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put_cpu_ptr(chan->buf);
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}
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/**
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* relay_reserve - reserve slot in channel buffer
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* @chan: relay channel
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* @length: number of bytes to reserve
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*
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* Returns pointer to reserved slot, NULL if full.
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*
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* Reserves a slot in the current cpu's channel buffer.
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* Does not protect the buffer at all - caller must provide
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* appropriate synchronization.
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*/
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static inline void *relay_reserve(struct rchan *chan, size_t length)
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{
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void *reserved = NULL;
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struct rchan_buf *buf = *get_cpu_ptr(chan->buf);
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if (unlikely(buf->offset + length > buf->chan->subbuf_size)) {
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length = relay_switch_subbuf(buf, length);
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if (!length)
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goto end;
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}
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reserved = buf->data + buf->offset;
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buf->offset += length;
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end:
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put_cpu_ptr(chan->buf);
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return reserved;
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}
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/**
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* subbuf_start_reserve - reserve bytes at the start of a sub-buffer
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* @buf: relay channel buffer
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* @length: number of bytes to reserve
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*
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* Helper function used to reserve bytes at the beginning of
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* a sub-buffer in the subbuf_start() callback.
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*/
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static inline void subbuf_start_reserve(struct rchan_buf *buf,
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size_t length)
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{
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BUG_ON(length >= buf->chan->subbuf_size - 1);
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buf->offset = length;
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}
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/*
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* exported relay file operations, kernel/relay.c
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*/
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extern const struct file_operations relay_file_operations;
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#ifdef CONFIG_RELAY
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int relay_prepare_cpu(unsigned int cpu);
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#else
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#define relay_prepare_cpu NULL
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#endif
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#endif /* _LINUX_RELAY_H */
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