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![Tejun Heo](/assets/img/avatar_default.png)
cic is association between io_context and request_queue. A cic is linked from both ioc and q and should be destroyed when either one goes away. As ioc and q both have their own locks, locking becomes a bit complex - both orders work for removal from one but not from the other. Currently, cfq tries to circumvent this locking order issue with RCU. ioc->lock nests inside queue_lock but the radix tree and cic's are also protected by RCU allowing either side to walk their lists without grabbing lock. This rather unconventional use of RCU quickly devolves into extremely fragile convolution. e.g. The following is from cfqd going away too soon after ioc and q exits raced. general protection fault: 0000 [#1] PREEMPT SMP CPU 2 Modules linked in: [ 88.503444] Pid: 599, comm: hexdump Not tainted 3.1.0-rc10-work+ #158 Bochs Bochs RIP: 0010:[<ffffffff81397628>] [<ffffffff81397628>] cfq_exit_single_io_context+0x58/0xf0 ... Call Trace: [<ffffffff81395a4a>] call_for_each_cic+0x5a/0x90 [<ffffffff81395ab5>] cfq_exit_io_context+0x15/0x20 [<ffffffff81389130>] exit_io_context+0x100/0x140 [<ffffffff81098a29>] do_exit+0x579/0x850 [<ffffffff81098d5b>] do_group_exit+0x5b/0xd0 [<ffffffff81098de7>] sys_exit_group+0x17/0x20 [<ffffffff81b02f2b>] system_call_fastpath+0x16/0x1b The only real hot path here is cic lookup during request initialization and avoiding extra locking requires very confined use of RCU. This patch makes cic removal from both ioc and request_queue perform double-locking and unlink immediately. * From q side, the change is almost trivial as ioc->lock nests inside queue_lock. It just needs to grab each ioc->lock as it walks cic_list and unlink it. * From ioc side, it's a bit more difficult because of inversed lock order. ioc needs its lock to walk its cic_list but can't grab the matching queue_lock and needs to perform unlock-relock dancing. Unlinking is now wholly done from put_io_context() and fast path is optimized by using the queue_lock the caller already holds, which is by far the most common case. If the ioc accessed multiple devices, it tries with trylock. In unlikely cases of fast path failure, it falls back to full double-locking dance from workqueue. Double-locking isn't the prettiest thing in the world but it's *far* simpler and more understandable than RCU trick without adding any meaningful overhead. This still leaves a lot of now unnecessary RCU logics. Future patches will trim them. -v2: Vivek pointed out that cic->q was being dereferenced after cic->release() was called. Updated to use local variable @this_q instead. Signed-off-by: Tejun Heo <tj@kernel.org> Cc: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
235 lines
4.8 KiB
C
235 lines
4.8 KiB
C
/*
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* fs/ioprio.c
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*
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* Copyright (C) 2004 Jens Axboe <axboe@kernel.dk>
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*
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* Helper functions for setting/querying io priorities of processes. The
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* system calls closely mimmick getpriority/setpriority, see the man page for
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* those. The prio argument is a composite of prio class and prio data, where
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* the data argument has meaning within that class. The standard scheduling
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* classes have 8 distinct prio levels, with 0 being the highest prio and 7
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* being the lowest.
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*
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* IOW, setting BE scheduling class with prio 2 is done ala:
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*
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* unsigned int prio = (IOPRIO_CLASS_BE << IOPRIO_CLASS_SHIFT) | 2;
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*
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* ioprio_set(PRIO_PROCESS, pid, prio);
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*
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* See also Documentation/block/ioprio.txt
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*
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*/
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#include <linux/gfp.h>
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#include <linux/kernel.h>
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#include <linux/export.h>
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#include <linux/ioprio.h>
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#include <linux/blkdev.h>
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#include <linux/capability.h>
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#include <linux/syscalls.h>
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#include <linux/security.h>
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#include <linux/pid_namespace.h>
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int set_task_ioprio(struct task_struct *task, int ioprio)
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{
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int err;
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struct io_context *ioc;
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const struct cred *cred = current_cred(), *tcred;
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rcu_read_lock();
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tcred = __task_cred(task);
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if (tcred->uid != cred->euid &&
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tcred->uid != cred->uid && !capable(CAP_SYS_NICE)) {
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rcu_read_unlock();
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return -EPERM;
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}
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rcu_read_unlock();
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err = security_task_setioprio(task, ioprio);
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if (err)
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return err;
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ioc = get_task_io_context(task, GFP_ATOMIC, NUMA_NO_NODE);
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if (ioc) {
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ioc_ioprio_changed(ioc, ioprio);
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put_io_context(ioc, NULL);
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}
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return err;
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}
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EXPORT_SYMBOL_GPL(set_task_ioprio);
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SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
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{
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int class = IOPRIO_PRIO_CLASS(ioprio);
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int data = IOPRIO_PRIO_DATA(ioprio);
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struct task_struct *p, *g;
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struct user_struct *user;
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struct pid *pgrp;
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int ret;
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switch (class) {
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case IOPRIO_CLASS_RT:
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if (!capable(CAP_SYS_ADMIN))
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return -EPERM;
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/* fall through, rt has prio field too */
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case IOPRIO_CLASS_BE:
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if (data >= IOPRIO_BE_NR || data < 0)
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return -EINVAL;
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break;
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case IOPRIO_CLASS_IDLE:
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break;
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case IOPRIO_CLASS_NONE:
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if (data)
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return -EINVAL;
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break;
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default:
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return -EINVAL;
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}
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ret = -ESRCH;
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rcu_read_lock();
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switch (which) {
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case IOPRIO_WHO_PROCESS:
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if (!who)
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p = current;
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else
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p = find_task_by_vpid(who);
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if (p)
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ret = set_task_ioprio(p, ioprio);
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break;
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case IOPRIO_WHO_PGRP:
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if (!who)
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pgrp = task_pgrp(current);
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else
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pgrp = find_vpid(who);
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do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
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ret = set_task_ioprio(p, ioprio);
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if (ret)
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break;
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} while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
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break;
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case IOPRIO_WHO_USER:
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if (!who)
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user = current_user();
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else
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user = find_user(who);
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if (!user)
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break;
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do_each_thread(g, p) {
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if (__task_cred(p)->uid != who)
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continue;
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ret = set_task_ioprio(p, ioprio);
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if (ret)
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goto free_uid;
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} while_each_thread(g, p);
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free_uid:
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if (who)
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free_uid(user);
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break;
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default:
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ret = -EINVAL;
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}
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rcu_read_unlock();
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return ret;
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}
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static int get_task_ioprio(struct task_struct *p)
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{
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int ret;
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ret = security_task_getioprio(p);
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if (ret)
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goto out;
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ret = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, IOPRIO_NORM);
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if (p->io_context)
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ret = p->io_context->ioprio;
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out:
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return ret;
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}
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int ioprio_best(unsigned short aprio, unsigned short bprio)
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{
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unsigned short aclass = IOPRIO_PRIO_CLASS(aprio);
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unsigned short bclass = IOPRIO_PRIO_CLASS(bprio);
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if (aclass == IOPRIO_CLASS_NONE)
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aclass = IOPRIO_CLASS_BE;
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if (bclass == IOPRIO_CLASS_NONE)
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bclass = IOPRIO_CLASS_BE;
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if (aclass == bclass)
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return min(aprio, bprio);
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if (aclass > bclass)
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return bprio;
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else
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return aprio;
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}
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SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
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{
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struct task_struct *g, *p;
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struct user_struct *user;
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struct pid *pgrp;
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int ret = -ESRCH;
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int tmpio;
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rcu_read_lock();
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switch (which) {
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case IOPRIO_WHO_PROCESS:
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if (!who)
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p = current;
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else
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p = find_task_by_vpid(who);
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if (p)
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ret = get_task_ioprio(p);
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break;
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case IOPRIO_WHO_PGRP:
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if (!who)
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pgrp = task_pgrp(current);
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else
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pgrp = find_vpid(who);
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do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
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tmpio = get_task_ioprio(p);
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if (tmpio < 0)
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continue;
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if (ret == -ESRCH)
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ret = tmpio;
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else
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ret = ioprio_best(ret, tmpio);
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} while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
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break;
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case IOPRIO_WHO_USER:
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if (!who)
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user = current_user();
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else
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user = find_user(who);
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if (!user)
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break;
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do_each_thread(g, p) {
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if (__task_cred(p)->uid != user->uid)
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continue;
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tmpio = get_task_ioprio(p);
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if (tmpio < 0)
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continue;
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if (ret == -ESRCH)
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ret = tmpio;
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else
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ret = ioprio_best(ret, tmpio);
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} while_each_thread(g, p);
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if (who)
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free_uid(user);
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break;
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default:
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ret = -EINVAL;
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}
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rcu_read_unlock();
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return ret;
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}
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