From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>,
syzkaller-bugs@googlegroups.com, jack@suse.cz
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
viro@zeniv.linux.org.uk, axboe@kernel.dk, tj@kernel.org,
david@fromorbit.com, linux-block@vger.kernel.org
Subject: Re: general protection fault in wb_workfn (2)
Date: Sun, 27 May 2018 09:47:54 +0900 [thread overview]
Message-ID: <0c7c5dea-7312-8a59-9d1b-5467f69719bf@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <000000000000cbd959056d1851ca@google.com>
Forwarding http://lkml.kernel.org/r/201805251915.FGH64517.HVFJOOLFFMQStO@I-love.SAKURA.ne.jp .
Jan Kara wrote:
> > void delayed_work_timer_fn(struct timer_list *t)
> > {
> > struct delayed_work *dwork = from_timer(dwork, t, timer);
> >
> > /* should have been called from irqsafe timer with irq already off */
> > __queue_work(dwork->cpu, dwork->wq, &dwork->work);
> > }
> >
> > Then, wb_workfn() is after all scheduled even if we check for
> > WB_registered bit, isn't it?
>
> It can be queued after WB_registered bit is cleared but it cannot be queued
> after mod_delayed_work(bdi_wq, &wb->dwork, 0) has finished. That function
> deletes the pending timer (the timer cannot be armed again because
> WB_registered is cleared) and queues what should be the last round of
> wb_workfn().
mod_delayed_work() deletes the pending timer but does not wait for already
invoked timer handler to complete because it is using del_timer() rather than
del_timer_sync(). Then, what happens if __queue_work() is almost concurrently
executed from two CPUs, one from mod_delayed_work(bdi_wq, &wb->dwork, 0) from
wb_shutdown() path (which is called without spin_lock_bh(&wb->work_lock)) and
the other from delayed_work_timer_fn() path (which is called without checking
WB_registered bit under spin_lock_bh(&wb->work_lock)) ?
wb_wakeup_delayed() {
spin_lock_bh(&wb->work_lock);
if (test_bit(WB_registered, &wb->state)) // succeeds
queue_delayed_work(bdi_wq, &wb->d_work, timeout) {
queue_delayed_work_on(WORK_CPU_UNBOUND, bdi_wq, &wb->d_work, timeout) {
if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&wb->d_work.work))) { // succeeds
__queue_delayed_work(WORK_CPU_UNBOUND, bdi_wq, &wb->d_work, timeout) {
add_timer(timer); // schedules for delayed_work_timer_fn()
}
}
}
}
spin_unlock_bh(&wb->work_lock);
}
delayed_work_timer_fn() {
// del_timer() already returns false at this point because this timer
// is already inside handler. But something took long here enough to
// wait for __queue_work() from wb_shutdown() path to finish?
__queue_work(WORK_CPU_UNBOUND, bdi_wq, &wb->d_work.work) {
insert_work(pwq, work, worklist, work_flags);
}
}
wb_shutdown() {
mod_delayed_work(bdi_wq, &wb->dwork, 0) {
mod_delayed_work_on(WORK_CPU_UNBOUND, bdi_wq, &wb->dwork, 0) {
ret = try_to_grab_pending(&wb->dwork.work, true, &flags) {
if (likely(del_timer(&wb->dwork.timer))) // fails because already in delayed_work_timer_fn()
return 1;
if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&wb->dwork.work))) // fails because already set by queue_delayed_work()
return 0;
// Returns 1 or -ENOENT after doing something?
}
if (ret >= 0)
__queue_delayed_work(WORK_CPU_UNBOUND, bdi_wq, &wb->dwork, 0) {
__queue_work(WORK_CPU_UNBOUND, bdi_wq, &wb->dwork.work) {
insert_work(pwq, work, worklist, work_flags);
}
}
}
}
}
next prev parent reply other threads:[~2018-05-27 0:49 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-26 9:15 general protection fault in wb_workfn (2) syzbot
2018-05-27 0:47 ` Tetsuo Handa [this message]
2018-05-27 2:21 ` [PATCH] bdi: Fix another oops in wb_workfn() Tetsuo Handa
2018-05-27 2:36 ` Tejun Heo
2018-05-28 13:35 ` general protection fault in wb_workfn (2) Jan Kara
2018-05-30 16:00 ` Tetsuo Handa
2018-05-31 11:42 ` Jan Kara
2018-05-31 13:19 ` Tetsuo Handa
2018-05-31 13:42 ` Jan Kara
2018-05-31 16:56 ` Jens Axboe
2018-06-05 13:45 ` Tetsuo Handa
2018-06-07 18:46 ` Dmitry Vyukov
2018-06-08 2:31 ` Tetsuo Handa
2018-06-08 14:45 ` Dmitry Vyukov
2018-06-08 15:16 ` Dmitry Vyukov
2018-06-08 16:53 ` Dmitry Vyukov
2018-06-08 17:14 ` Dmitry Vyukov
2018-06-09 5:30 ` Tetsuo Handa
2018-06-09 14:00 ` [PATCH] bdi: Fix another oops in wb_workfn() Tetsuo Handa
2018-06-11 9:12 ` Jan Kara
2018-06-11 16:01 ` Tejun Heo
2018-06-11 16:29 ` Jan Kara
2018-06-11 17:20 ` Tejun Heo
2018-06-12 15:57 ` Jan Kara
2018-06-13 10:43 ` Tetsuo Handa
2018-06-13 11:51 ` Tetsuo Handa
2018-06-13 14:06 ` Linus Torvalds
2018-06-13 14:46 ` Jan Kara
2018-06-13 14:55 ` Linus Torvalds
2018-06-13 16:20 ` Tetsuo Handa
2018-06-13 16:25 ` Linus Torvalds
2018-06-13 16:45 ` Jan Kara
2018-06-13 21:04 ` Tetsuo Handa
2018-06-14 10:11 ` Jan Kara
2018-06-13 14:33 ` Tejun Heo
2018-06-15 12:06 ` Jan Kara
2018-06-18 12:27 ` Jan Kara
2018-06-01 2:30 ` general protection fault in wb_workfn (2) Dave Chinner
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