From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Long Gao <gaolong@kylinos.com.cn>,
Al Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Patch for lost wakeups
Date: Thu, 8 Aug 2013 21:17:49 +0200 [thread overview]
Message-ID: <20130808191749.GA12062@redhat.com> (raw)
In-Reply-To: <CA+55aFwAnWZGkXbyLmL3RxQGW-dX_jjDBPfA5PjxeVWn==a6tA@mail.gmail.com>
On 08/08, Linus Torvalds wrote:
>
> As a result, doing a "recalc_sigpending_and_wake(()"
and btw it should die, I think.
> is definitely
> incorrect, because sigpending state cannot actually have changed.
Yes, if we need to wakeup in this case something is already wrong.
> - somebody setting TASK_SLEEPING -> __schedule() testing the
> signal_pending_state()
>
> and as far as I can tell we have proper barriers for those (the
> scheduler gets the rq lock
Yes, but... ttwu() takse another lock, ->pi_lock to test ->state.
This looks racy, even if wmb() actually acts as mb(), we don't
have mb() on the other side and schedule() can miss SIGPENDING?
Unless the task does set_current_state(TASK_INTERRUPTIBLE) which
adds mb(). But, just for example, sigsuspend() relies on schedule().
> smp_wmb();
> raw_spin_lock_irqsave(&p->pi_lock, flags);
> if (!(p->state & state))
>
> before it tests the task state. And the wmb() *together* with the
> spinlock really should be a full memory barrier (nothing can get out
> from the spinlock, and any writes before this had better be serialized
> by the wmb and the write inherent in the spinlock itself). But this is
> definitely some subtle stuff.
So perhaps it makes sense to re-test after s/smp_wmb/smp_mb/ ?
And perhaps we can add smp_mb__before_lock(), we alredy have
smp_mb__after_lock().
And of course, there could be another bug. I just did
"grep recalc_sigpending" and immediately found at least one buggy
user, fs/dlm/user.c which calls it lockless.
> > Every time Xorg hangs up, the status of Xorg is read as following(cat /proc/2597/status):
Gao, could you show /proc/pid/stack just in case?
Oleg.
next prev parent reply other threads:[~2013-08-08 19:23 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <tencent_26310211398C21034BD3B2F9@qq.com>
2013-08-08 18:19 ` Patch for lost wakeups Linus Torvalds
2013-08-08 19:17 ` Oleg Nesterov [this message]
2013-08-08 19:51 ` Linus Torvalds
2013-08-09 13:04 ` Oleg Nesterov
2013-08-09 18:21 ` Linus Torvalds
2013-08-11 17:25 ` Oleg Nesterov
2013-08-11 17:27 ` Oleg Nesterov
[not found] ` <tencent_293B72F26D71A4191C7C999A@qq.com>
2013-08-11 17:39 ` Oleg Nesterov
2013-08-11 23:52 ` James Bottomley
2013-08-12 17:02 ` [PATCH] sched: fix the theoretical signal_wake_up() vs schedule() race Oleg Nesterov
2013-08-13 7:55 ` Peter Zijlstra
2013-08-13 14:33 ` Oleg Nesterov
2013-08-16 18:46 ` [tip:sched/core] sched: Fix the theoretical signal_wake_up() vs. " tip-bot for Oleg Nesterov
2013-08-17 15:05 ` Oleg Nesterov
2013-08-19 7:13 ` Ingo Molnar
2013-08-09 15:18 ` [PATCH 0/1] dlm: kill the unnecessary and wrong device_close()->recalc_sigpending() Oleg Nesterov
2013-08-09 15:19 ` [PATCH 1/1] " Oleg Nesterov
2013-08-12 20:26 ` David Teigland
2013-08-09 13:28 ` Patch for lost wakeups Oleg Nesterov
2013-08-09 15:31 ` block_all_signals() must die (Was: Patch for lost wakeups) Oleg Nesterov
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