From: Oleg Nesterov <oleg@redhat.com>
To: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org,
waiman.long@hp.com, peterz@infradead.org,
raghavendra.kt@linux.vnet.ibm.com
Subject: Re: BUG: spinlock bad magic on CPU#0, migration/0/9
Date: Fri, 13 Feb 2015 19:53:28 +0100 [thread overview]
Message-ID: <20150213185328.GA19746@redhat.com> (raw)
In-Reply-To: <20150213181752.GB11953@opentech.at>
On 02/13, Nicholas Mc Guire wrote:
>
> On Thu, 12 Feb 2015, Oleg Nesterov wrote:
>
> > Nicholas, sorry, I sent the patch but forgot to CC you.
> > See https://lkml.org/lkml/2015/2/12/587
> >
> > And please note that "completion" was specially designed to guarantee
> > that complete() can't play with this memory after wait_for_completion/etc
> > returns.
> >
>
> hmmm.... I guess that "falling out of context" can happen in a number of cases
> with completion - any of the timeout/interruptible variants e.g:
>
> void xxx(void)
> {
> struct completion c;
>
> init_completion(&c);
>
> expose_this_completion(&c);
>
> wait_for_completion_timeout(&c,A_FEW_JIFFIES);
> }
>
> and if the other side did not call complete() within A_FEW_JIFFIES then
> it would result in the same failure - I don't think the API can prevent
> this type of bug.
Yes sure, but in this case the user of wait_for_completion_timeout() should
blame itself, it is simply buggy.
> Tt has to be ensured by additional locking
Yes, but
> drivers/misc/tifm_7xx1.c:tifm_7xx1_resume() resolve this issue by resetting
> the completion to NULL and testing for !NULL before calling complete()
> with appropriate locking protection access.
I don't understand this code, I can be easily wrong. but at first glance it
doesn't need completion at all. Exactly because it relies on the additional
fm->lock. ->finish_me could be "task_struct *", the tifm_7xx1_resume() could
simply do schedule_timeout(), tifm_7xx1_isr() could do wake_up_process().
Nevermind, this is off-topic and most probably I misread this code.
> Never the less of course the proposed change in completion_done() was a bug -
> many thanks for catching that so quickly !
OK, perhaps you can ack the fix I sent?
Oleg.
next prev parent reply other threads:[~2015-02-13 18:55 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-12 0:34 BUG: spinlock bad magic on CPU#0, migration/0/9 Paul E. McKenney
2015-02-12 3:15 ` Davidlohr Bueso
2015-02-12 3:43 ` Paul E. McKenney
2015-02-12 17:28 ` Oleg Nesterov
2015-02-12 17:41 ` Oleg Nesterov
2015-02-12 17:58 ` Davidlohr Bueso
2015-02-12 19:10 ` Nicholas Mc Guire
2015-02-12 19:37 ` Oleg Nesterov
2015-02-12 21:27 ` Oleg Nesterov
2015-02-13 18:17 ` Nicholas Mc Guire
2015-02-13 18:53 ` Oleg Nesterov [this message]
2015-02-14 8:35 ` Nicholas Mc Guire
2015-02-14 14:00 ` Oleg Nesterov
2015-02-12 19:59 ` Davidlohr Bueso
2015-02-12 19:32 ` Nicholas Mc Guire
2015-02-12 19:39 ` Oleg Nesterov
2015-02-12 19:59 ` [PATCH] sched/completion: completion_done() should serialize with complete() Oleg Nesterov
2015-02-13 21:09 ` Paul E. McKenney
2015-02-13 21:56 ` Davidlohr Bueso
2015-02-13 22:02 ` Davidlohr Bueso
2015-02-16 8:21 ` Peter Zijlstra
2015-02-16 16:51 ` Oleg Nesterov
2015-02-18 17:06 ` [tip:sched/core] sched/completion: Serialize completion_done() " tip-bot for Oleg Nesterov
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