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: Thu, 12 Feb 2015 20:37:34 +0100 [thread overview]
Message-ID: <20150212193734.GA28499@redhat.com> (raw)
In-Reply-To: <20150212191009.GA26275@opentech.at>
On 02/12, Nicholas Mc Guire wrote:
>
> On Thu, 12 Feb 2015, Oleg Nesterov wrote:
>
> > No, sorry, only the 2nd one.
> >
> > > Unless at least document how
> > > you can use these helpers.
> > >
> > > Consider this code:
> > >
> > > void xxx(void)
> > > {
> > > struct completion c;
> > >
> > > init_completion(&c);
> > >
> > > expose_this_completion(&c);
> > >
> > > while (!completion_done(&c)
> > > schedule_timeout_uninterruptible(1);
>
> But that would not break due to the change - even if completion_done() had a
> problem - complete_done() is not consuming x->done it is only checking it?
Nicholas, looks like you didn't read the text below:
> > > Before that change this code was correct, now it is not. Hmm and note that
> > > this is what stop_machine_from_inactive_cpu() does although I do not know
> > > if this is related or not.
> > >
> > > Because completion_done() can now race with complete(), the final
> > > spin_unlock() can write to the memory after it was freed/reused. In this
> > > case it can write to the stack after return.
Or I misunderstood you.
> > bool completion_done(struct completion *x)
> > {
> > - return !!ACCESS_ONCE(x->done);
> > + if (!READ_ONCE(x->done))
> > + return false;
> > +
> > + smp_rmb();
> > + spin_unlock_wait(&x->wait.lock);
> > + return true;
>
> what would be the sense of the spin_unlock_wait here ?
> all you are interested in here is the state of x->done
No. Please see above.
> regarding the smp_rmb() where would the counterpart to that be ?
to avoid the reordering, we should not read ->wait.lock before ->done.
Oleg.
next prev parent reply other threads:[~2015-02-12 19:39 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 [this message]
2015-02-12 21:27 ` Oleg Nesterov
2015-02-13 18:17 ` Nicholas Mc Guire
2015-02-13 18:53 ` Oleg Nesterov
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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