linux-kernel - Re: BUG: spinlock bad magic on CPU#0, migration/0/9

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Message-ID: <20150214140048.GA28535@redhat.com>
Date:	Sat, 14 Feb 2015 15:00:48 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	Davidlohr Bueso <dave@...olabs.net>, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, waiman.long@...com,
	peterz@...radead.org, raghavendra.kt@...ux.vnet.ibm.com
Subject: Re: BUG: spinlock bad magic on CPU#0, migration/0/9

On 02/14, Nicholas Mc Guire wrote:
>
> Basically if you call wait_for_completion_timeout and the timeout condition
> occures you always need some way of notifying the completing end that it
> should no longer call complete()/complete_all().

Sure. "struct completion" doesn't differ from any other object when it comes
to use-after-free.

> > OK, perhaps you can ack the fix I sent?
>
> the only question I still have is that there would be no matching
> smp_wmb() to the smp_rmb() you are using (atleast I did not figure out where).

You seem to assume that every rmb() must be paired with wmb(). This is not
always true.

But as for completion_done(), its rmb() pairs with "release" semantics of
complete()->spin_unlock(), which is a "one way" barrier.

Oleg.

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