linux-kernel - Re: sched: spinlock recursion in sched_rr_get

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Message-ID: <549ED5D7.8070007@oracle.com>
Date:	Sat, 27 Dec 2014 10:52:55 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Davidlohr Bueso <dave@...olabs.net>,
	Li Bin <huawei.libin@...wei.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, rui.xiang@...wei.com,
	wengmeiling.weng@...wei.com
Subject: Re: sched: spinlock recursion in sched_rr_get_interval

On 12/27/2014 04:52 AM, Davidlohr Bueso wrote:
>> Hello，
>> > Does ACCESS_ONCE() can help this issue? I have no evidence that its lack is
>> > responsible for the issue, but I think here need it indeed. Is that right?
>> > 
>> > SPIN_BUG_ON(ACCESS_ONCE(lock->owner) == current, "recursion");
> Hmm I guess on a contended spinlock, there's a chance that lock->owner
> can change, if the contended lock is acquired, right between the 'cond'
> and spin_debug(), which would explain the bogus ->owner related
> messages. Of course the same applies to ->owner_cpu. Your ACCESS_ONCE,
> however, doesn't really change anything since we still read ->owner
> again in spin_debug; How about something like this (untested)?

There's a chance that lock->owner would change, but how would you explain
it changing to 'current'?

That is, what race condition specifically creates the
'lock->owner == current' situation in the debug check?

Thanks,
Sasha
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