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Subject: Re: [PATCH] smp_call_function_many SMP race
From: Peter Zijlstra <peterz@infradead.org>
To: paulmck@linux.vnet.ibm.com
Cc: Anton Blanchard <anton@samba.org>,
       Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>,
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       Linus Torvalds <torvalds@linux-foundation.org>,
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Date: Tue, 23 Mar 2010 16:49:18 +0100
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On Tue, 2010-03-23 at 08:33 -0700, Paul E. McKenney wrote:
> On Tue, Mar 23, 2010 at 01:26:43PM +0100, Peter Zijlstra wrote:
> > On Tue, 2010-03-23 at 22:15 +1100, Anton Blanchard wrote:
> > > 
> > > It turns out commit c0f68c2fab4898bcc4671a8fb941f428856b4ad5 (generic-ipi:
> > > cleanup for generic_smp_call_function_interrupt()) is at fault. It removes
> > > locking from smp_call_function_many and in doing so creates a rather
> > > complicated race. 
> > 
> > A rather simple question since my brain isn't quite ready processing the
> > content here..
> > 
> > Isn't reverting that one patch a simpler solution than adding all that
> > extra logic? If not, then the above statement seems false and we had a
> > bug even with that preempt_enable/disable() pair.
> > 
> > Just wondering.. :-)
> 
> If I understand correctly, if you want to fix it by reverting patches,
> you have to revert back to simple locking (up to and including
> 54fdade1c3332391948ec43530c02c4794a38172).  And I believe that the poor
> performance of simple locking was whole reason for the series of patches.

Right, then c0f68c2 did not in fact cause this bug..