linux-kernel - Re: [PATCH] smp_call_function

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Message-ID: <20100323153359.GM2517@linux.vnet.ibm.com>
Date:	Tue, 23 Mar 2010 08:33:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Anton Blanchard <anton@...ba.org>,
	Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>,
	Jens Axboe <jens.axboe@...cle.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Milton Miller <miltonm@....com>, Nick Piggin <npiggin@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] smp_call_function_many SMP race

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.

							Thanx, Paul
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