linux-kernel - Re: [PATCH -v3] use per cpu data for single cpu ipi calls

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Message-Id: <1233319731.4495.177.camel@laptop>
Date:	Fri, 30 Jan 2009 13:48:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>, npiggin@...e.de,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH -v3] use per cpu data for single cpu ipi calls

On Fri, 2009-01-30 at 13:38 +0100, Jens Axboe wrote:

> >  		if (!wait) {
> > +			/*
> > +			 * We are calling a function on a single CPU
> > +			 * and we are not going to wait for it to finish.
> > +			 * We first try to allocate the data, but if we
> > +			 * fail, we fall back to use a per cpu data to pass
> > +			 * the information to that CPU. Since all callers
> > +			 * of this code will use the same data, we must
> > +			 * synchronize the callers to prevent a new caller
> > +			 * from corrupting the data before the callee
> > +			 * can access it.
> > +			 *
> > +			 * The CSD_FLAG_LOCK is used to let us know when
> > +			 * the IPI handler is done with the data.
> > +			 * The first caller will set it, and the callee
> > +			 * will clear it. The next caller must wait for
> > +			 * it to clear before we set it again. This
> > +			 * will make sure the callee is done with the
> > +			 * data before a new caller will use it.
> > +			 * We use spinlocks to manage the callers.
> > +			 */
> 
> That last sentence appears stale now, since there's no locking involved
> for the per-cpu csd.

Gah, I edited that, and generated the patch using

# git diff --stat -p 4f4b6c1a94a8735bbdc030a2911cf395495645b6.. kernel/smp.c | xclip

on Ingo's tip/master. However it seems such a diff does not include
non-commited changes, even though a regular git diff does:

# git diff kernel/smp.c
diff --git a/kernel/smp.c b/kernel/smp.c
index 9eead6c..bbedbb7 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -251,7 +251,6 @@ int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
                         * it to clear before we set it again. This
                         * will make sure the callee is done with the
                         * data before a new caller will use it.
-                        * We use spinlocks to manage the callers.
                         */
                        data = kmalloc(sizeof(*data), GFP_ATOMIC);
                        if (data)

What is the recommended way to do such a diff?

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