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

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Message-ID: <20090130123858.GQ30821@kernel.dk>
Date:	Fri, 30 Jan 2009 13:38:58 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, Jan 30 2009, Peter Zijlstra wrote:
> On Fri, 2009-01-30 at 12:23 +0100, Jens Axboe wrote:
> 
> > Peter, can you post a final complete patch for review and acks?
> 
> Sure,

Looks good, you can add my acked-by to that. One small comment:

>  		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.

-- 
Jens Axboe

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