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Subject: Re: [PATCH RFC] perf_counter: Don't swap contexts containing
 locked mutex
From: Peter Zijlstra <peterz@infradead.org>
To: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org
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	 <1243584388.23657.156.camel@twins>  <1243584793.23657.168.camel@twins>
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Date: Fri, 29 May 2009 10:28:41 +0200
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On Fri, 2009-05-29 at 10:13 +0200, Peter Zijlstra wrote:
> On Fri, 2009-05-29 at 10:06 +0200, Peter Zijlstra wrote:
> 
> > static struct perf_counter_ctx *pin_ctx(struct perf_counter *counter, u64 *old_gen)
> > {
> > 	struct perf_counter_context *ctx;
> > 	unsigned long flags;
> > 
> > 	rcu_read_lock();
> > retry:
> > 	ctx = rcu_dereference(counter->ctx);
> > 	spin_lock_irqsave(&ctx->lock, flags);
> > 	if (ctx != rcu_dereference(counter->ctx))
> > 		goto retry;
> > 
> > 	*old_gen = ctx->generation;
> > 	ctx->generation = ~0ULL;
> > 	spin_unlock_irqrestore(&ctx->lock, flags);
> > 	rcu_read_unlock();
> > 
> > 	return ctx;
> > }
> > 
> > static void unpin_ctx(struct perf_counter_ctx *ctx, u64 old_gen)
> > {
> > 	unsigned long flags;
> > 
> > 	spin_lock_irqsave(&ctx->lock, flags);
> > 	ctx->generation = old_gen;
> > 	spin_unlock_irqrestore(&ctx->lock, flags);
> > }
> 
> OK, I think I got this wrong, counter->ctx isn't the problem.
> task->perf_counter_ctx is.
> 
> Still would be nice to write it in the above form. I'll go over the code
> again to see who else might want it.

OK, I went over the code, and your patch does indeed cover the few spots
we need. It was just my brain going haywire and auditing the wrong
pattern.