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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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Date: Fri, 29 May 2009 10:13:13 +0200
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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.