linux-kernel - Re: [PATCH RFC] perf_counter: Don't swap contexts containing locked mutex

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Message-Id: <1243584793.23657.168.camel@twins>
Date:	Fri, 29 May 2009 10:13:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] perf_counter: Don't swap contexts containing
 locked mutex

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.
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