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

[Peter Zijlstra <peterz@infradead.org>]

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.