linux-kernel - Re: [PATCH v4] perf_counter: Fix race in attaching counters to tasks and exiting

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Message-ID: <18974.32901.99290.162863@cargo.ozlabs.ibm.com>
Date:	Thu, 28 May 2009 22:16:05 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4] perf_counter: Fix race in attaching counters to
 tasks and exiting

Peter Zijlstra writes:

> The poor task will now have to wait for yet another RCU grace period,
> but yeah :-)

I moved the put_task_struct into put_ctx, inside the if, so the poor
task won't get impatient. :)

> Looks good, although lockdep will complain, the proper annotation would
> be spin_lock_nested_lock(&ctx->lock, &rq->lock), but since we don't have
> access to the rq here, we should change the second spin_lock, to:
> 
>   spin_lock_nested(&next_ctx->lock, SINGLE_DEPTH_NESTING);

Ah OK, thanks.

> You jump into an rcu_read_lock() section there.

Oops.  I'll fix that.  I also need to use spin_[un]lock_irq in there.
New version coming.  Thanks for the review.

Paul.
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