linux-kernel - Re: [PATCH v2] perf/core: fix RCU issues with cgroup monitoring mode

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Message-ID: <1445991431.7476.62.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 27 Oct 2015 17:17:11 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH v2] perf/core: fix RCU issues with cgroup monitoring mode

On Tue, 2015-10-27 at 16:37 -0700, Stephane Eranian wrote:

> But that macro is already embedded into the  task_css_set() macro.
> What we are saying here is that we have another way of ensuring the
> cgroup cannot disappear here, and thus the rcu lockdep detects we
> are not holding the rcu read lock but we know this is okay. We are
> trying to avoid grabbing the rcu read lock when it is not really needed
> to guarantee correct execution. So we are just shutting up the lockdep
> in these particular cases by passing true as the safe argument value.

Sure, but in lockdep verbs, the following :

+       /* holding ctx->lock, so cgroup access is safe */
+       cgrp = perf_cgroup_from_task(task, true);

would map to :

cgrp = perf_cgroup_from_task(task, lockdep_is_held(&ctx->lock));

Notice the comment becomes useless.



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