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

From: Eric Dumazet
Date: Tue Oct 27 2015 - 20:17:18 EST

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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