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

From: Eric Dumazet
Date: Tue Oct 27 2015 - 16:24:13 EST

On Tue, 2015-10-27 at 20:25 +0100, Stephane Eranian wrote:
> This patch eliminates all known RCU violations detected
> by the RCU checker (PROVE_RCU). The impact code paths
> were all related to cgroup mode monitoring and involved
> access a task's cgrp.
> V2 is updated to include suggestions from PeterZ to eliminate
> some of the warnings without grabbing the rcu_read lock because
> we know we are already holding the ctx->lock which prevents
> the cgroup from disappearing while we are accessing it.
> The trick, as suggested by Peter, is to modify the
> perf_cgroup_from_task() to take an extra boolean parameter
> to allow bypassing the lockdep test in the task_subsys_cstate()
> macros. This patch uses this approach to update all calls the
> perf_cgroup_from_task().
> V2 Patch relative to:
> 8b3c8e6 Revert "rculist: Make list_entry_rcu() use lockless_dereference()"
> Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
> ---

Instead trusting caller to provide correct 'safe' boolean,
what about using lockdep_is_held() ?

This way, you keep full lockep support.

A random example is

#define rcu_dereference_rtnl(p) rcu_dereference_check(p, lockdep_rtnl_is_held())

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