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

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Message-ID: <1445977445.7476.52.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Tue, 27 Oct 2015 13:24:05 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
	ak@...ux.intel.com, edumazet@...gle.com, acme@...hat.com
Subject: Re: [PATCH v2] perf/core: fix RCU issues with cgroup monitoring mode

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@...gle.com>
> ---


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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