linux-kernel - Re: [PATCH] perf: Fix missing RCU reader protection in perf_event_clear

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Message-ID: <fd5f90b2-62b1-4984-8969-41e87322b677@paulmck-laptop>
Date: Wed, 16 Oct 2024 11:23:43 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Liang, Kan" <kan.liang@...ux.intel.com>
Cc: peterz@...radead.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
	linux-next@...r.kernel.org, linux-perf-users@...r.kernel.org,
	kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] perf: Fix missing RCU reader protection in
 perf_event_clear_cpumask()

On Fri, Sep 13, 2024 at 11:14:41AM -0700, Paul E. McKenney wrote:
> On Fri, Sep 13, 2024 at 12:25:59PM -0400, Liang, Kan wrote:
> > 
> > 
> > On 2024-09-13 12:23 p.m., kan.liang@...ux.intel.com wrote:
> > > From: Kan Liang <kan.liang@...ux.intel.com>
> > > 
> > > Running rcutorture scenario TREE05, the below warning is triggered.
> > > 
> > > [   32.604594] WARNING: suspicious RCU usage
> > > [   32.605928] 6.11.0-rc5-00040-g4ba4f1afb6a9 #55238 Not tainted
> > > [   32.607812] -----------------------------
> > > [   32.609140] kernel/events/core.c:13946 RCU-list traversed in non-reader section!!
> > > [   32.611595] other info that might help us debug this:
> > > [   32.614247] rcu_scheduler_active = 2, debug_locks = 1
> > > [   32.616392] 3 locks held by cpuhp/4/35:
> > > [   32.617687]  #0: ffffffffb666a650 (cpu_hotplug_lock){++++}-{0:0}, at: cpuhp_thread_fun+0x4e/0x200
> > > [   32.620563]  #1: ffffffffb666cd20 (cpuhp_state-down){+.+.}-{0:0}, at: cpuhp_thread_fun+0x4e/0x200
> > > [   32.623412]  #2: ffffffffb677c288 (pmus_lock){+.+.}-{3:3}, at: perf_event_exit_cpu_context+0x32/0x2f0
> > > 
> > > In perf_event_clear_cpumask(), uses list_for_each_entry_rcu() without an
> > > obvious RCU read-side critical section.
> > > 
> > > Either pmus_srcu or pmus_lock is good enough to protect the pmus list.
> > > In the current context, pmus_lock is already held. The
> > > list_for_each_entry_rcu() is not required.
> > > 
> > > Fixes: 4ba4f1afb6a9 ("perf: Generic hotplug support for a PMU with a scope")
> > > Reported-by: Paul E. McKenney <paulmck@...nel.org>
> > > Closes: https://lore.kernel.org/lkml/2b66dff8-b827-494b-b151-1ad8d56f13e6@paulmck-laptop/
> > > Tested-by: Paul E. McKenney <paulmck@...nel.org>
> > > Reported-by: kernel test robot <oliver.sang@...el.com>
> > > Closes: https://lore.kernel.org/oe-lkp/202409131559.545634cc-oliver.sang@intel.com
> > 
> > Forgot to add the below tag, please fold it.
> > 
> > Suggested-by: Peter Zijlstra <peterz@...radead.org>
> 
> Have one of these to go along with it.  ;-)
> 
> Tested-by: Paul E. McKenney <paulmck@...nel.org>

Just following up, seeing how this is not yet in -next.

Is this on its way upstream?

							Thanx, Paul

> > Thanks,
> > Kan
> > > Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
> > > ---
> > >  kernel/events/core.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > index 20e97c1aa4d6..5ba9934b49df 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -13912,7 +13912,7 @@ static void perf_event_clear_cpumask(unsigned int cpu)
> > >  	}
> > >  
> > >  	/* migrate */
> > > -	list_for_each_entry_rcu(pmu, &pmus, entry, lockdep_is_held(&pmus_srcu)) {
> > > +	list_for_each_entry(pmu, &pmus, entry) {
> > >  		if (pmu->scope == PERF_PMU_SCOPE_NONE ||
> > >  		    WARN_ON_ONCE(pmu->scope >= PERF_PMU_MAX_SCOPE))
> > >  			continue;