Re: [PATCH] perf: Fix missing RCU reader protection in perf_event_clear_cpumask()

[Paul E. McKenney]

On Fri, Sep 13, 2024 at 12:25:59PM -0400, Liang, Kan wrote:
> 
> 
> On 2024-09-13 12:23 p.m., kan.liang@xxxxxxxxxxxxxxx wrote:
> > From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/lkml/2b66dff8-b827-494b-b151-1ad8d56f13e6@paulmck-laptop/
> > Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-lkp/202409131559.545634cc-oliver.sang@xxxxxxxxx
> 
> Forgot to add the below tag, please fold it.
> 
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

Have one of these to go along with it.  ;-)

Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> Thanks,
> Kan
> > Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > ---
> >  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;