RE: [PATCH] perf/core: remove a meaningless pair of rcu_read_{,un}lock()

[Xu, Yanfei]

> -----Original Message-----
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Sent: Wednesday, June 8, 2022 5:43 PM
> To: Xu, Yanfei <yanfei.xu@xxxxxxxxx>
> Cc: mingo@xxxxxxxxxx; acme@xxxxxxxxxx; mark.rutland@xxxxxxx;
> alexander.shishkin@xxxxxxxxxxxxxxx; jolsa@xxxxxxxxxx; namhyung@xxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linux-perf-users@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] perf/core: remove a meaningless pair of
> rcu_read_{,un}lock()
> 
> On Wed, Jun 08, 2022 at 05:09:38PM +0800, Yanfei Xu wrote:
> > Per the codes, this pair of rcu_read_{,un}lock() protects nothing.
> > Let's remove it.
> >
> > Further, the variable "pmu" is safe as it is in the SRCU read-side
> > critical scope of "pmus_srcu" and perf_pmu_unregister() delete it
> > after calling synchronize_srcu.
> >
> > Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxx>
> > ---
> >  kernel/events/core.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c index
> > 950b25c3f210..36b0df6feab4 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -11296,9 +11296,7 @@ static struct pmu *perf_init_event(struct
> perf_event *event)
> >  	}
> >
> >  again:
> > -	rcu_read_lock();
> >  	pmu = idr_find(&pmu_idr, type);
> > -	rcu_read_unlock();
> 
> You're mistaken, this is required for the radix tree internal nodes.
> Without it we can't safely traverse the radix tree without full serialization
> against the modifiers.
Thanks your reminder! And after digging and learning these commits: f9c46d6ea5ce ("idr: make idr_find rcu-safe ") and 452a68d0ef34("KVM: hyperv: idr_find needs RCU protection "). Now I understand it.

Thanks,
Yanfei