lkml.org 
[lkml]   [2020]   [Jan]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 2/2] events: callchain: Use RCU API to access RCU pointer
On Thu, Jan 30, 2020 at 09:23:21AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 29, 2020 at 05:19:09PM -0500, Joel Fernandes wrote:
> > On Wed, Jan 29, 2020 at 09:38:13PM +0530, Amol Grover wrote:
> > > callchain_cpus_entries is annotated as an RCU pointer.
> > > Hence rcu_dereference_protected or similar RCU API is
> > > required to dereference the pointer.
> > >
> > > This fixes the following sparse warning
> > > kernel/events/callchain.c:65:17: warning: incorrect type in assignment
>
> Seems silly to have this two patches; the first introduces the second
> issue, might as well fix it all in one go.
>

Got it. I'll combine them into a single patch and re-send.

> Also look at the output of:
>
> git log --oneline kernel/events/
>
> and then at your $subject.
>
> > > Signed-off-by: Amol Grover <frextrite@gmail.com>
> > > ---
> > > kernel/events/callchain.c | 3 ++-
> > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> > > index f91e1f41d25d..a672d02a1b3a 100644
> > > --- a/kernel/events/callchain.c
> > > +++ b/kernel/events/callchain.c
> > > @@ -62,7 +62,8 @@ static void release_callchain_buffers(void)
> > > {
> > > struct callchain_cpus_entries *entries;
> > >
> > > - entries = callchain_cpus_entries;
> > > + entries = rcu_dereference_protected(callchain_cpus_entries,
> > > + lockdep_is_held(&callchain_mutex));
> >
> >
> > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>
> Do we really need that smp_read_barrier_depends() here? Then again, I
> don't suppose this is a fast path.
>

rcu_dereference_protected is actually a lightweight API and IIRC it
omits the READ_ONCE() and hence the memory barriers.

Thanks
Amol

> IIRC even Alpha got the dependent write ordering right.
>
> > > RCU_INIT_POINTER(callchain_cpus_entries, NULL);
> > > call_rcu(&entries->rcu_head, release_callchain_buffers_rcu);
> > > }
> > > --
> > > 2.24.1
> > >

\
 
 \ /
  Last update: 2020-01-30 11:15    [W:0.063 / U:0.644 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site