Subject: Re: [PATCH v4 1/3] make jump_labels wait while updates are in place

From: Jason Baron <[email-masked]>
Date: Fri, 27 Apr 2012 09:53:21 -0400

On Thu, Apr 26, 2012 at 08:43:06PM -0400, Steven Rostedt wrote:
> On Thu, Apr 26, 2012 at 07:51:05PM -0300, Glauber Costa wrote:
> > In mem cgroup, we need to guarantee that two concurrent updates
> > of the jump_label interface wait for each other. IOW, we can't have
> > other updates returning while the first one is still patching the
> > kernel around, otherwise we'll race.
> 
> But it shouldn't. The code as is should prevent that.
> 
> > 
> > I believe this is something that can fit well in the static branch
> > API, without noticeable disadvantages:
> > 
> > * in the common case, it will be a quite simple lock/unlock operation
> > * Every context that calls static_branch_slow* already expects to be
> >   in sleeping context because it will mutex_lock the unlikely case.
> > * static_key_slow_inc is not expected to be called in any fast path,
> >   otherwise it would be expected to have quite a different name. Therefore
> >   the mutex + atomic combination instead of just an atomic should not kill
> >   us.
> > 
> > Signed-off-by: Glauber Costa <[email-masked]>
> > CC: Tejun Heo <[email-masked]>
> > CC: Li Zefan <[email-masked]>
> > CC: Kamezawa Hiroyuki <[email-masked]>
> > CC: Johannes Weiner <[email-masked]>
> > CC: Michal Hocko <[email-masked]>
> > CC: Ingo Molnar <[email-masked]>
> > CC: Jason Baron <[email-masked]>
> > ---
> >  kernel/jump_label.c |   21 +++++++++++----------
> >  1 files changed, 11 insertions(+), 10 deletions(-)
> > 
> > diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> > index 4304919..5d09cb4 100644
> > --- a/kernel/jump_label.c
> > +++ b/kernel/jump_label.c
> > @@ -57,17 +57,16 @@ static void jump_label_update(struct static_key *key, int enable);
> >  
> >  void static_key_slow_inc(struct static_key *key)
> >  {
> > +	jump_label_lock();
> >  	if (atomic_inc_not_zero(&key->enabled))
> > -		return;
> 
> If key->enabled is not zero, there's nothing to be done. As the jump
> label has already been enabled. Note, the key->enabled doesn't get set
> until after the jump label is updated. Thus, if two tasks were to come
> in, they both would be locked on the jump_label_lock().
> 

Right, for x86 which uses stop_machine currently, we guarantee that all
cpus are going to see the updated code, before the inc of key->enabled.
However, other arches (sparc, mips, powerpc, for example), seem to be
using much lighter weight updates, which I hope are ok :)

Thanks,

-Jason

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