On Tue, Apr 23, 2019 at 09:55:59PM +0200, Peter Zijlstra wrote: > On Tue, Apr 23, 2019 at 03:41:32PM -0400, Waiman Long wrote: > > On 4/23/19 3:34 PM, Peter Zijlstra wrote: > > > On Tue, Apr 23, 2019 at 03:12:16PM -0400, Waiman Long wrote: > > > >> You are right on that. However, there is a variant called > > >> preempt_enable_no_resched() that doesn't have this side effect. So I am > > >> going to use that one instead. > > > Only if the very next line is schedule(). Otherwise you're very much not > > > going to use that function. > > > > May I know the reason why. > > Because it can 'consume' a need_resched and introduces arbitrary delays > before the schedule() eventually happens, breaking the very notion of > PREEMPT=y (and the fundamentals RT relies on). > > > I saw a number of instances of > > preempt_enable_no_resched() without right next a schedule(). > > Look more closely.. and let me know, if true, those are bugs that need > fixing. > > Argghhh.. BPF... > > Also, with the recent RCU rework, we can probably drop that > rcu_read_lock()/rcu_read_unlock() from there if we're disabling > preemption anyway. > > --- > Subject: bpf: Fix preempt_enable_no_resched() abuse > > Unless the very next line is schedule(), or implies it, one must not use > preempt_enable_no_resched(). It can cause a preemption to go missing and > thereby cause arbitrary delays, breaking the PREEMPT=y invariant. > > Cc: Roman Gushchin <g...@fb.com> > Cc: Alexei Starovoitov <a...@kernel.org> > Cc: Daniel Borkmann <dan...@iogearbox.net> > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > --- > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h > index f02367faa58d..944ccc310201 100644 > --- a/include/linux/bpf.h > +++ b/include/linux/bpf.h > @@ -510,7 +510,7 @@ int bpf_prog_array_copy(struct bpf_prog_array __rcu > *old_array, > } \ > _out: \ > rcu_read_unlock(); \ > - preempt_enable_no_resched(); \ > + preempt_enable(); \ > _ret; \
Applied to bpf tree. Thanks! It should have been fixed long ago. Not sure how we kept forgetting about it.
