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 <[email protected]> Cc: Alexei Starovoitov <[email protected]> Cc: Daniel Borkmann <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> --- 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; \ })
