On Tue, Mar 7, 2017 at 4:27 PM, Paul E. McKenney <paul...@linux.vnet.ibm.com> wrote: >> > [...] >> >> >> >> >> >> What is that mutex? And what locks/unlocks provide synchronization? I >> >> >> see that one uses exp_mutex and another -- exp_wake_mutex. >> >> > >> >> > Both of them. >> >> > >> >> > ->exp_mutex is acquired by the task requesting the grace period, and >> >> > the counter's first increment is done by that task under that mutex. >> >> > This task then schedules a workqueue, which drives forward the grace >> >> > period. Upon grace-period completion, the workqueue handler does the >> >> > second increment (the one that your patch addressed). The workqueue >> >> > handler then acquires ->exp_wake_mutex and wakes the task that holds >> >> > ->exp_mutex (along with all other tasks waiting for this grace period), >> >> > and that task releases ->exp_mutex, which allows the next grace period >> >> > to >> >> > start (and the first increment for that next grace period to be carried >> >> > out under that lock). The workqueue handler releases ->exp_wake_mutex >> >> > after finishing its wakeups. >> >> >> >> Then we need the following for the case when task requesting the grace >> >> period does not block, right? >> > >> > Won't be necessary I think, as the smp_mb() in rcu_seq_end() and the >> > smp_mb__before_atomic() in sync_exp_work_done() already provide the >> > required ordering, no? >> >> smp_mb() is probably fine, but smp_mb__before_atomic() is release not >> acquire. If we want to play that game, then I guess we also need >> smp_mb__after_atomic() there. But it would be way easier to understand >> what's happens there and prove that it's correct, if we use >> store_release/load_acquire. > > Fair point, how about the following?
I am not qualified enough to reason about these smp_mb__after_atomic. >From practical point of view there may be enough barriers in the resulting machine code already, but re formal semantics of adding smp_mb__after_atomic after an unrelated subsequent atomic RMW op I gave up. You must be the best candidate for this now :) > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 6fd8074f1976596898e39f5b7ea1755652533906 > Author: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Date: Tue Mar 7 07:21:23 2017 -0800 > > rcu: Add smp_mb__after_atomic() to sync_exp_work_done() > > The sync_exp_work_done() function needs to fully order the counter-check > operation against anything happening after the corresponding grace period. > This is a theoretical bug, as all current architectures either provide > full ordering for atomic operation on the one hand or implement, > however, a little future-proofing is a good thing. This commit > therefore adds smp_mb__after_atomic() after the atomic_long_inc() > in sync_exp_work_done(). > > Reported-by: Dmitry Vyukov <dvyu...@google.com> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com> > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > index 027e123d93c7..652071abd9b4 100644 > --- a/kernel/rcu/tree_exp.h > +++ b/kernel/rcu/tree_exp.h > @@ -247,6 +247,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, > atomic_long_t *stat, > /* Ensure test happens before caller kfree(). */ > smp_mb__before_atomic(); /* ^^^ */ > atomic_long_inc(stat); > + smp_mb__after_atomic(); /* ^^^ */ > return true; > } > return false;
