(sorry for cross-posting to moderated lists btw, I've since acquired a patch to get_maintainers.pl that wil exclude them in the future)
On Tue, Jun 25, 2019 at 08:51:01AM +0100, David Howells wrote: > Peter Zijlstra <pet...@infradead.org> wrote: > > > I tried using wake_up_var() today and accidentally noticed that it > > didn't imply an smp_mb() and specifically requires it through > > wake_up_bit() / waitqueue_active(). > > Thinking about it again, I'm not sure why you need to add the barrier when > wake_up() (which this is a wrapper around) is required to impose a barrier at > the front if there's anything to wake up (ie. the wait queue isn't empty). > > If this is insufficient, does it make sense just to have wake_up*() functions > do an unconditional release or full barrier right at the front, rather than it > being conditional on something being woken up? The curprit is __wake_up_bit()'s usage of waitqueue_active(); it is this latter (see its comment) that requires the smp_mb(). wake_up_bit() and wake_up_var() are wrappers around __wake_up_bit(). Without this barrier it is possible for the waitqueue_active() load to be hoisted over the cond=true store and the remote end can miss the store and we can miss its enqueue and we'll all miss a wakeup and get stuck. Adding an smp_mb() (or use wq_has_sleeper()) in __wake_up_bit() would be nice, but I fear some people will complain about overhead, esp. since about half the sites don't need the barrier due to being behind test_and_clear_bit() and the other half using smp_mb__after_atomic() after some clear_bit*() variant. There's a few sites that seem to open-code wait_var_event()/wake_up_var() and those actually need the full smp_mb(), but then maybe they should be converted to var instread of bit anyway. > > @@ -619,9 +614,7 @@ static int dvb_usb_fe_sleep(struct dvb_frontend *fe) > > err: > > if (!adap->suspend_resume_active) { > > adap->active_fe = -1; > > I'm wondering if there's a missing barrier here. Should the clear_bit() on > the next line be clear_bit_unlock() or clear_bit_release()? That looks reasonable, but I'd like to hear from the DVB folks on that. > > - clear_bit(ADAP_SLEEP, &adap->state_bits); > > - smp_mb__after_atomic(); > > - wake_up_bit(&adap->state_bits, ADAP_SLEEP); > > + clear_and_wake_up_bit(ADAP_SLEEP, &adap->state_bits); > > } > > > > dev_dbg(&d->udev->dev, "%s: ret=%d\n", __func__, ret); > > diff --git a/fs/afs/fs_probe.c b/fs/afs/fs_probe.c > > index cfe62b154f68..377ee07d5f76 100644 > > --- a/fs/afs/fs_probe.c > > +++ b/fs/afs/fs_probe.c > > @@ -18,6 +18,7 @@ static bool afs_fs_probe_done(struct afs_server *server) > > > > wake_up_var(&server->probe_outstanding); > > clear_bit_unlock(AFS_SERVER_FL_PROBING, &server->flags); > > + smp_mb__after_atomic(); > > wake_up_bit(&server->flags, AFS_SERVER_FL_PROBING); > > return true; > > } > > Looking at this and the dvb one, does it make sense to stick the release > semantics of clear_bit_unlock() into clear_and_wake_up_bit()? I was thinking of adding another helper, maybe unlock_and_wake_up_bit() that included that extra barrier, but maybe making it unconditional isn't the worst idea. > Also, should clear_bit_unlock() be renamed to clear_bit_release() (and > similarly test_and_set_bit_lock() -> test_and_set_bit_acquire()) if we seem to > be trying to standardise on that terminology. That definitely makes sense to me, there's only 157 clear_bit_unlock() and 76 test_and_set_bit_lock() users (note the asymetry of that).