[RFC][PATCH] wake_up_var() memory ordering

David Howells dhowells at redhat.com
Tue Jun 25 00:51:01 PDT 2019

Peter Zijlstra <peterz at 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?

> @@ -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()?

> -		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()?

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.


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