Re: [PATCH 1/3] sysfs: simplify handling for s_active refcount

[Neil Brown <neilb@suse.de>]

On Mon, 29 Mar 2010 15:43:25 +1100
Neil Brown <neilb@suse.de> wrote:

> However if we do consider memory ordering guarantees we can describe a clear
> limit to the possibly delay between SYSFS_FLAG_REMOVED being set, and being
> seen.  The atomic_inc_not_zero serves as a memory barrier in exactly the same
> way that the current code requires atomic_dec_return.  So while
> 
> 	if (likely(sd)
> 	    && (sd->s_flags & SYSFS_FLAG_REMOVED) == 0
> 	    && atomic_inc_not_zero(&sd->s_active)) {
> 
> could possibly gain a reference even 'after' SYS_FLAG_REMOVED as been set,
> a second call to this on the same processor will see SYSFS_FLAG_REMOVED.
> So at the absolute most, we could see NCPUS active references gained and
> dropped after SYSFS_FLAG_REMOVED was set - a clear limit which is all we need.

It just occurred to me that this 'proof' isn't quite complete in itself.  I
need to also show that there is a suitable memory barrier after
SYSFS_FLAG_REMOVED is set.  There is as it is always set under sysfs_mutex,
so the mutex_unlock provides a barrier.
So after sysfs_mutex is unlocked, it is conceivable that each CPU could grant
one active reference against the sysfs_dirent before SYSFS_FLAG_REMOVED was
globally visible.

> I'm still not sure we even need to argue in terms of memory barriers to be
> sure the code is correct, but it seems they are sufficient to give a simple
> proof.

NeilBrown