Re: [PATCH v3 04/19] VFS: use wait_var_event for waiting in d_alloc_parallel()

[Al Viro]

On Tue, Apr 28, 2026 at 09:18:39PM +1000, NeilBrown wrote:
> > d_must_wait() conditional, though - ->d_flags and ->d_lock are in different
> > cachelines and there's no need to dirty both every time we are called.
> > IOW, have d_must_wait() do this:
> > 	if (!d_in_lookup(dentry))
> > 		return false;
> > 	if (!(dentry->d_flags & DCACHE_LOCK_WAITER))
> > 		dentry->d_flags |= DCACHE_LOCK_WAITER;
> > 	return true;
> 
> The only time that DCACHE_LOCK_WAITER will already be set is when there
> are two (or more) waiters as well as the thread they are waiting on.
> That means three (or more) threads all accessing the same name at the
> same time.  How often does that happen?  Is the micro-optimisation worth
> the small increase in code size?

Depends upon the load, obviously - a bunch of threads hitting the same
pathname at the same time... not impossible.

More to the point, why not set DCACHE_LOCK_WAITER as soon as we grab ->d_lock
there?  Then waiting becomes simply "wait until !d_in_lookup()".  Sure, we
might end up setting DCACHE_LOCK_WAITER on a dentry that has just dropped
DCACHE_PAR_LOOKUP - who cares?

While we are at it, do we need to drop it when we clear PAR_LOOKUP?  Because
if we do not, the whole "what do we return from __d_lookup_unhash()" thing
disappears - we simply pass the dentry to end_dir_add() and have it check
->d_flags & DCACHE_LOCK_WAITER to decide whether to bother with wakeup.