Re: [PATCH v8 11/17] zsmalloc: make zspage lock preemptible

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (25/02/24 09:59), Sebastian Andrzej Siewior wrote:
> > +/*
> > + * The zspage lock can be held from atomic contexts, but it needs to remain
> > + * preemptible when held for reading because it remains held outside of those
> > + * atomic contexts, otherwise we unnecessarily lose preemptibility.
> > + *
> > + * To achieve this, the following rules are enforced on readers and writers:
> > + *
> > + * - Writers are blocked by both writers and readers, while readers are only
> > + *   blocked by writers (i.e. normal rwlock semantics).
> > + *
> > + * - Writers are always atomic (to allow readers to spin waiting for them).
> > + *
> > + * - Writers always use trylock (as the lock may be held be sleeping readers).
> > + *
> > + * - Readers may spin on the lock (as they can only wait for atomic writers).
> > + *
> > + * - Readers may sleep while holding the lock (as writes only use trylock).
> > + */
> > +static void zspage_read_lock(struct zspage *zspage)
> > +{
> > +	struct zspage_lock *zsl = &zspage->zsl;
> > +
> > +	rwsem_acquire_read(zsl_dep_map(zsl), 0, 0, _RET_IP_);
> > +
> > +	spin_lock(&zsl->lock);
> > +	zsl->cnt++;
> > +	spin_unlock(&zsl->lock);
> 
> How is this working given that a read_lock always increments the
> counter? If it is write_locked then a read_lock makes it UNLOCKED.

If zspage is write-locked then zsl->lock is also locked, because
write-lock returns with zsl->lock acquired and releases it in
write-unlock.