On (25/02/27 13:05), Sebastian Andrzej Siewior wrote:
> > > > +static void zram_slot_lock_init(struct zram *zram, u32 index)
> > > > {
> > > > - return spin_trylock(&zram->table[index].lock);
> > > > + lockdep_init_map(slot_dep_map(zram, index),
> > > > + "zram->table[index].lock",
> > > > + zram_lock_class(zram), 0);
> > > > +}
> > > Why do need zram_lock_class and slot_dep_map? As far as I can tell, you
> > > init both in the same place and you acquire both in the same place.
> > > Therefore it looks like you tell lockdep that you acquire two locks
> > > while it would be enough to do it with one.
> >
> > Sorry, I'm not that familiar with lockdep, can you elaborate?
> > I don't think we can pass NULL as lock-class to lockdep_init_map(),
> > this should trigger `if (DEBUG_LOCKS_WARN_ON(!key))` as far as I
> > can tell. I guess it's something else that you are suggesting?
>
> ach. Got it. What about
>
> | static void zram_slot_lock_init(struct zram *zram, u32 index)
> | {
> | static struct lock_class_key __key;
> |
> | lockdep_init_map(slot_dep_map(zram, index),
> | "zram->table[index].lock",
> | &__key, 0);
> | }
>
> So every lock coming from zram belongs to the same class. Otherwise each
> lock coming from zram_slot_lock_init() would belong to a different class
> and for lockdep it would look like they are different locks. But they
> are used always in the same way.
I see. I thought that they key was "shared" between zram meta table
entries because the key is per-zram device, which sort of made sense
(we can have different zram devices in a system - one swap, a bunch
mounted with various file-systems on them).
I can do a 'static key', one for all zram devices.
> > > > static void zram_slot_lock(struct zram *zram, u32 index)
> > > > {
> > > > - spin_lock(&zram->table[index].lock);
> > > > + unsigned long *lock = &zram->table[index].flags;
> > > > +
> > > > + mutex_acquire(slot_dep_map(zram, index), 0, 0, _RET_IP_);
> > > > + wait_on_bit_lock(lock, ZRAM_ENTRY_LOCK, TASK_UNINTERRUPTIBLE);
> > > > + lock_acquired(slot_dep_map(zram, index), _RET_IP_);
> > >
> > > This looks odd. The first mutex_acquire() can be invoked twice by two
> > > threads, right? The first thread gets both (mutex_acquire() and
> > > lock_acquired()) while, the second gets mutex_acquire() and blocks on
> > > wait_on_bit_lock()).
> >
> > Hmm why is this a problem? ... and I'm pretty sure it was you who
> > suggested to put mutex_acquire() before wait_on_bit_lock() [1] ;)
>
> Sure. I was confused that you issue it twice. I didn't noticed the d in
> lock_acquired(). So you have one for lockdep and one for lockstat. That
> is okay ;)
Cool!
