Re: [PATCH v5 06/14] futex: Add helper which include the put of a hb after end of operation.
From: Sebastian Andrzej Siewior
Date: Tue Dec 17 2024 - 12:08:10 EST
On 2024-12-16 19:48:07 [+0100], Thomas Gleixner wrote:
> So something like this might be more to the point:
>
> futex: Prepare for reference counting of the process private hash
>
> To support runtime resizing of the process private hash, it's
> required to add a reference count to the hash structure. The
> reference count ensures that the hash cannot be resized or freed
> while a task is operating on it.
>
> The reference count will be obtained within futex_hash() and dropped
> once the hash bucket is unlocked and not longer required for the
> particular operation (queue, unqueue, wakeup etc.).
>
> This is achieved by:
>
> - appending _put() to existing functions so it's clear that they
> also put the hash reference and fixing up the usage sites
>
> - providing new helpers, which combine common operations (unlock,
> put), and using them at the appropriate places
>
> - providing new helper for standalone reference counting
> functionality and using them at places, where the unlock operation
> needs to be separate.
>
> Hmm?
much better.
> > -void futex_q_unlock(struct futex_hash_bucket *hb)
> > +void futex_q_unlock_put(struct futex_hash_bucket *hb)
> > __releases(&hb->lock)
> > {
> > spin_unlock(&hb->lock);
> > futex_hb_waiters_dec(hb);
> > + futex_hash_put(hb);
>
> You missed a place to move the waiter_dec() before the unlock. Actually
This is fine because futex_hb_waiters_dec() happens before the reference
drop. However it is better to keep it symmetrical so I going to move it.
> this move should be in a separate preparatory patch, which does only
> that. It also needs a proper change log which explains why this done,
> why it is equivalent and not introducing a change in terms of ordering
> rules. This:
>
> > Move futex_hb_waiters_dec() before the reference drop, if needed
> > before the unlock.
>
> does not really give any clue at all.
>
> > if (unlikely(ret)) {
> > - double_unlock_hb(hb1, hb2);
> > futex_hb_waiters_dec(hb2);
> > + double_unlock_hb_put(hb1, hb2);
>
> And having this move before the _put() change makes the latter a purely
> mechanical change which let's the reader/reviewer focus on the reference
> count rules and not be distracted by the waiter count changes.
Okay, moving this into its own patch.
> > - /* futex_queue and wait for wakeup, timeout, or a signal. */
> > + /* futex_queue_put and wait for wakeup, timeout, or a signal. */
>
> When you fix up these comments, can you please use the fn() notation?
sure
> Thanks,
>
> tglx
Sebastian