linux-kernel - Re: [PATCH 11/20] locking/osq: Export osq

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Date:   Wed, 2 Aug 2023 16:44:07 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Waiman Long <longman@...hat.com>
Cc:     linux-bcachefs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 11/20] locking/osq: Export osq_(lock|unlock)

On Wed, Aug 02, 2023 at 04:16:12PM -0400, Waiman Long wrote:
> On 7/12/23 17:11, Kent Overstreet wrote:
> > These are used by bcachefs's six locks.
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Waiman Long <longman@...hat.com>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > ---
> >   kernel/locking/osq_lock.c | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
> > index d5610ad52b..b752ec5cc6 100644
> > --- a/kernel/locking/osq_lock.c
> > +++ b/kernel/locking/osq_lock.c
> > @@ -203,6 +203,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
> >   	return false;
> >   }
> > +EXPORT_SYMBOL_GPL(osq_lock);
> >   void osq_unlock(struct optimistic_spin_queue *lock)
> >   {
> > @@ -230,3 +231,4 @@ void osq_unlock(struct optimistic_spin_queue *lock)
> >   	if (next)
> >   		WRITE_ONCE(next->locked, 1);
> >   }
> > +EXPORT_SYMBOL_GPL(osq_unlock);
> 
> Have you considered extending the current rw_semaphore to support a SIX lock
> semantics? There are a number of instances in the kernel that a up_read() is
> followed by a down_write(). Basically, the code try to upgrade the lock from
> read to write. I have been thinking about adding a upgrade_read() API to do
> that. However, the concern that I had was that another writer may come in
> and make modification before the reader can be upgraded to have exclusive
> write access and will make the task to repeat what has been done in the read
> lock part. By adding a read with intent to upgrade to write, we can have
> that guarantee.

It's been discussed, Linus had the same thought.

But it'd be a massive change to the rw semaphore code; this "read with
intent" really is a third lock state which needs all the same
lock/trylock/unlock paths, and with the way rw semaphore has separate
entry points for read and write it'd be a _ton_ of new code. It really
touches everything - waitlist handling included.

And six locks have several other features that bcachefs needs, and other
users may also end up wanting, that rw semaphores don't have; the two
main features being a percpu read lock mode and support for an external
cycle detector (which requires exposing lock waitlists, with some
guarantees about how those waitlists are used).

> With that said, I would prefer to keep osq_{lock/unlock} for internal use by
> some higher level locking primitives - mutex, rwsem and rt_mutex.

Yeah, I'm aware, but it seems like exposing osq_(lock|unlock) is the
most palatable solution for now. Long term, I'd like to get six locks
promoted to kernel/locking.