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@xxxxxxxxx> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > > Cc: Waiman Long <longman@xxxxxxxxxx> > > Cc: Boqun Feng <boqun.feng@xxxxxxxxx> > > --- > > 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.