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SubjectRe: [PATCH] [RFC] bcachefs: SIX locks (shared/intent/exclusive)
On Mon, May 21, 2018 at 08:04:16PM -0700, Matthew Wilcox wrote:
> On Mon, May 21, 2018 at 10:19:51PM -0400, Kent Overstreet wrote:
> > New lock for bcachefs, like read/write locks but with a third state,
> > intent.
> >
> > Intent locks conflict with each other, but not with read locks; taking a
> > write lock requires first holding an intent lock.
> Can you put something in the description that these are sleeping locks
> (like mutexes), not spinning locks (like spinlocks)? (Yeah, I know
> there's the opportunistic spin, but conceptually, they're sleeping locks).

Yup, I'll add that

> Some other things I'd like documented:
> - Any number of readers can hold the lock
> - Once one thread acquires the lock for intent, further intent acquisitions
> will block. May new readers acquire the lock?

I think I should have that covered already - "Intent does not block read, but
does block other intent locks"

> - You cannot acquire the lock for write directly, you must acquire it for
> intent first, then upgrade to write.
> - Can you downgrade to read from intent, or downgrade from write back to
> intent?

You hold both write and intent, like so:


> - Once you are trying to upgrade from intent to write, are new read
> acquisitions blocked? (can readers starve writers?)

Readers can starve writers in the current implementation, but that's something
that should probably be fixed...

> - When you drop the lock as a writer, do we prefer reader acquisitions
> over intent acquisitions? That is, if we have a queue of RRIRIRIR,
> and we drop the lock, does the queue look like II or IRIR?

Separate queues per lock type, so dropping a write lock will wake up everyone
trying to take a read lock, and dropping an intent lock wakes up everyone trying
to take an intent lock.


Here's the new documentation I just wrote:

* Shared/intent/exclusive locks: sleepable read/write locks, much like rw
* semaphores, except with a third intermediate state, intent. Basic operations
* are:
* six_lock_read(&foo->lock);
* six_unlock_read(&foo->lock);
* six_lock_intent(&foo->lock);
* six_unlock_intent(&foo->lock);
* six_lock_write(&foo->lock);
* six_unlock_write(&foo->lock);
* Intent locks block other intent locks, but do not block read locks, and you
* must have an intent lock held before taking a write lock, like so:
* six_lock_intent(&foo->lock);
* six_lock_write(&foo->lock);
* six_unlock_write(&foo->lock);
* six_unlock_intent(&foo->lock);
* Other operations:
* six_trylock_read()
* six_trylock_intent()
* six_trylock_write()
* six_lock_downgrade(): convert from intent to read
* six_lock_tryupgrade(): attempt to convert from read to intent
* Locks also embed a sequence number, which is incremented when the lock is
* locked or unlocked for write. The current sequence number can be grabbed
* while a lock is held from lock->state.seq; then, if you drop the lock you can
* use six_relock_(read|intent_write)(lock, seq) to attempt to retake the lock
* iff it hasn't been locked for write in the meantime.
* There are also operations that take the lock type as a parameter, where the
* type is one of SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write:
* six_lock_type(lock, type)
* six_unlock_type(lock, type)
* six_relock(lock, type, seq)
* six_trylock_type(lock, type)
* six_trylock_convert(lock, from, to)
* A lock may be held multiple types by the same thread (for read or intent,
* not write) - up to SIX_LOCK_MAX_RECURSE. However, the six locks code does
* _not_ implement the actual recursive checks itself though - rather, if your
* code (e.g. btree iterator code) knows that the current thread already has a
* lock held, and for the correct type, six_lock_increment() may be used to
* bump up the counter for that type - the only effect is that one more call to
* unlock will be required before the lock is unlocked.

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