From: Kent Overstreet <email@example.com> To: Matthew Wilcox <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH] [RFC] bcachefs: SIX locks (shared/intent/exclusive) Date: Mon, 21 May 2018 23:49:06 -0400 [thread overview] Message-ID: <20180522034906.GA4535@kmo-pixel> (raw) In-Reply-To: <20180522030416.GB18682@bombadil.infradead.org> 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: six_lock_intent(&foo->lock); six_lock_write(&foo->lock); six_unlock_write(&foo->lock); six_unlock_intent(&foo->lock); > - 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. * */
prev parent reply other threads:[~2018-05-22 3:49 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-05-22 2:19 Kent Overstreet 2018-05-22 3:04 ` Matthew Wilcox 2018-05-22 3:49 ` Kent Overstreet [this message]
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