From: Kent Overstreet <kent.overstreet@gmail.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-xfs@vger.kernel.org, linux-btrfs@vger.kernel.org,
peterz@infradead.org
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 [PATCH] [RFC] bcachefs: SIX locks (shared/intent/exclusive) Kent Overstreet
2018-05-22 3:04 ` Matthew Wilcox
2018-05-22 3:49 ` Kent Overstreet [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180522034906.GA4535@kmo-pixel \
--to=kent.overstreet@gmail.com \
--cc=linux-btrfs@vger.kernel.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-xfs@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).