locking/ww_mutex/Documentation: Update the design document

Document the invariants we maintain for the wait list of ww_mutexes.

Signed-off-by: Nicolai Hähnle <Nicolai.Haehnle@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <dev@mblankhorst.nl>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dri-devel@lists.freedesktop.org
Link: http://lkml.kernel.org/r/1482346000-9927-13-git-send-email-nhaehnle@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Nicolai Hähnle 2016-12-21 19:46:40 +01:00 committed by Ingo Molnar
parent 977625a693
commit 27bd57aa81

View File

@ -309,11 +309,15 @@ Design:
normal mutex locks, which are far more common. As such there is only a small
increase in code size if wait/wound mutexes are not used.
We maintain the following invariants for the wait list:
(1) Waiters with an acquire context are sorted by stamp order; waiters
without an acquire context are interspersed in FIFO order.
(2) Among waiters with contexts, only the first one can have other locks
acquired already (ctx->acquired > 0). Note that this waiter may come
after other waiters without contexts in the list.
In general, not much contention is expected. The locks are typically used to
serialize access to resources for devices. The only way to make wakeups
smarter would be at the cost of adding a field to struct mutex_waiter. This
would add overhead to all cases where normal mutexes are used, and
ww_mutexes are generally less performance sensitive.
serialize access to resources for devices.
Lockdep:
Special care has been taken to warn for as many cases of api abuse