linux-kernel - [PATCH v3 12/12] Documentation/locking/ww

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Message-Id: <1482346000-9927-13-git-send-email-nhaehnle@gmail.com>
Date:   Wed, 21 Dec 2016 19:46:40 +0100
From:   Nicolai Hähnle <nhaehnle@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Nicolai Hähnle <Nicolai.Haehnle@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Maarten Lankhorst <dev@...ankhorst.nl>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        dri-devel@...ts.freedesktop.org
Subject: [PATCH v3 12/12] Documentation/locking/ww_mutex: Update the design document

From: Nicolai Hähnle <Nicolai.Haehnle@....com>

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

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Maarten Lankhorst <dev@...ankhorst.nl>
Cc: Daniel Vetter <daniel@...ll.ch>
Cc: Chris Wilson <chris@...is-wilson.co.uk>
Cc: dri-devel@...ts.freedesktop.org
Signed-off-by: Nicolai Hähnle <Nicolai.Haehnle@....com>
---
 Documentation/locking/ww-mutex-design.txt | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/locking/ww-mutex-design.txt b/Documentation/locking/ww-mutex-design.txt
index 8a112dc..34c3a1b 100644
--- a/Documentation/locking/ww-mutex-design.txt
+++ b/Documentation/locking/ww-mutex-design.txt
@@ -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
-- 
2.7.4