linux-kernel - [PATCH v4 3/4] futex: Document ordering guarantees

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Message-Id: <1387485950-7320-4-git-send-email-davidlohr@hp.com>
Date:	Thu, 19 Dec 2013 12:45:49 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...hat.com, dvhart@...ux.intel.com, peterz@...radead.org,
	tglx@...utronix.de, paulmck@...ux.vnet.ibm.com, efault@....de,
	jeffm@...e.com, torvalds@...ux-foundation.org, jason.low2@...com,
	Waiman.Long@...com, tom.vaden@...com, scott.norton@...com,
	aswin@...com, davidlohr@...com, Ingo Molnar <mingo@...nel.org>
Subject: [PATCH v4 3/4] futex: Document ordering guarantees

From: Thomas Gleixner <tglx@...utronix.de>

That's essential, if you want to hack on futexes.

Cc: Ingo Molnar <mingo@...nel.org>
Cc: Darren Hart <dvhart@...ux.intel.com>
Acked-by: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Mike Galbraith <efault@....de>
Cc: Jeff Mahoney <jeffm@...e.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Scott Norton <scott.norton@...com>
Cc: Tom Vaden <tom.vaden@...com>
Cc: Aswin Chandramouleeswaran <aswin@...com>
Cc: Waiman Long <Waiman.Long@...com>
Cc: Jason Low <jason.low2@...com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Davidlohr Bueso <davidlohr@...com>
---
 kernel/futex.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 577481d..af1fc31 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -69,6 +69,63 @@
 
 #include "locking/rtmutex_common.h"
 
+/*
+ * Basic futex operation and ordering guarantees:
+ *
+ * The waiter reads the futex value in user space and calls
+ * futex_wait(). It computes the hash bucket and acquires the hash
+ * bucket lock. After that it reads the futex user space value again
+ * and verifies that the data has not changed. If it has not changed
+ * it enqueues itself into the hash bucket, releases the hash
+ * bucket lock and schedules.
+ *
+ * The waker side modifies the user space value of the futex and calls
+ * futex_wake(). It computes the hash bucket and acquires the hash
+ * bucket lock. Then it looks for waiters on that futex in the hash
+ * bucket and wakes them.
+ *
+ * Note that the spin_lock serializes waiters and wakers, so that the
+ * following scenario is avoided:
+ *
+ * CPU 0                               CPU 1
+ * val = *futex;
+ * sys_futex(WAIT, futex, val);
+ *   futex_wait(futex, val);
+ *   uval = *futex;
+ *                                     *futex = newval;
+ *                                     sys_futex(WAKE, futex);
+ *                                       futex_wake(futex);
+ *                                       if (queue_empty())
+ *                                         return;
+ *   if (uval == val)
+ *      lock(hash_bucket(futex));
+ *      queue();
+ *     unlock(hash_bucket(futex));
+ *     schedule();
+ *
+ * This would cause the waiter on CPU 0 to wait forever because it
+ * missed the transition of the user space value from val to newval
+ * and the waker did not find the waiter in the hash bucket queue.
+ * The spinlock serializes that:
+ *
+ * CPU 0                               CPU 1
+ * val = *futex;
+ * sys_futex(WAIT, futex, val);
+ *   futex_wait(futex, val);
+ *   lock(hash_bucket(futex));
+ *   uval = *futex;
+ *                                     *futex = newval;
+ *                                     sys_futex(WAKE, futex);
+ *                                       futex_wake(futex);
+ *                                       lock(hash_bucket(futex));
+ *   if (uval == val)
+ *      queue();
+ *     unlock(hash_bucket(futex));
+ *     schedule();                       if (!queue_empty())
+ *                                         wake_waiters(futex);
+ *                                       unlock(hash_bucket(futex));
+ */
+
 int __read_mostly futex_cmpxchg_enabled;
 
 /*
-- 
1.8.1.4

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