Commit-ID: 99b60ce69734dfeda58c6184a326b9475ce1dba3 Gitweb: http://git.kernel.org/tip/99b60ce69734dfeda58c6184a326b9475ce1dba3 Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx> AuthorDate: Sun, 12 Jan 2014 15:31:24 -0800 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Mon, 13 Jan 2014 11:45:19 +0100 futexes: Document multiprocessor ordering guarantees That's essential, if you want to hack on futexes. Reviewed-by: Darren Hart <dvhart@xxxxxxxxxxxxxxx> Reviewed-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx> Cc: Mike Galbraith <efault@xxxxxx> Cc: Jeff Mahoney <jeffm@xxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Cc: Scott Norton <scott.norton@xxxxxx> Cc: Tom Vaden <tom.vaden@xxxxxx> Cc: Aswin Chandramouleeswaran <aswin@xxxxxx> Cc: Waiman Long <Waiman.Long@xxxxxx> Cc: Jason Low <jason.low2@xxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/1389569486-25487-4-git-send-email-davidlohr@xxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- kernel/futex.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 57 insertions(+) diff --git a/kernel/futex.c b/kernel/futex.c index 577481d..fcc6850 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(). This function 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(). This functions 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; /* -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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