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author | Thomas Gleixner <tglx@linutronix.de> | 2014-01-12 15:31:24 -0800 |
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committer | Ingo Molnar <mingo@kernel.org> | 2014-01-13 11:45:19 +0100 |
commit | 99b60ce69734dfeda58c6184a326b9475ce1dba3 (patch) | |
tree | f9e4c7f179859f3e20bdd5e7c501196f0a07eddf /kernel | |
parent | a52b89ebb6d4499be38780db8d176c5d3a6fbc17 (diff) | |
download | linux-99b60ce69734dfeda58c6184a326b9475ce1dba3.tar.gz linux-99b60ce69734dfeda58c6184a326b9475ce1dba3.tar.bz2 linux-99b60ce69734dfeda58c6184a326b9475ce1dba3.zip |
futexes: Document multiprocessor ordering guarantees
That's essential, if you want to hack on futexes.
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Reviewed-by: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Scott Norton <scott.norton@hp.com>
Cc: Tom Vaden <tom.vaden@hp.com>
Cc: Aswin Chandramouleeswaran <aswin@hp.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/1389569486-25487-4-git-send-email-davidlohr@hp.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/futex.c | 57 |
1 files changed, 57 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index 577481d5c59d..fcc6850483fb 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; /* |