From: Thomas Gleixner <t...@linutronix.de> That's essential, if you want to hack on futexes.
Cc: Ingo Molnar <mi...@kernel.org> Cc: Darren Hart <dvh...@linux.intel.com> Acked-by: Peter Zijlstra <pet...@infradead.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com> Cc: Mike Galbraith <efa...@gmx.de> Cc: Jeff Mahoney <je...@suse.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Scott Norton <scott.nor...@hp.com> Cc: Tom Vaden <tom.va...@hp.com> Cc: Aswin Chandramouleeswaran <as...@hp.com> Cc: Waiman Long <waiman.l...@hp.com> Cc: Jason Low <jason.l...@hp.com> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Signed-off-by: Davidlohr Bueso <davidl...@hp.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
