On Thu, 2014-01-02 at 07:05 -0800, Davidlohr Bueso wrote: > From: Thomas Gleixner <[email protected]> > > That's essential, if you want to hack on futexes. > > Cc: Ingo Molnar <[email protected]> > Cc: Darren Hart <[email protected]>
Reviewed-by: Darren Hart <[email protected]> > Acked-by: Peter Zijlstra <[email protected]> > Cc: Thomas Gleixner <[email protected]> > Cc: Paul E. McKenney <[email protected]> > Cc: Mike Galbraith <[email protected]> > Cc: Jeff Mahoney <[email protected]> > Cc: Linus Torvalds <[email protected]> > Cc: Randy Dunlap <[email protected]> > Cc: Scott Norton <[email protected]> > Cc: Tom Vaden <[email protected]> > Cc: Aswin Chandramouleeswaran <[email protected]> > Cc: Waiman Long <[email protected]> > Cc: Jason Low <[email protected]> > Signed-off-by: Thomas Gleixner <[email protected]> > Signed-off-by: Davidlohr Bueso <[email protected]> > --- > 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; > > /* -- Darren Hart Intel Open Source Technology Center Yocto Project - Linux Kernel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
