Commit 993b2ff2 authored by Davidlohr Bueso's avatar Davidlohr Bueso Committed by Thomas Gleixner
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futex: Mention key referencing differences between shared and private futexes 



Update our documentation as of fix 76835b0e (futex: Ensure
get_futex_key_refs() always implies a barrier). Explicitly
state that we don't do key referencing for private futexes.

Signed-off-by: default avatarDavidlohr Bueso <dbueso@suse.de>
Cc: Matteo Franchin <Matteo.Franchin@arm.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: default avatarCatalin Marinas <catalin.marinas@arm.com>
Link: http://lkml.kernel.org/r/1414121220.817.0.camel@linux-t7sj.site


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent f114040e

kernel/futex.c

+10 −4
Original line number Diff line number Diff line
@@ -143,9 +143,8 @@ 
 *

 * Where (A) orders the waiters increment and the futex value read through

 * atomic operations (see hb_waiters_inc) and where (B) orders the write

 * to futex and the waiters read -- this is done by the barriers in

 * get_futex_key_refs(), through either ihold or atomic_inc, depending on the

 * futex type.

 * to futex and the waiters read -- this is done by the barriers for both

 * shared and private futexes in get_futex_key_refs().

 *

 * This yields the following case (where X:=waiters, Y:=futex):

 *
@@ -344,13 +343,20 @@ static void get_futex_key_refs(union futex_key *key) 
		futex_get_mm(key); /* implies MB (B) */

		break;

	default:

		/*

		 * Private futexes do not hold reference on an inode or

		 * mm, therefore the only purpose of calling get_futex_key_refs

		 * is because we need the barrier for the lockless waiter check.

		 */

		smp_mb(); /* explicit MB (B) */

	}

}



/*

 * Drop a reference to the resource addressed by a key.

 * The hash bucket spinlock must not be held.

 * The hash bucket spinlock must not be held. This is

 * a no-op for private futexes, see comment in the get

 * counterpart.

 */

static void drop_futex_key_refs(union futex_key *key)

{