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futex: Mention key referencing differences between shared and private futexes

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Update our documentation as of fix 76835b0ebf (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: Davidlohr 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: Catalin Marinas <catalin.marinas@arm.com>
Link: http://lkml.kernel.org/r/1414121220.817.0.camel@linux-t7sj.site
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
This commit is contained in:
Davidlohr Bueso 2014-10-23 20:27:00 -07:00 committed by Thomas Gleixner
parent f114040e3e
commit 993b2ff221
1 changed files with 10 additions and 4 deletions
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14
kernel/futex.c
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@ -143,9 +143,8 @@
* *
* Where (A) orders the waiters increment and the futex value read through * Where (A) orders the waiters increment and the futex value read through
* atomic operations (see hb_waiters_inc) and where (B) orders the write * 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 * to futex and the waiters read -- this is done by the barriers for both
* get_futex_key_refs(), through either ihold or atomic_inc, depending on the * shared and private futexes in get_futex_key_refs().
* futex type.
* *
* This yields the following case (where X:=waiters, Y:=futex): * 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) */ futex_get_mm(key); /* implies MB (B) */
break; break;
default: 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) */ smp_mb(); /* explicit MB (B) */
} }
} }
/* /*
* Drop a reference to the resource addressed by a key. * 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) static void drop_futex_key_refs(union futex_key *key)
{ {