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authorDavidlohr Bueso <dave@stgolabs.net>2014-10-23 20:27:00 -0700
committerThomas Gleixner <tglx@linutronix.de>2014-10-26 16:16:18 +0100
commit993b2ff221999066fcff231590593d0b98f45d32 (patch)
tree7cd1c4dcfed58b3785ca2849993e0e004e7d690c
parentf114040e3ea6e07372334ade75d1ee0775c355e1 (diff)
futex: Mention key referencing differences between shared and private futexes
Update our documentation as of fix 76835b0ebf8 (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>
-rw-r--r--kernel/futex.c14
1 files changed, 10 insertions, 4 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index f3a3a071283c..bbf071f325b8 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -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)
{