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| author | Davidlohr Bueso <dave@stgolabs.net> | 2014-10-23 20:27:00 -0700 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2014-10-26 16:16:18 +0100 |
| commit | 993b2ff221999066fcff231590593d0b98f45d32 (patch) | |
| tree | 7cd1c4dcfed58b3785ca2849993e0e004e7d690c /kernel | |
| parent | f114040e3ea6e07372334ade75d1ee0775c355e1 (diff) | |
| download | linux-993b2ff221999066fcff231590593d0b98f45d32.tar.bz2 | |
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>
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/futex.c | 14 |
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) { |