linux-kernel - Re: [PATCH] futex: Ensure get_futex_key

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Message-ID: <1413663590.17869.14.camel@linux-t7sj.site>
Date:	Sat, 18 Oct 2014 13:19:50 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org,
	Matteo Franchin <Matteo.Franchin@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Darren Hart <dvhart@...radead.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Subject: Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a
 barrier

On Sat, 2014-10-18 at 14:32 -0500, Darren Hart wrote:
> Which is not incomplete (lacking the explicit smp_mb()) added by this
> patch. Perhaps the MB implementation of get_futex_key_refs() need not be
> explicitly enumerated here?

Agreed, how about this:

diff --git a/kernel/futex.c b/kernel/futex.c
index 21f7e41..7a0805a 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):
  *



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