futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

From: Matthieu CASTET <matthieu.castet@parrot.com>
To: <linux-kernel@vger.kernel.org>,
	Michael Kerrisk <mtk.manpages@gmail.com>,
	Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op
Date: Mon, 20 Jun 2016 16:26:52 +0200	[thread overview]
Message-ID: <20160620162652.2c4619c3@perruche.parrot.biz> (raw)

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Hi,

the commit 337f13046ff03717a9e99675284a817527440a49 is saying that it
change to syscall to an equivalent to FUTEX_WAIT_BITSET |
FUTEX_CLOCK_REALTIME with a bitset of FUTEX_BITSET_MATCH_ANY.

It seems wrong to me, because in case of FUTEX_WAIT, in
"SYSCALL_DEFINE6(futex", we convert relative timeout to absolute
timeout [1].

So FUTEX_CLOCK_REALTIME | FUTEX_WAIT is expecting a relative timeout
when FUTEX_WAIT_BITSET take an absolute timeout.

To make it work you have to use something like the (untested) attached
patch.

Matthieu

[1]
        if (cmd == FUTEX_WAIT)
            t = ktime_add_safe(ktime_get(), t);

[-- Attachment #2: diff --]
[-- Type: text/x-patch, Size: 394 bytes --]

diff --git a/kernel/futex.c b/kernel/futex.c
index 33664f7..4bee915 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3230,7 +3230,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
 			return -EINVAL;
 
 		t = timespec_to_ktime(ts);
-		if (cmd == FUTEX_WAIT)
+		if (cmd == FUTEX_WAIT && !(op & FUTEX_CLOCK_REALTIME))
 			t = ktime_add_safe(ktime_get(), t);
 		tp = &t;
 	}
