linux-kernel - futex: Allow FUTEX_CLOCK_REALTIME with FUTEX

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Message-ID: <20160620162652.2c4619c3@perruche.parrot.biz>
Date:	Mon, 20 Jun 2016 16:26:52 +0200
From:	Matthieu CASTET <matthieu.castet@...rot.com>
To:	<linux-kernel@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Darren Hart <dvhart@...ux.intel.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op

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);
View attachment "diff" of type "text/x-patch" (395 bytes)