linux-kernel - Re: Revised futex(2) man page for review

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Message-ID: <20150328120337.GB32047@worktop.ger.corp.intel.com>
Date:	Sat, 28 Mar 2015 13:03:37 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <dvhart@...ux.intel.com>,
	Carlos O'Donell <carlos@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Jakub Jelinek <jakub@...hat.com>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Arnd Bergmann <arnd@...db.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux API <linux-api@...r.kernel.org>,
	Torvald Riegel <triegel@...hat.com>,
	Roland McGrath <roland@...k.frob.com>,
	Darren Hart <dvhart@...radead.org>,
	Anton Blanchard <anton@...ba.org>,
	Eric Dumazet <edumazet@...gle.com>,
	bill o gallmeister <bgallmeister@...il.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Daniel Wagner <wagi@...om.org>, Rich Felker <dalias@...c.org>,
	Andy Lutomirski <luto@...capital.net>,
	bert hubert <bert.hubert@...herlabs.nl>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Heinrich Schuchardt <xypron.glpk@....de>
Subject: Re: Revised futex(2) man page for review

On Sat, Mar 28, 2015 at 12:47:25PM +0100, Peter Zijlstra wrote:
>        FUTEX_WAIT (since Linux 2.6.0)
>               This operation tests that the value at the futex word pointed to
>               by the address uaddr still contains the expected value val,  and
>               if  so,  then sleeps awaiting FUTEX_WAKE on the futex word.  The
>               load of the value of the futex word is an atomic  memory  access
>               (i.e.,  using  atomic  machine  instructions  of  the respective
>               architecture).  This load,  the  comparison  with  the  expected
>               value,  and  starting  to  sleep  are  performed  atomically and
>               totally ordered with respect to other futex  operations  on  the
>               same  futex  word.  If the thread starts to sleep, it is consid‐
>               ered a waiter on this futex word.  If the futex value  does  not
>               match  val,  then  the  call  fails  immediately  with the error
>               EAGAIN.
> 
>               The purpose of the comparison with the expected value is to pre‐
>               vent  lost  wake-ups: If another thread changed the value of the
>               futex word after the calling thread decided to  block  based  on
>               the  prior  value, and if the other thread executed a FUTEX_WAKE
>               operation (or similar wake-up) after the value change and before
>               this  FUTEX_WAIT  operation,  then  the  latter will observe the
>               value change and will not start to sleep.
> 
>               If the timeout argument is non-NULL, its contents specify a rel‐
>               ative   timeout   for   the  wait,  measured  according  to  the
>               CLOCK_MONOTONIC clock.  (This interval will be rounded up to the
>               system clock granularity, and kernel scheduling delays mean that
>               the blocking interval may overrun by a small amount.)  If  time‐
>               out is NULL, the call blocks indefinitely.

Would it not be better to only state that the wait will not return
before the timeout -- unless woken -- and not bother with clock
granularity and scheduling delays?
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