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From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Adhemerval Zanella <adhemerval.zanella@linaro.org>,
	Lukasz Majewski <lukma@denx.de>,
	Florian Weimer <fweimer@redhat.com>,
	Carlos O'Donell <carlos@redhat.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Ingo Molnar <mingo@kernel.org>,
	Darren Hart <dvhart@infradead.org>,
	Andrei Vagin <avagin@gmail.com>,
	Kurt Kanzenbach <kurt@linutronix.de>
Subject: Re: [patch 0/6] futex: Bugfixes and FUTEX_LOCK_PI2
Date: Wed, 5 May 2021 14:59:35 +0200	[thread overview]
Message-ID: <YJKWt2vlr74WR5tx@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210422194417.866740847@linutronix.de>

On Thu, Apr 22, 2021 at 09:44:17PM +0200, Thomas Gleixner wrote:
> The following series started off looking into supporting selectable clocks
> for FUTEX_LOCK_PI which is hardcoded to CLOCK_REALTIME and cannot be
> changed.
> 
> On the way I found two bugs related to the timeout handling:
> 
>   - The allowance for FUTEX_WAIT to use FUTEX_CLOCK_REALTIME is broken and
>     never worked.
> 
>   - The recent time namespace support wreckaged FUTEX_LOCK_PI timeouts when
>     the task belongs to a namespace which has an CLOCK_MONOTONIC offset.
> 
> Both should have been caught by that Gleixner dude when merging them,
> but it seems he's getting old.
> 
> Not having a selectable clock for PI futexes is inconsistent because all
> other interfaces have it. Unfortunately this was figured out by glibc folks
> quite some time ago, but nobody told us :(
> 
> The nasty hack to support it would be to treat FUTEX_CLOCK_REALTIME inverse
> for FUTEX_LOCK_PI, but that's a horrible idea. Adding a new flag to the
> futex op, i.e. FUTEX_CLOCK_MONOTONIC would be possible, but that's yet
> another variant which makes is harder for libraries to have a consistent
> clock selection handling.
> 
> So I went the way to let FUTEX_LOCK_PI alone and to add FUTEX_LOCK_PI2
> which handles the clocks the same way as the other operands.
> 
> Thoughts?

With the missing FUTEX_LOCK_PI2 in #6, as spotted by André Almeida, fixed:

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

It's all somewhat sad, but I don't see any other way out of this. Using
LOCK_PI2 will be a fairly horrible pile of hacks on the userspace side
of things given they need to first detect it's presence etc., but that
seems unavoidable whatever we do :/

  parent reply	other threads:[~2021-05-05 12:59 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-22 19:44 [patch 0/6] futex: Bugfixes and FUTEX_LOCK_PI2 Thomas Gleixner
2021-04-22 19:44 ` [patch 1/6] Revert 337f13046ff0 ("futex: Allow FUTEX_CLOCK_REALTIME with FUTEX_WAIT op") Thomas Gleixner
2021-05-06 18:14   ` [tip: locking/urgent] " tip-bot2 for Thomas Gleixner
2021-04-22 19:44 ` [patch 2/6] futex: Do not apply time namespace adjustment on FUTEX_LOCK_PI Thomas Gleixner
2021-05-06 18:14   ` [tip: locking/urgent] " tip-bot2 for Thomas Gleixner
2021-04-22 19:44 ` [patch 3/6] futex: Get rid of the val2 conditional dance Thomas Gleixner
2021-04-23 21:40   ` André Almeida
2021-04-23 22:34     ` Thomas Gleixner
2021-04-23 23:21       ` André Almeida
2021-05-06 18:20   ` [tip: locking/urgent] " tip-bot2 for Thomas Gleixner
2021-04-22 19:44 ` [patch 4/6] futex: Make syscall entry points less convoluted Thomas Gleixner
2021-05-06 18:20   ` [tip: locking/urgent] " tip-bot2 for Thomas Gleixner
2021-04-22 19:44 ` [patch 5/6] futex: Prepare futex_lock_pi() for runtime clock selection Thomas Gleixner
2021-04-23  9:34   ` Lukasz Majewski
2021-04-23 10:08     ` Thomas Gleixner
2021-06-23  8:19   ` [tip: locking/core] " tip-bot2 for Thomas Gleixner
2021-04-22 19:44 ` [patch 6/6] futex: Provide FUTEX_LOCK_PI2 to support " Thomas Gleixner
2021-04-23 22:20   ` André Almeida
2021-04-23 22:36     ` Thomas Gleixner
2021-06-23  8:19   ` [tip: locking/core] " tip-bot2 for Thomas Gleixner
2021-05-05 12:59 ` Peter Zijlstra [this message]
2021-05-05 13:51   ` [patch 0/6] futex: Bugfixes and FUTEX_LOCK_PI2 Kurt Kanzenbach

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