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Date:   Sun, 6 May 2018 12:03:32 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
cc:     Daniel Colascione <dancol@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        boqun.feng@gmail.com, luto@amacapital.net, davejwatson@fb.com,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Andrew Hunter <ahh@google.com>, andi@firstfloor.org,
        cl@linux.com, bmaurer@fb.com, rostedt@goodmis.org,
        josh@joshtriplett.org, torvalds@linux-foundation.org,
        catalin.marinas@arm.com, will.deacon@arm.com,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        Joel Fernandes <joelaf@google.com>
Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences
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On Wed, 2 May 2018, Peter Zijlstra wrote:

> On Wed, May 02, 2018 at 06:27:22PM +0000, Daniel Colascione wrote:
> > On Wed, May 2, 2018 at 10:22 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >> On Wed, May 02, 2018 at 03:53:47AM +0000, Daniel Colascione wrote:
> > > > Suppose we make a userspace mutex implemented with a lock word having
> > three
> > > > bits: acquired, sleep_mode, and wait_pending, with the rest of the word
> > not
> > > > being relevant at the moment.
> > 
> > > So ideally we'd kill FUTEX_WAIT/FUTEX_WAKE for mutexes entirely, and go
> > > with FUTEX_LOCK/FUTEX_UNLOCK that have the same semantics as the
> > > existing FUTEX_LOCK_PI/FUTEX_UNLOCK_PI, namely, the word contains the
> > > owner TID.
> > 
> > That doesn't work if you want to use the rest of the word for something
> > else, like a recursion count. With FUTEX_WAIT and FUTEX_WAKE, you can make
> > a lock with two bits.
> 
> Recursive locks are teh most horrible crap ever. And having the tid in
> the word allows things like kernel based optimistic spins and possibly
> PI related things.

FWIW, robust futex have also the TID requirement.

Thanks,

	tglx