linux-kernel - Re: [RFC PATCH for 4.18 00/14] Restartable Sequences

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Date:   Wed, 02 May 2018 18:27:22 +0000
From:   Daniel Colascione <dancol@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        boqun.feng@...il.com, luto@...capital.net, davejwatson@...com,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux@....linux.org.uk, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, Andrew Hunter <ahh@...gle.com>, andi@...stfloor.org,
        cl@...ux.com, bmaurer@...com, rostedt@...dmis.org,
        josh@...htriplett.org, torvalds@...ux-foundation.org,
        catalin.marinas@....com, will.deacon@....com,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences

On Wed, May 2, 2018 at 10:22 AM Peter Zijlstra <peterz@...radead.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.

> As brought up in the last time we talked about spin loops, why do we
> care if the spin loop is in userspace or not? Aside from the whole PTI
> thing, the syscall cost was around 150 cycle or so, while a LOCK CMPXCHG
> is around 20 cycles. So ~7 spins gets you the cost of entry.

That's pre-KPTI, isn't it?