Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20180430224433.17407-1-mathieu.desnoyers@efficios.com>
 <CAKOZuesa338sc_=w6-wvro25idrSN_zx_zrHFyo-bDiORs7FZw@mail.gmail.com> <20180502172218.GL12180@hirez.programming.kicks-ass.net>
In-Reply-To: <20180502172218.GL12180@hirez.programming.kicks-ass.net>
From:   Daniel Colascione <dancol@google.com>
Date:   Wed, 02 May 2018 18:27:22 +0000
Message-ID: <CAKOZuesjF8QxfphgsyRKctO89jR9VAM727dspTe7PX1z8KFpyw@mail.gmail.com>
Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     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>,
        linux@arm.linux.org.uk, tglx@linutronix.de, mingo@redhat.com,
        hpa@zytor.com, 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>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

> 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?