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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20180502172218.GL12180@hirez.programming.kicks-ass.net>
Date:   Wed, 2 May 2018 19:22:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     mathieu.desnoyers@...icios.com, paulmck@...ux.vnet.ibm.com,
        boqun.feng@...il.com, luto@...capital.net, davejwatson@...com,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        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,
        mtk.manpages@...il.com, Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences

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.

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.