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

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Date:   Thu, 3 May 2018 10:49:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Colascione <dancol@...gle.com>
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 02, 2018 at 08:37:13PM +0000, Daniel Colascione wrote:
> > Recursive locks are teh most horrible crap ever. And having the tid in
> 
> What happened to providing mechanism, not policy?
> 
> You can't wish away recursive locking. It's baked into Java and the CLR,
> and it's enshrined in POSIX. It's not going away, and there's no reason not
> to support it efficiently.

You can implement recursive locks just fine with a TID based word, just
keep the recursion counter external to the futex word. If owner==self,
increment etc..

> > the word allows things like kernel based optimistic spins and possibly
> > PI related things.
> 
> Sure. A lot of people don't want PI though, or at least they want to opt
> into it. And we shouldn't require an entry into the kernel for what we can
> in principle do efficiently in userspace.

Any additional PI would certainly be opt-in, but the kernel based
spinning might make sense unconditionally.