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Date:   Thu, 3 May 2018 10:49:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Daniel Colascione <dancol@google.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
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        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        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, 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.