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Thu, 3 May 2018 04:49:58 -0400 Date: Thu, 3 May 2018 10:49:06 +0200 From: Peter Zijlstra To: Daniel Colascione Cc: Mathieu Desnoyers , Paul McKenney , boqun.feng@gmail.com, luto@amacapital.net, davejwatson@fb.com, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Paul Turner , Andrew Morton , linux@arm.linux.org.uk, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, Andrew Hunter , 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 , Joel Fernandes Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences Message-ID: <20180503084906.GW12217@hirez.programming.kicks-ass.net> References: <20180430224433.17407-1-mathieu.desnoyers@efficios.com> <20180502172218.GL12180@hirez.programming.kicks-ass.net> <20180502202233.GV12217@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: 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.