linux-kernel - Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

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Message-ID: <CA+55aFwzRqPHKp67EhJ9hsX4v5=p3uQxvnaVZko2ZXDf6CQAPQ@mail.gmail.com>
Date:	Sat, 23 Nov 2013 19:46:31 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Davidlohr Bueso <davidlohr@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	"Norton, Scott J" <scott.norton@...com>, tom.vaden@...com,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	Jason Low <jason.low2@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 4/5] futex: Avoid taking hb lock if nothing to wakeup

On Sat, Nov 23, 2013 at 5:16 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>
> Now the question is why we queue the waiter _AFTER_ reading the user
> space value. The comment in the code is pretty non sensical:
>
>    * On the other hand, we insert q and release the hash-bucket only
>    * after testing *uaddr.  This guarantees that futex_wait() will NOT
>    * absorb a wakeup if *uaddr does not match the desired values
>    * while the syscall executes.
>
> There is no reason why we cannot queue _BEFORE_ reading the user space
> value. We just have to dequeue in all the error handling cases, but
> for the fast path it does not matter at all.
>
> CPU 0                                   CPU 1
>
>     val = *futex;
>     futex_wait(futex, val);
>
>     spin_lock(&hb->lock);
>
>     plist_add(hb, self);
>     smp_wmb();
>
>     uval = *futex;
>                                         *futex = newval;
>                                         futex_wake();
>
>                                         smp_rmb();
>                                         if (plist_empty(hb))
>                                            return;
> ...

This would seem to be a nicer approach indeed, without needing the
extra atomics.

Davidlohr, mind trying Thomas' approach?

             Linus
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