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

[Linus Torvalds]

On Sat, Nov 23, 2013 at 5:16 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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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