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

Linus Torvalds
2013-11-24T03:46:34Z

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