Re: [PATCH 5/6] futex: handle timeout inside adaptive lock spin

[Darren Hart]

Thomas Gleixner wrote:
> On Mon, 5 Apr 2010, Darren Hart wrote:
>
> > Signed-off-by: Darren Hart <dvhltc@xxxxxxxxxx>
> > ---
> >  kernel/futex.c |   24 +++++++++++++++++++++---
> >  1 files changed, 21 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/futex.c b/kernel/futex.c
> > index c33ac2a..af61dcd 100644
> > --- a/kernel/futex.c
> > +++ b/kernel/futex.c
> > @@ -2385,6 +2385,7 @@ out:
> >  /**
> >   * trylock_futex_adaptive() - Try to acquire the futex lock in a busy loop
> >   * @uaddr: the futex user address
> > + * @timeout: absolute timeout or NULL if none
> >   *
> >   * Try to acquire a futex lock in a loop until the owner changes or the owner
> >   * is descheduled. To lock the futex, set the value to the current TID.
> > @@ -2394,10 +2395,11 @@ out:
> >   *  1 - Futex lock acquired
> >   * <0 - On error
> >   */
> > -static int trylock_futex_adaptive(u32 __user *uaddr)
> > +static int trylock_futex_adaptive(u32 __user *uaddr, ktime_t *timeout)
> >  {
> >  	int ret = 0;
> >  	u32 curval;
> > +	ktime_t now;
> >  
> >  	for (;;) {
> >  		struct thread_info *owner;
> > @@ -2433,6 +2435,22 @@ static int trylock_futex_adaptive(u32 __user *uaddr)
> >  		if (need_resched())
> >  			break;
> >  
> > +		if (timeout) {
> > +			now = ktime_get();
>
>   Hmm. Calling that in every iteration might hurt especially on non
>   TSC systems, but well...

I haven't come across a better alternative since arming the timer before 
setting TASK_INTERRUPTIBLE isn't appropriate.

> > +/* FIXME: consider creating ktime_less_than(lhs, rhs) */
>
>   No need. The .tv64 comparison works in both cases. :)

Ah, for some reason I was thinking that was only the case if 
CONFIG_KTIME_SCALAR was set. Very nice, thanks.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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