Re: [PATCH] futex: add FUTEX_SET_WAIT operation

[Darren Hart]

Peter Zijlstra wrote:

Hey Peter,

Some thoughts on adaptive futexes ...

> Subject: futex: implement adaptive spin
> From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Date: Tue Jan 20 14:40:36 CET 2009
>
> Implement kernel side adaptive spining for futexes.
>
> This is implemented as a new futex op: FUTEX_WAIT_ADAPTIVE, because it
> requires the futex lock field to contain a TID and regular futexes do
> not have that restriction.
>
> FUTEX_WAIT_ADAPTIVE will spin while the lock owner is running (on a 
> different cpu) or until the task gets preempted. After that it behaves
> like FUTEX_WAIT.
>
> The spin loop tries to acquire the lock by cmpxchg(lock, 0, tid) == tid
> on the lower 30 bits (TID_MASK) of the lock field -- leaving the
> WAITERS and OWNER_DIED flags in tact.
>
> NOTE: we could fold mutex_spin_on_owner() and futex_spin_on_owner() by
> adding a lock_owner function argument.
>
>   void lock_spin_on_owner(struct thread_info *(*lock_owner)(void *lock),
>                           void *lock, struct thread_info *owner);
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> ...

> Index: linux-2.6/kernel/futex.c
> ===================================================================
> --- linux-2.6.orig/kernel/futex.c
> +++ linux-2.6/kernel/futex.c
> @@ -1158,10 +1158,40 @@ handle_fault:
>   */
>  #define FLAGS_SHARED		0x01
>  #define FLAGS_CLOCKRT		0x02
> +#define FLAGS_ADAPTIVE		0x03
>
>  static long futex_wait_restart(struct restart_block *restart);
>
> -static int futex_wait(u32 __user *uaddr, int fshared,
> +#ifdef CONFIG_SMP
> +struct thread_info *futex_owner(u32 __user *uaddr)
> +{
> +	struct thread_info *ti = NULL;
> +	struct task_struct *p;
> +	pid_t pid;
> +	u32 uval;
> +
> +	if (get_futex_value_locked(&uval, uaddr))
> +		return NULL;

Just give up if it would cause a fault?

> +
> +	pid = uval & FUTEX_TID_MASK;
> +	rcu_read_lock();
> +	p = find_taks_by_vpid(pid);

I'm impressed that you can create such a solid patch without compiling it!

> +	if (p) {
> +		const struct cred *cred, *pcred;
> +
> +		cread = current_cred();
> +		pcred = __task_cred(p);
> +		if (cred->euid == pcred->euid ||
> +		    cred->euid == pcred->uid)
> +			ti = task_thread_info(p);
> +	}
> +	rcu_read_unlock();
> +
> +	return ti;
> +}
> +#endif
> +
> +static int futex_wait(u32 __user *uaddr, int flags,
>  		      u32 val, ktime_t *abs_time, u32 bitset, int clockrt)
>  {
>  	struct task_struct *curr = current;
> @@ -1176,11 +1206,43 @@ static int futex_wait(u32 __user *uaddr,
>  	if (!bitset)
>  		return -EINVAL;
>
> +#ifdef CONFIG_SMP
> +	if (!futex_cmpxchg_enabled || !(flags & FLAGS_ADAPTIVE))
> +		goto skip_adaptive;
> +
> +	preempt_disable();
> +	for (;;) {
> +		struct thread_info *owner;
> +		u32 curval = 0, newval = task_pid_vnr(curr);

Do we need to lookup newval every iteration?

> +
> +		owner = futex_owner(uaddr);
> +		if (owner && futex_spin_on_owner(uaddr, owner))
> +			break;
> +
> +		if (get_futex_value_locked(&uval, uaddr))
> +			break;
> +
> +		curval |= uval & ~FUTEX_TID_MASK;
> +		newval |= uval & ~FUTEX_TID_MASK;
> +
> +		if (cmpxchg_futex_value_locked(uaddr, curval, newval)
> +				== newval)

"== curval" isn't it? futex_atomic_cmpxchg_inatomic() returns the 
oldval, not the newval.

> +			return 0;
> +
> +		if (!owner && (need_resched() || rt_task(curr)))
> +			break;

Hrm... why go through the loop at all for an rt_task if we bail on the 
first iteration?

> +
> +		cpu_relax();
> +	}
> +	preempt_enable();
> +skip_adaptive:

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