Re: [PATCH v5 4/4] futex: Avoid taking hb lock if nothing to wakeup

[Paul E. McKenney]

On Thu, Jan 02, 2014 at 07:05:20AM -0800, Davidlohr Bueso wrote:
> From: Davidlohr Bueso <davidlohr@xxxxxx>
> 
> In futex_wake() there is clearly no point in taking the hb->lock if we know
> beforehand that there are no tasks to be woken. While the hash bucket's plist
> head is a cheap way of knowing this, we cannot rely 100% on it as there is a
> racy window between the futex_wait call and when the task is actually added to
> the plist. To this end, we couple it with the spinlock check as tasks trying to
> enter the critical region are most likely potential waiters that will be added
> to the plist, thus preventing tasks sleeping forever if wakers don't acknowledge
> all possible waiters.
> 
> Furthermore, the futex ordering guarantees are preserved, ensuring that waiters
> either observe the changed user space value before blocking or is woken by a
> concurrent waker. For wakers, this is done by relying on the barriers in
> get_futex_key_refs() -- for archs that do have implicit mb in atomic_inc() we
> explicitly add them through a new futex_get_mm function. For waiters we rely
> on the fact that spin_lock calls already update the head counter, so spinners
> are visible even if the lock hasn't been acquired yet.
> 
> For more details please refer to the updated comments in the code and related
> discussion: https://lkml.org/lkml/2013/11/26/556
> 
> Special thanks to tglx for careful review and feedback.
> 
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Mike Galbraith <efault@xxxxxx>
> Cc: Jeff Mahoney <jeffm@xxxxxxxx>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Scott Norton <scott.norton@xxxxxx>
> Cc: Tom Vaden <tom.vaden@xxxxxx>
> Cc: Aswin Chandramouleeswaran <aswin@xxxxxx>
> Cc: Waiman Long <Waiman.Long@xxxxxx>
> Cc: Jason Low <jason.low2@xxxxxx>
> Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>

A couple of comments below.

							Thanx, Paul

> ---
>  kernel/futex.c | 113 +++++++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 90 insertions(+), 23 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index fcc6850..5b4d09e 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -75,17 +75,20 @@
>   * The waiter reads the futex value in user space and calls
>   * futex_wait(). This function computes the hash bucket and acquires
>   * the hash bucket lock. After that it reads the futex user space value
> - * again and verifies that the data has not changed. If it has not
> - * changed it enqueues itself into the hash bucket, releases the hash
> - * bucket lock and schedules.
> + * again and verifies that the data has not changed. If it has not changed
> + * it enqueues itself into the hash bucket, releases the hash bucket lock
> + * and schedules.
>   *
>   * The waker side modifies the user space value of the futex and calls
> - * futex_wake(). This functions computes the hash bucket and acquires
> - * the hash bucket lock. Then it looks for waiters on that futex in the
> - * hash bucket and wakes them.
> + * futex_wake(). This function computes the hash bucket and acquires the
> + * hash bucket lock. Then it looks for waiters on that futex in the hash
> + * bucket and wakes them.
>   *
> - * Note that the spin_lock serializes waiters and wakers, so that the
> - * following scenario is avoided:
> + * In scenarios where wakeups are called and no tasks are blocked on a futex,
> + * taking the hb spinlock can be avoided and simply return. In order for this
> + * optimization to work, ordering guarantees must exist so that the waiter
> + * being added to the list is acknowledged when the list is concurrently being
> + * checked by the waker, avoiding scenarios like the following:
>   *
>   * CPU 0                               CPU 1
>   * val = *futex;
> @@ -106,24 +109,50 @@
>   * This would cause the waiter on CPU 0 to wait forever because it
>   * missed the transition of the user space value from val to newval
>   * and the waker did not find the waiter in the hash bucket queue.
> - * The spinlock serializes that:
> + *
> + * The correct serialization ensures that a waiter either observes
> + * the changed user space value before blocking or is woken by a
> + * concurrent waker:
>   *
>   * CPU 0                               CPU 1
>   * val = *futex;
>   * sys_futex(WAIT, futex, val);
>   *   futex_wait(futex, val);
> - *   lock(hash_bucket(futex));
> - *   uval = *futex;
> - *                                     *futex = newval;
> - *                                     sys_futex(WAKE, futex);
> - *                                       futex_wake(futex);
> - *                                       lock(hash_bucket(futex));
> + *
> + *   waiters++;
> + *   mb(); (A) <-- paired with -.
> + *                              |
> + *   lock(hash_bucket(futex));  |
> + *                              |
> + *   uval = *futex;             |
> + *                              |        *futex = newval;
> + *                              |        sys_futex(WAKE, futex);
> + *                              |          futex_wake(futex);
> + *                              |
> + *                              `------->   mb(); (B)
>   *   if (uval == val)
> - *      queue();
> + *     queue();
>   *     unlock(hash_bucket(futex));
> - *     schedule();                       if (!queue_empty())
> - *                                         wake_waiters(futex);
> - *                                       unlock(hash_bucket(futex));
> + *     schedule();                         if (waiters)
> + *                                           lock(hash_bucket(futex));
> + *                                           wake_waiters(futex);
> + *                                           unlock(hash_bucket(futex));
> + *
> + * Where (A) orders the waiters increment and the futex value read -- this
> + * is guaranteed by the head counter in the hb spinlock; and where (B)
> + * orders the write to futex and the waiters read.
> + *
> + * This yields the following case (where X:=waiters, Y:=futex):
> + *
> + *	X = Y = 0
> + *
> + *	w[X]=1		w[Y]=1
> + *	MB		MB
> + *	r[Y]=y		r[X]=x
> + *
> + * Which guarantees that x==0 && y==0 is impossible; which translates back into
> + * the guarantee that we cannot both miss the futex variable change and the
> + * enqueue.
>   */
> 
>  int __read_mostly futex_cmpxchg_enabled;
> @@ -211,6 +240,38 @@ static unsigned long __read_mostly futex_hashsize;
> 
>  static struct futex_hash_bucket *futex_queues;
> 
> +static inline void futex_get_mm(union futex_key *key)
> +{
> +	atomic_inc(&key->private.mm->mm_count);
> +#ifdef CONFIG_SMP

You don't need the #ifdef -- smp_mb__after_atomic_inc() does not emit
code if !SMP.

> +	/*
> +	 * Ensure futex_get_mm() implies a full barrier such that
> +	 * get_futex_key() implies a full barrier. This is relied upon
> +	 * as full barrier (B), see the ordering comment above.
> +	 */
> +	smp_mb__after_atomic_inc();
> +#endif
> +}
> +
> +static inline bool hb_waiters_pending(struct futex_hash_bucket *hb)
> +{
> +#ifdef CONFIG_SMP
> +	/*
> +	 * Tasks trying to enter the critical region are most likely
> +	 * potential waiters that will be added to the plist. Ensure
> +	 * that wakers won't miss to-be-slept tasks in the window between
> +	 * the wait call and the actual plist_add.
> +	 */
> +	if (spin_is_locked(&hb->lock))
> +		return true;
> +	smp_rmb(); /* Make sure we check the lock state first */
> +
> +	return !plist_head_empty(&hb->chain);
> +#else
> +	return true;
> +#endif
> +}
> +
>  /*
>   * We hash on the keys returned from get_futex_key (see below).
>   */
> @@ -245,10 +306,10 @@ static void get_futex_key_refs(union futex_key *key)
> 
>  	switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
>  	case FUT_OFF_INODE:
> -		ihold(key->shared.inode);
> +		ihold(key->shared.inode); /* implies MB (B) */
>  		break;
>  	case FUT_OFF_MMSHARED:
> -		atomic_inc(&key->private.mm->mm_count);
> +		futex_get_mm(key); /* implies MB (B) */
>  		break;
>  	}
>  }
> @@ -322,7 +383,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
>  	if (!fshared) {
>  		key->private.mm = mm;
>  		key->private.address = address;
> -		get_futex_key_refs(key);
> +		get_futex_key_refs(key);  /* implies MB (B) */
>  		return 0;
>  	}
> 
> @@ -1052,6 +1113,11 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
>  		goto out;
> 
>  	hb = hash_futex(&key);
> +
> +	/* Make sure we really have tasks to wakeup */
> +	if (!hb_waiters_pending(hb))
> +		goto out_put_key;

Nice optimization, especially in the (hopefully) common case of low
contention!

> +
>  	spin_lock(&hb->lock);
> 
>  	plist_for_each_entry_safe(this, next, &hb->chain, list) {
> @@ -1072,6 +1138,7 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
>  	}
> 
>  	spin_unlock(&hb->lock);
> +out_put_key:
>  	put_futex_key(&key);
>  out:
>  	return ret;
> @@ -1535,7 +1602,7 @@ static inline struct futex_hash_bucket *queue_lock(struct futex_q *q)
>  	hb = hash_futex(&q->key);
>  	q->lock_ptr = &hb->lock;
> 
> -	spin_lock(&hb->lock);
> +	spin_lock(&hb->lock); /* implies MB (A) */

You need smp_mb__before_spinlock() before the spin_lock() to get a
full memory barrier.

>  	return hb;
>  }
> 
> -- 
> 1.8.1.4
> 

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