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From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, mingo@kernel.org,
	boqun.feng@gmail.com, paulmck@linux.vnet.ibm.com,
	longman@redhat.com
Subject: Re: [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath
Date: Thu, 26 Apr 2018 17:53:35 +0200	[thread overview]
Message-ID: <20180426155335.GL4064@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1524738868-31318-6-git-send-email-will.deacon@arm.com>

On Thu, Apr 26, 2018 at 11:34:19AM +0100, Will Deacon wrote:
> @@ -290,58 +312,50 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	}
>  
>  	/*
> +	 * If we observe any contention; queue.
> +	 */
> +	if (val & ~_Q_LOCKED_MASK)
> +		goto queue;
> +
> +	/*
>  	 * trylock || pending
>  	 *
>  	 * 0,0,0 -> 0,0,1 ; trylock
>  	 * 0,0,1 -> 0,1,1 ; pending
>  	 */
> +	val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val);
> +	if (!(val & ~_Q_LOCKED_MASK)) {
>  		/*
> +		 * we're pending, wait for the owner to go away.
> +		 *
> +		 * *,1,1 -> *,1,0

Tail must be 0 here, right?

> +		 *
> +		 * this wait loop must be a load-acquire such that we match the
> +		 * store-release that clears the locked bit and create lock
> +		 * sequentiality; this is because not all
> +		 * clear_pending_set_locked() implementations imply full
> +		 * barriers.
>  		 */
> +		if (val & _Q_LOCKED_MASK) {
> +			smp_cond_load_acquire(&lock->val.counter,
> +					      !(VAL & _Q_LOCKED_MASK));
> +		}
>  
>  		/*
> +		 * take ownership and clear the pending bit.
> +		 *
> +		 * *,1,0 -> *,0,1
>  		 */

Idem.

> +		clear_pending_set_locked(lock);
>  		return;
> +	}
>  
>  	/*
> +	 * If pending was clear but there are waiters in the queue, then
> +	 * we need to undo our setting of pending before we queue ourselves.
>  	 */
> +	if (!(val & _Q_PENDING_MASK))
> +		clear_pending(lock);

This is the branch for when we have !0 tail.

>  
>  	/*
>  	 * End of pending bit optimistic spinning and beginning of MCS

> @@ -445,15 +459,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>  	 * claim the lock:
>  	 *
>  	 * n,0,0 -> 0,0,1 : lock, uncontended
> +	 * *,*,0 -> *,*,1 : lock, contended
>  	 *
> +	 * If the queue head is the only one in the queue (lock value == tail)
> +	 * and nobody is pending, clear the tail code and grab the lock.
> +	 * Otherwise, we only need to grab the lock.
>  	 */
>  	for (;;) {
>  		/* In the PV case we might already have _Q_LOCKED_VAL set */
> +		if ((val & _Q_TAIL_MASK) != tail || (val & _Q_PENDING_MASK)) {
>  			set_locked(lock);
>  			break;
>  		}

This one hunk is terrible on the brain. I'm fairly sure I get it, but I
feel that comment can use help. Or at least, I need help reading it.

I'll try and cook up something when my brain starts working again.

  reply	other threads:[~2018-04-26 15:53 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-26 10:34 [PATCH v3 00/14] kernel/locking: qspinlock improvements Will Deacon
2018-04-26 10:34 ` [PATCH v3 01/14] barriers: Introduce smp_cond_load_relaxed and atomic_cond_read_relaxed Will Deacon
2018-04-27  9:36   ` [tip:locking/core] locking/barriers: Introduce smp_cond_load_relaxed() and atomic_cond_read_relaxed() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 02/14] locking/qspinlock: Merge struct __qspinlock into struct qspinlock Will Deacon
2018-04-27  9:37   ` [tip:locking/core] locking/qspinlock: Merge 'struct __qspinlock' into 'struct qspinlock' tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 03/14] locking/qspinlock: Bound spinning on pending->locked transition in slowpath Will Deacon
2018-04-27  9:37   ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 04/14] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound Will Deacon
2018-04-27  9:38   ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Will Deacon
2018-04-26 15:53   ` Peter Zijlstra [this message]
2018-04-26 16:55     ` Will Deacon
2018-04-28 12:45       ` Peter Zijlstra
2018-04-30  8:53         ` Will Deacon
2018-04-26 20:16   ` Waiman Long
2018-04-27 10:16     ` Will Deacon
2018-04-27 11:01       ` [tip:locking/core] locking/qspinlock: Remove duplicate clear_pending() function from PV code tip-bot for Will Deacon
2018-04-27 13:09       ` [PATCH v3 05/14] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath Waiman Long
2018-04-27  9:39   ` [tip:locking/core] locking/qspinlock: Remove unbounded cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 06/14] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue Will Deacon
2018-04-27  9:39   ` [tip:locking/core] locking/qspinlock: Kill cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 07/14] locking/qspinlock: Use atomic_cond_read_acquire Will Deacon
2018-04-27  9:40   ` [tip:locking/core] locking/qspinlock: Use atomic_cond_read_acquire() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 08/14] locking/mcs: Use smp_cond_load_acquire() in mcs spin loop Will Deacon
2018-04-27  9:40   ` [tip:locking/core] locking/mcs: Use smp_cond_load_acquire() in MCS " tip-bot for Jason Low
2018-04-26 10:34 ` [PATCH v3 09/14] locking/qspinlock: Use smp_cond_load_relaxed to wait for next node Will Deacon
2018-04-27  9:41   ` [tip:locking/core] locking/qspinlock: Use smp_cond_load_relaxed() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 10/14] locking/qspinlock: Make queued_spin_unlock use smp_store_release Will Deacon
2018-04-27  9:42   ` [tip:locking/core] locking/qspinlock: Use smp_store_release() in queued_spin_unlock() tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 11/14] locking/qspinlock: Elide back-to-back RELEASE operations with smp_wmb() Will Deacon
2018-04-27  9:42   ` [tip:locking/core] " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 12/14] locking/qspinlock: Use try_cmpxchg instead of cmpxchg when locking Will Deacon
2018-04-27  9:43   ` [tip:locking/core] locking/qspinlock: Use try_cmpxchg() instead of cmpxchg() " tip-bot for Will Deacon
2018-04-26 10:34 ` [PATCH v3 13/14] locking/qspinlock: Add stat tracking for pending vs slowpath Will Deacon
2018-04-27  9:43   ` [tip:locking/core] locking/qspinlock: Add stat tracking for pending vs. slowpath tip-bot for Waiman Long
2018-04-26 10:34 ` [PATCH v3 14/14] MAINTAINERS: Add myself as a co-maintainer for LOCKING PRIMITIVES Will Deacon
2018-04-26 15:55   ` Peter Zijlstra
2018-04-27  9:44   ` [tip:locking/core] MAINTAINERS: Add myself as a co-maintainer for the locking subsystem tip-bot for Will Deacon
2018-04-26 15:54 ` [PATCH v3 00/14] kernel/locking: qspinlock improvements Peter Zijlstra
2018-04-27  9:33   ` Ingo Molnar
2018-04-26 20:18 ` Waiman Long

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