linux-kernel - Re: [PATCH 2/6] locking: Introduce range reader/writer lock

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Date:   Thu, 20 Apr 2017 09:01:33 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     mingo@...nel.org, peterz@...radead.org, akpm@...ux-foundation.org,
        jack@...e.cz, kirill.shutemov@...ux.intel.com, mhocko@...e.com,
        mgorman@...hsingularity.net, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/6] locking: Introduce range reader/writer lock

On Tue, 18 Apr 2017, Laurent Dufour wrote:

>On 06/04/2017 10:46, Davidlohr Bueso wrote:
>> +__range_read_lock_common(struct range_rwlock_tree *tree,
>> +			 struct range_rwlock *lock, long state)
>> +{
>> +	struct interval_tree_node *node;
>> +	unsigned long flags;
>> +
>> +	spin_lock_irqsave(&tree->lock, flags);
>> +	range_lock_set_reader(lock);
>> +
>> +	if (!__range_intersects_intree(tree, lock))
>> +		goto insert;
>> +
>> +	range_interval_tree_foreach(node, &tree->root,
>> +				    lock->node.start, lock->node.last) {
>> +		struct range_rwlock *blocked_lock;
>> +		blocked_lock = range_entry(node, struct range_rwlock, node);
>> +
>> +		if (!range_lock_is_reader(blocked_lock))
>> +			lock->blocking_ranges++;
>> +	}
>> +insert:
>> +	__range_tree_insert(tree, lock);
>> +
>> +	lock->waiter = current;
>
>Hi Davidlohr,
>
>Setting lock->waiter after calling range_lock_set_reader() is resetting
>the reader flag. Moving the call to range_lock_set_reader() here fixes that.

Yeah, I moved the set_reader() call to after setting lock->waiter.

>
>> +	spin_unlock_irqrestore(&tree->lock, flags);
>> +
>> +	return wait_for_ranges(tree, lock, state);
>> +}

[...]

>> +int range_read_trylock(struct range_rwlock_tree *tree, struct range_rwlock *lock)
>> +{
>> +	int ret = true;
>> +	unsigned long flags;
>> +	struct interval_tree_node *node;
>> +
>> +	spin_lock_irqsave(&tree->lock, flags);
>> +
>> +	if (!__range_intersects_intree(tree, lock))
>> +		goto insert;
>> +
>> +	/*
>> +	 * We have overlapping ranges in the tree, ensure that we can
>> +	 * in fact share the lock.
>> +	 */
>> +	range_interval_tree_foreach(node, &tree->root,
>> +				    lock->node.start, lock->node.last) {
>> +		struct range_rwlock *blocked_lock;
>> +		blocked_lock = range_entry(node, struct range_rwlock, node);
>> +
>> +		if (!range_lock_is_reader(blocked_lock)) {
>> +			ret = false;
>> +			goto unlock;
>> +		}
>> +	}
>> +insert:
>> +	range_lock_set_reader(lock);
>
>Here, the lock->waiter field should have been set to current before
>calling range_lock_set_reader()

But this is a trylock attempt, there is no waiting going on.

Thanks,
Davidlohr