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

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Message-Id: <d6378a2f-acf2-6771-970f-b0fbf2d3a120@linux.vnet.ibm.com>
Date:   Fri, 21 Apr 2017 09:00:41 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Davidlohr Bueso <dave@...olabs.net>
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 20/04/2017 18:01, Davidlohr Bueso wrote:
>>> +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.

Agreed, this one is useless.