Re: [PATCH] locking/rwsem: Make rwsem_is_contended() track status of OSQ

[Waiman Long <longman@redhat.com>]

On 03/07/2017 12:45 PM, Peter Zijlstra wrote:
> On Tue, Mar 07, 2017 at 11:03:48AM -0500, Waiman Long wrote:
>> It was found that the current rwsem_is_contended() function did not
>> look at the status of the OSQ and hence would miss waiters on OSQ. So
>> that function is now modified to look at the OSQ as well.
> Ideally I'd kill the entire function.
>
> 	if (need_resched() ||
> 	    rwsem_is_contended(&fs_info->commit_root_sem)) {
> 		if (wakeup)
> 			caching_ctl->progress = last;
> 		btrfs_release_path(path);
> 		up_read(&fs_info->commit_root_sem);
> 		mutex_unlock(&caching_ctl->mutex);
> 		cond_resched();
> 		mutex_lock(&caching_ctl->mutex);
> 		down_read(&fs_info->commit_root_sem);
> 		goto next;
> 	}
>
> is the only user of it in the entire tree and it makes no bloody sense
> what so ever. rwsem is a preemptible lock after all.

That works for me too. I do realize that there is only one user in the
kernel, but I am hesitant to change it as I am not familiar with that
piece of code.

Cheers,
Longman