Re: [PATCH v5 03/22] cachefiles: introduce on-demand read mode

[JeffleXu]

On 3/21/22 10:26 PM, Matthew Wilcox wrote:
> On Mon, Mar 21, 2022 at 10:08:47PM +0800, JeffleXu wrote:
>> reqs_lock is also used to protect the check of cache->flags. Please
>> refer to patch 4 [1] of this patchset.
> 
> Yes, that's exactly what I meant by "bad idea".
> 
>> ```
>> +	/*
>> +	 * Enqueue the pending request.
>> +	 *
>> +	 * Stop enqueuing the request when daemon is dying. So we need to
>> +	 * 1) check cache state, and 2) enqueue request if cache is alive.
>> +	 *
>> +	 * The above two ops need to be atomic as a whole. @reqs_lock is used
>> +	 * here to ensure that. Otherwise, request may be enqueued after xarray
>> +	 * has been flushed, in which case the orphan request will never be
>> +	 * completed and thus netfs will hang there forever.
>> +	 */
>> +	read_lock(&cache->reqs_lock);
>> +
>> +	/* recheck dead state under lock */
>> +	if (test_bit(CACHEFILES_DEAD, &cache->flags)) {
>> +		read_unlock(&cache->reqs_lock);
>> +		ret = -EIO;
>> +		goto out;
>> +	}
> 
> So this is an error path.  We're almost always going to take the xa_lock
> immediately after taking the read_lock.  In other words, you've done two
> atomic operations instead of one.

Right.

> 
>> +	xa_lock(xa);
>> +	ret = __xa_alloc(xa, &id, req, xa_limit_32b, GFP_KERNEL);
>> +	if (!ret)
>> +		__xa_set_mark(xa, id, CACHEFILES_REQ_NEW);
>> +	xa_unlock(xa);
>> +
>> +	read_unlock(&cache->reqs_lock);
>> ```
>>
>> It's mainly used to protect against the xarray flush.
>>
>> Besides, IMHO read-write lock shall be more performance friendly, since
>> most cases are the read side.
> 
> That's almost never true.  rwlocks are usually a bad idea because you
> still have to bounce the cacheline, so you replace lock contention
> (which you can see) with cacheline contention (which is harder to
> measure).  And then you have questions about reader/writer fairness
> (should new readers queue behind a writer if there's one waiting, or
> should a steady stream of readers be able to hold a writer off
> indefinitely?)

Interesting, I didn't notice it before. Thanks for explaining it.


BTW what I want is just

```
PROCESS 1		PROCESS 2
=========		=========
#lock			#lock
set DEAD state		if (not DEAD)
flush xarray		   enqueue into xarray
#unlock			#unlock
```

I think it is a generic paradigm. So it seems that the spinlock inside
xarray is already adequate for this job?

-- 
Thanks,
Jeffle