Re: [fuse-devel] fuse: when are release requests queued?

[Nikolaus Rath <Nikolaus@rath.org>]

On May 31 2017, Maxim Patlasov <mpatlasov@virtuozzo.com> wrote:
> On 05/31/2017 12:19 PM, Nikolaus Rath wrote:
>
>> On May 31 2017, Maxim Patlasov <mpatlasov@virtuozzo.com> wrote:
>>>>>> Can someone tell me at which point the fuse kernel module will send a
>>>>>> RELEASE request to userspace?
>>>>> Anytime after fuse_release(). It only puts request to background
>>>>> queue. Later, the request will be transferred to pending queue. And
>>>>> later, the userspace will fetch it by fuse_dev_do_read().
>>>>>
>>>>>> Is it possible that this is delayed until
>>>>>> after the close() syscall for the last fd has returned and userspace has
>>>>>> submitted a different fuse request for the same fs?
>>>>> I think it's possible. See how flush_bg_queue() do nothing if
>>>>> fc->active_background > fc->max_background.
>>>> Thanks Maxim! Not sure what I'd do with these issues without you :-).
>>>>
>>>>
>>>> Is there a way to deliberate trigger this behavior for debugging? For
>>>> example, is there a kernel equivalent of sleep(1) that I could put into
>>>> fuse_release()?
>>> schedule_timeout_interruptible(HZ).
>> Hmm. I made the following change in linux 4.10:
>>
>> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
>> index 2401c5..3568a8 100644
>> --- a/fs/fuse/file.c
>> +++ b/fs/fuse/file.c
>> @@ -252,6 +252,9 @@ void fuse_release_common(struct file *file, int opcode)
>>          if (unlikely(!ff))
>>                  return;
>>   +        // Wait a little to force race condition in userspace
>> +        schedule_timeout_interruptible(1);
>> +
>>          req = ff->reserved_req;
>>          fuse_prepare_release(ff, file->f_flags, opcode);
>>   
>>
>> But when doing e.g. "echo test > newfile", the RELEASE request still
>> comes right away (judging from the libfuse debugging output).
>>
>> Do I need to do something else?
>
> Try HZ*10 instead of 1 as an argument of
> schedule_timeout_interruptible.

Ok, now the RELEASE comes a lot later. But now userspace is also
blocking until RELEASE comes in.

>>> But it's better to instrument fuse
>>> userspace to postpone processing some i/o requests. Then you'll keep
>>> fc->active_background > fc->max_background for a while. During that
>>> period fuse_release may succeed with FUSE_RELEASE queued, but not
>>> passed to the userspace. Then you cat try to sneak another request --
>>> something not involving fuse background queue.
>>
>> I don't know.. why is this better? It seems a lot more complicated. I
>> need to generate the extra request, add some switch to tell libfuse when
>> to start processing again, synchronize this with sneaking in the other
>> request...
>
> I thought it's better because it would trigger delayed processing of
> FUSE_RELEASE: last __fput() succeeded, but fuse userspace will see
> FUSE_RELEASE only later. Adding sleep to fuse_release_common would
> only extend processing time of last __fput(), is that what you need?

I do not fully understand the difference you describe. What I would like
to construct is the following scenario:

1. Userspace calls close()
2. Userspace close() returns
3. Userspace calls unlink()
4. Userspace unlink() returns
5. libfuse reads UNLINK request from kernel pipe
6. libfuse reads RELEASE request from kernel pipe

What would be the simplest way to do that?

Thanks!
-Nikolaus

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