On 05/29/2017 09:49 AM, Nikolaus Rath wrote:
Hi Maxim,
On May 26 2017, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
On 05/25/2017 04:08 PM, Nikolaus Rath wrote:
Hello,
I am trying to debug a sporadic test failure in libfuse
(https://github.com/libfuse/libfuse/issues/157).
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). 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.
Looking at fs/fuse/file.c, it looks as if fuse_release() directly calls
fuse_request_send_background() to send the request. But at that point I
can no longer follow the code. Is it possible for another request to
sneak in at this point?
Yes, but not for that given fuse_file that we're closing now.
I assume that a fuse_file refers to the (formerly) opened file, right?
So e.g. a unlink() request for the same directly entry could still go
through before RELEASE has been transferred to the pending queue?
Yes. But FUSE_UNLINK works on file path, it doesn't depend on fuse_file
at all. Hence unlink request can go anytime.
Best,
-Nikolaus
