Message-ID: <9544f973-8ffd-43f9-b943-f7398b40f952@bsbernd.com>
Date: Mon, 10 Nov 2025 23:09:05 +0100
From: Bernd Schubert <bernd@...ernd.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: Joanne Koong <joannelkoong@...il.com>, miklos@...redi.hu, neal@...pa.dev,
 linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/5] fuse: flush pending fuse events before aborting the
 connection



On 11/10/25 19:54, Darrick J. Wong wrote:
> On Mon, Nov 10, 2025 at 07:22:57PM +0100, Bernd Schubert wrote:
>>
>>
>> On 11/10/25 18:56, Darrick J. Wong wrote:
>>> On Fri, Nov 07, 2025 at 04:02:54PM -0800, Darrick J. Wong wrote:
>>>> On Fri, Nov 07, 2025 at 11:03:24PM +0100, Bernd Schubert wrote:
>>>>>
>>>>>
>>>>> On 11/7/25 05:26, Darrick J. Wong wrote:
>>>>>> [I read this email backwards, like I do]
>>>>>>
>>>>>> On Thu, Nov 06, 2025 at 10:37:41AM -0800, Joanne Koong wrote:
>>>>>>> On Wed, Nov 5, 2025 at 4:17 PM Darrick J. Wong <djwong@...nel.org> wrote:
>>>>>>>>
>>>>>>>> On Tue, Nov 04, 2025 at 11:22:26AM -0800, Joanne Koong wrote:
>>>>>>>>
>>>>>>>> <snipping here because this thread has gotten very long>
>>>>>>>>
>>>>>>>>>>>> +       while (wait_event_timeout(fc->blocked_waitq,
>>>>>>>>>>>> +                       !fc->connected || atomic_read(&fc->num_waiting) == 0,
>>>>>>>>>>>> +                       HZ) == 0) {
>>>>>>>>>>>> +               /* empty */
>>>>>>>>>>>> +       }
>>>>>>>>>>>
>>>>>>>>>>> I'm wondering if it's necessary to wait here for all the pending
>>>>>>>>>>> requests to complete or abort?
>>>>>>>>>>
>>>>>>>>>> I'm not 100% sure what the fuse client shutdown sequence is supposed to
>>>>>>>>>> be.  If someone kills a program with a large number of open unlinked
>>>>>>>>>> files and immediately calls umount(), then the fuse client could be in
>>>>>>>>>> the process of sending FUSE_RELEASE requests to the server.
>>>>>>>>>>
>>>>>>>>>> [background info, feel free to speedread this paragraph]
>>>>>>>>>> For a non-fuseblk server, unmount aborts all pending requests and
>>>>>>>>>> disconnects the fuse device.  This means that the fuse server won't see
>>>>>>>>>> all the FUSE_REQUESTs before libfuse calls ->destroy having observed the
>>>>>>>>>> fusedev shutdown.  The end result is that (on fuse2fs anyway) you end up
>>>>>>>>>> with a lot of .fuseXXXXX files that nobody cleans up.
>>>>>>>>>>
>>>>>>>>>> If you make ->destroy release all the remaining open files, now you run
>>>>>>>>>> into a second problem, which is that if there are a lot of open unlinked
>>>>>>>>>> files, freeing the inodes can collectively take enough time that the
>>>>>>>>>> FUSE_DESTROY request times out.
>>>>>>>>>>
>>>>>>>>>> On a fuseblk server with libfuse running in multithreaded mode, there
>>>>>>>>>> can be several threads reading fuse requests from the fusedev.  The
>>>>>>>>>> kernel actually sends its own FUSE_DESTROY request, but there's no
>>>>>>>>>> coordination between the fuse workers, which means that the fuse server
>>>>>>>>>> can process FUSE_DESTROY at the same time it's processing FUSE_RELEASE.
>>>>>>>>>> If ->destroy closes the filesystem before the FUSE_RELEASE requests are
>>>>>>>>>> processed, you end up with the same .fuseXXXXX file cleanup problem.
>>>>>>>>>
>>>>>>>>> imo it is the responsibility of the server to coordinate this and make
>>>>>>>>> sure it has handled all the requests it has received before it starts
>>>>>>>>> executing the destruction logic.
>>>>>>>>
>>>>>>>> I think we're all saying that some sort of fuse request reordering
>>>>>>>> barrier is needed here, but there's at least three opinions about where
>>>>>>>> that barrier should be implemented.  Clearly I think the barrier should
>>>>>>>> be in the kernel, but let me think more about where it could go if it
>>>>>>>> were somewhere else.
>>>>>>>>
>>>>>>>> First, Joanne's suggestion for putting it in the fuse server itself:
>>>>>>>>
>>>>>>>> I don't see how it's generally possible for the fuse server to know that
>>>>>>>> it's processed all the requests that the kernel might have sent it.
>>>>>>>> AFAICT each libfuse thread does roughly this:
>>>>>>>>
>>>>>>>> 1. read() a request from the fusedev fd
>>>>>>>> 2. decode the request data and maybe do some allocations or transform it
>>>>>>>> 3. call fuse server with request
>>>>>>>> 4. fuse server does ... something with the request
>>>>>>>> 5. fuse server finishes, hops back to libfuse / calls fuse_reply_XXX
>>>>>>>>
>>>>>>>> Let's say thread 1 is at step 4 with a FUSE_DESTROY.  How does it find
>>>>>>>> out if there are other fuse worker threads that are somewhere in steps
>>>>>>>> 2 or 3?  AFAICT the library doesn't keep track of the number of threads
>>>>>>>> that are waiting in fuse_session_receive_buf_internal, so fuse servers
>>>>>>>> can't ask the library about that either.
>>>>>>>>
>>>>>>>> Taking a narrower view, it might be possible for the fuse server to
>>>>>>>> figure this out by maintaining an open resource count.  It would
>>>>>>>> increment this counter when a FUSE_{OPEN,CREATE} request succeeds and
>>>>>>>> decrement it when FUSE_RELEASE comes in.  Assuming that FUSE_RELEASE is
>>>>>>>> the only kind of request that can be pending when a FUSE_DESTROY comes
>>>>>>>> in, then destroy just has to wait for the counter to hit zero.
>>>>>>>
>>>>>>> I was thinking this logic could be in libfuse's fuse_loop_mt.c. Where
>>>>>>> if there are X worker threads that are all running fuse_do_work( )
>>>>>>> then if you get a FUSE_DESTROY on one of those threads that thread can
>>>>>>> set some se->destroyed field. At this point the other threads will
>>>>>>> have already called fuse_session_receive_buf_internal() on all the
>>>>>>> flushed background requests, so after they process it and return from
>>>>>>> fuse_session_process_buf_internal(), then they check if se->destroyed
>>>>>>> was set, and if it is they exit the thread, while in the thread that
>>>>>>> got the FUSE_DESTROY it sleeps until all the threads have completed
>>>>>>> and then it executes the destroy logic.That to me seems like the
>>>>>>> cleanest approach.
>>>>>>
>>>>>> Hrm.  Well now (scrolling to the bottom and back) that I know that the
>>>>>> FUSE_DESTROY won't get put on the queue ahead of the FUSE_RELEASEs, I
>>>>>> think that /could/ work.
>>>>>>
>>>>>> One tricky thing with having worker threads check a flag and exit is
>>>>>> that they can be sleeping in the kernel (from _fuse_session_receive_buf)
>>>>>> when the "just go away" flag gets set.  If the thread never wakes up,
>>>>>> then it'll never exit.  In theory you could have the FUSE_DESTROY thread
>>>>>> call pthread_cancel on all the other worker threads to eliminate them
>>>>>> once they emerge from PTHREAD_CANCEL_DISABLE state, but I still have
>>>>>> nightmares from adventures in pthread_cancel at Sun in 2002. :P
>>>>>>
>>>>>> Maybe an easier approach would be to have fuse_do_work increment a
>>>>>> counter when it receives a buffer and decrement it when it finishes with
>>>>>> that buffer.  The FUSE_DESTROY thread merely has to wait for that
>>>>>> counter to reach 1, at which point it's the only thread with a request
>>>>>> to process, so it can call do_destroy.  That at least would avoid adding
>>>>>> a new user of pthread_cancel() into the mt loop code.
>>>>>
>>>>> I will read through the rest (too tired right now) durig the weekend. 
>>>>> I was also thinking about counter. And let's please also do this right
>>>>> also handling io-uring. I.e. all CQEs needs to have been handled.
>>>>> Without io-uring it would be probably a counter in decreased in 
>>>>> fuse_free_req(), with io-uring it is a bit more complex.
>>>>
>>>> Oh right, the uring backend.
>>>>
>>>> Assuming that it's really true that the only requests pending during an
>>>> unmount are going to be FUSE_RELEASE (nobody's actually said that's
>>>> true) then it's *much* easier to count the number of open files in
>>>> fuse_session and make _do_destroy in the lowlevel library wait until the
>>>> open file count reaches zero.
>>>
>>> FWIW I tried this out over the weekend with the patch below and the
>>> wait_event() turned off in the kernel.  It seems to work (though I only
>>> tried it cursorily with iouring) so if the kernel fuse developers would
>>> rather not have a wait_event() in the unmount path then I suppose this
>>> is a way to move ahead with this topic.
>>>
>>> --D
>>>
>>> From: Darrick J. Wong <djwong@...nel.org>
>>> Subject: [PATCH] libfuse: wait in do_destroy until all open files are closed
>>>
>>> Joanne suggests that libfuse should defer a FUSE_DESTROY request until
>>> all FUSE_RELEASEs have completed.  Let's see if that works by tracking
>>> the count of open files.
>>>
>>> Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
>>> ---
>>>  lib/fuse_i.h        |    4 ++++
>>>  lib/fuse_lowlevel.c |   54 ++++++++++++++++++++++++++++++++++++++++++++++++---
>>>  2 files changed, 55 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/lib/fuse_i.h b/lib/fuse_i.h
>>> index 0ce2c0134ed879..dfe9d9f067498e 100644
>>> --- a/lib/fuse_i.h
>>> +++ b/lib/fuse_i.h
>>> @@ -117,6 +117,10 @@ struct fuse_session {
>>>  	 */
>>>  	uint32_t conn_want;
>>>  	uint64_t conn_want_ext;
>>> +
>>> +	/* destroy has to wait for all the open files to go away */
>>> +	pthread_cond_t zero_open_files;
>>> +	uint64_t open_files;
>>>  };
>>>  
>>>  struct fuse_chan {
>>> diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
>>> index 12724ed66bdcc8..f12c6db0eb0e60 100644
>>> --- a/lib/fuse_lowlevel.c
>>> +++ b/lib/fuse_lowlevel.c
>>> @@ -52,6 +52,30 @@
>>>  #define PARAM(inarg) (((char *)(inarg)) + sizeof(*(inarg)))
>>>  #define OFFSET_MAX 0x7fffffffffffffffLL
>>>  
>>> +static inline void inc_open_files(struct fuse_session *se)
>>> +{
>>> +	pthread_mutex_lock(&se->lock);
>>> +	se->open_files++;
>>> +	pthread_mutex_unlock(&se->lock);
>>> +}
>>> +
>>> +static inline void dec_open_files(struct fuse_session *se)
>>> +{
>>> +	pthread_mutex_lock(&se->lock);
>>> +	se->open_files--;
>>> +	if (!se->open_files)
>>> +		pthread_cond_broadcast(&se->zero_open_files);
>>> +	pthread_mutex_unlock(&se->lock);
>>> +}
>>
>> I think open files only decreases when destroy is received? Doesn't that
>> give us the chance to use an atomic (C11) and then to broadcast only
>> when FUSE_DESTROY is received? I.e. I would use an atomic for
>> 'open_files', set something like 'se->destroy_received' and then trigger
>> the broadcast only when that is set.
> 
> I'm sorry, but I'm not familiar enough with C11 atomics to know if that
> would work.  I suppose it /has/ been 3 years since the kernel went from
> C89 to C11 but some of us are still old dinosaurs who cut their teeth on
> C back when the paint was still fresh on C90. ;)

No worries, I'm personally catching up with C++ beyond C++98.

> 
> I think it would be ok to define open_files as _Atomic and then do
> something like:
> 
> 	if (atomic_fetch_sub(&se->open_files, -1) == 1)
> 		cnd_broadcast(&cse->zero_open_files);
> 
> without needing to take se->lock.  I'm not sure how you'd handle races
> between a thread setting destroy_received and dec_open_files testing
> the flag, though.  Maybe it'd be easier to bias open_files upward by one
> in the init method and downward by one in op_destroy so we'd never send
> the broadcast until unmount time?

I link that very much. Your patch didn't apply in fuse_reply_create,
because of your other changes, so I applied some parts manually.
I would like to test and merge this patch
as soon as possible, as we actually got an external ticket about it
around last week (tear down DLM issues) and generic/531 actually also
fails with passthrough_hp

generic/531       - output mismatch (see /home/fusetests/src/xfstests-dev/results//generic/531.out.bad)
    --- tests/generic/531.out   2023-02-02 15:37:23.979018084 +0100
    +++ /home/fusetests/src/xfstests-dev/results//generic/531.out.bad   2025-11-10 15:30:23.253431465 +0100
    @@ -1,2 +1,25 @@
     QA output created by 531
    +open?: Bad file descriptor
    +open?: Bad file descriptor
    +open?: Bad file descriptor
    +open?: Bad file descriptor
    +open?: Bad file descriptor
    +open?: Bad file descriptor
    ...
    (Run 'diff -u /home/fusetests/src/xfstests-dev/tests/generic/531.out /home/fusetests/src/xfstests-dev/res


My VM is currently busy with another patch set verification, I'm going
to try it after that.

Can I push the version below to libfuse for review/merge?

Subject: [PATCH] libfuse: wait in do_destroy until all open files are closed

From: Darrick J. Wong <djwong@...nel.org>

Joanne suggests that libfuse should defer a FUSE_DESTROY request until
all FUSE_RELEASEs have completed.  Let's see if that works by tracking
the count of open files.

Signed-off-by: "Darrick J. Wong" <djwong@...nel.org>
Signed-off-by: Bernd Schubert <bschubert@....com>
---
 lib/fuse_i.h        |    4 ++++
 lib/fuse_lowlevel.c |   51 +++++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/lib/fuse_i.h b/lib/fuse_i.h
index d35e1e51d823..b603e52b1b85 100644
--- a/lib/fuse_i.h
+++ b/lib/fuse_i.h
@@ -117,6 +117,10 @@ struct fuse_session {
 	 */
 	uint32_t conn_want;
 	uint64_t conn_want_ext;
+
+	/* destroy has to wait for all the open files to go away */
+	pthread_cond_t zero_open_files;
+	_Atomic uint64_t open_files;
 };
 
 struct fuse_chan {
diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index d420b257b9dd..2e09027e6022 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -52,6 +52,30 @@
 #define PARAM(inarg) (((char *)(inarg)) + sizeof(*(inarg)))
 #define OFFSET_MAX 0x7fffffffffffffffLL
 
+static inline void inc_open_files(struct fuse_session *se)
+{
+	se->open_files++;
+}
+
+static inline void dec_open_files(struct fuse_session *se)
+{
+	int prev = atomic_fetch_sub(&se->open_files, 1);
+	if (unlikely(prev == 1)) {
+		pthread_mutex_lock(&se->lock);
+		pthread_cond_broadcast(&se->zero_open_files);
+		pthread_mutex_unlock(&se->lock);
+	}
+}
+
+static inline void wait_for_zero_open_files(struct fuse_session *se)
+{
+	pthread_mutex_lock(&se->lock);
+	while (se->open_files > 0) {
+		pthread_cond_wait(&se->zero_open_files, &se->lock);
+	}
+	pthread_mutex_unlock(&se->lock);
+}
+
 struct fuse_pollhandle {
 	uint64_t kh;
 	struct fuse_session *se;
@@ -522,17 +546,23 @@ int fuse_reply_entry(fuse_req_t req, const struct fuse_entry_param *e)
 int fuse_reply_create(fuse_req_t req, const struct fuse_entry_param *e,
 		      const struct fuse_file_info *f)
 {
+	struct fuse_session *se = req->se;
 	alignas(uint64_t) char buf[sizeof(struct fuse_entry_out) + sizeof(struct fuse_open_out)];
 	size_t entrysize = req->se->conn.proto_minor < 9 ?
 		FUSE_COMPAT_ENTRY_OUT_SIZE : sizeof(struct fuse_entry_out);
 	struct fuse_entry_out *earg = (struct fuse_entry_out *) buf;
 	struct fuse_open_out *oarg = (struct fuse_open_out *) (buf + entrysize);
+	int error;
 
 	memset(buf, 0, sizeof(buf));
 	fill_entry(earg, e);
 	fill_open(oarg, f);
-	return send_reply_ok(req, buf,
+	error = send_reply_ok(req, buf,
 			     entrysize + sizeof(struct fuse_open_out));
+	if (!error)
+		inc_open_files(se);
+
+	return error;
 }
 
 int fuse_reply_attr(fuse_req_t req, const struct stat *attr,
@@ -583,10 +613,15 @@ int fuse_passthrough_close(fuse_req_t req, int backing_id)
 int fuse_reply_open(fuse_req_t req, const struct fuse_file_info *f)
 {
 	struct fuse_open_out arg;
+	struct fuse_session *se = req->se;
+	int error;
 
 	memset(&arg, 0, sizeof(arg));
 	fill_open(&arg, f);
-	return send_reply_ok(req, &arg, sizeof(arg));
+	error = send_reply_ok(req, &arg, sizeof(arg));
+	if (!error)
+		inc_open_files(se);
+	return error;
 }
 
 static int do_fuse_reply_write(fuse_req_t req, size_t count)
@@ -1854,6 +1889,7 @@ static void _do_release(fuse_req_t req, const fuse_ino_t nodeid,
 {
 	(void)in_payload;
 	const struct fuse_release_in *arg = op_in;
+	struct fuse_session *se = req->se;
 	struct fuse_file_info fi;
 
 	memset(&fi, 0, sizeof(fi));
@@ -1872,6 +1908,7 @@ static void _do_release(fuse_req_t req, const fuse_ino_t nodeid,
 		req->se->op.release(req, nodeid, &fi);
 	else
 		fuse_reply_err(req, 0);
+	dec_open_files(se);
 }
 
 static void do_release(fuse_req_t req, const fuse_ino_t nodeid,
@@ -1976,6 +2013,7 @@ static void _do_releasedir(fuse_req_t req, const fuse_ino_t nodeid,
 {
 	(void)in_payload;
 	struct fuse_release_in *arg = (struct fuse_release_in *)op_in;
+	struct fuse_session *se = req->se;
 	struct fuse_file_info fi;
 
 	memset(&fi, 0, sizeof(fi));
@@ -1986,6 +2024,7 @@ static void _do_releasedir(fuse_req_t req, const fuse_ino_t nodeid,
 		req->se->op.releasedir(req, nodeid, &fi);
 	else
 		fuse_reply_err(req, 0);
+	dec_open_files(se);
 }
 
 static void do_releasedir(fuse_req_t req, const fuse_ino_t nodeid,
@@ -2996,6 +3035,9 @@ static void _do_destroy(fuse_req_t req, const fuse_ino_t nodeid,
 	(void)op_in;
 	(void)in_payload;
 
+	se->open_files--;
+	wait_for_zero_open_files(se);
+
 	mountpoint = atomic_exchange(&se->mountpoint, NULL);
 	free(mountpoint);
 
@@ -3824,6 +3866,7 @@ void fuse_session_destroy(struct fuse_session *se)
 		fuse_ll_pipe_free(llp);
 	pthread_key_delete(se->pipe_key);
 	sem_destroy(&se->mt_finish);
+	pthread_cond_destroy(&se->zero_open_files);
 	pthread_mutex_destroy(&se->mt_lock);
 	pthread_mutex_destroy(&se->lock);
 	free(se->cuse_data);
@@ -4202,6 +4245,9 @@ fuse_session_new_versioned(struct fuse_args *args,
 	sem_init(&se->mt_finish, 0, 0);
 	pthread_mutex_init(&se->mt_lock, NULL);
 
+	pthread_cond_init(&se->zero_open_files, NULL);
+	se->open_files = 1; /* decreased by FUSE_DESTROY */
+
 	err = pthread_key_create(&se->pipe_key, fuse_ll_pipe_destructor);
 	if (err) {
 		fuse_log(FUSE_LOG_ERR, "fuse: failed to create thread specific key: %s\n",
@@ -4226,6 +4272,7 @@ fuse_session_new_versioned(struct fuse_args *args,
 
 out5:
 	sem_destroy(&se->mt_finish);
+	pthread_cond_destroy(&se->zero_open_files);
 	pthread_mutex_destroy(&se->mt_lock);
 	pthread_mutex_destroy(&se->lock);
 out4:


Thanks,
Bernd

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