FAILED: patch "[PATCH] fuse: fix livelock in synchronous file put from fuseblk" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 26e5c67deb2e1f42a951f022fdf5b9f7eb747b01
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025101628-exclude-hamstring-8d43@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 26e5c67deb2e1f42a951f022fdf5b9f7eb747b01 Mon Sep 17 00:00:00 2001
From: "Darrick J. Wong" <[email protected]>
Date: Mon, 15 Sep 2025 17:24:17 -0700
Subject: [PATCH] fuse: fix livelock in synchronous file put from fuseblk
workers

I observed a hang when running generic/323 against a fuseblk server. This test opens a file, initiates a lot of AIO writes to that file descriptor, and closes the file descriptor before the writes complete. Unsurprisingly, the AIO exerciser threads are mostly stuck waiting for responses from the fuseblk server:

# cat /proc/372265/task/372313/stack
[<0>] request_wait_answer+0x1fe/0x2a0 [fuse]
[<0>] __fuse_simple_request+0xd3/0x2b0 [fuse]
[<0>] fuse_do_getattr+0xfc/0x1f0 [fuse]
[<0>] fuse_file_read_iter+0xbe/0x1c0 [fuse]
[<0>] aio_read+0x130/0x1e0
[<0>] io_submit_one+0x542/0x860
[<0>] __x64_sys_io_submit+0x98/0x1a0
[<0>] do_syscall_64+0x37/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x4b/0x53

But the /weird/ part is that the fuseblk server threads are waiting for responses from itself:

# cat /proc/372210/task/372232/stack
[<0>] request_wait_answer+0x1fe/0x2a0 [fuse]
[<0>] __fuse_simple_request+0xd3/0x2b0 [fuse]
[<0>] fuse_file_put+0x9a/0xd0 [fuse]
[<0>] fuse_release+0x36/0x50 [fuse]
[<0>] __fput+0xec/0x2b0
[<0>] task_work_run+0x55/0x90
[<0>] syscall_exit_to_user_mode+0xe9/0x100
[<0>] do_syscall_64+0x43/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x4b/0x53

The fuseblk server is fuse2fs so there's nothing all that exciting in the server itself. So why is the fuse server calling fuse_file_put? The commit message for the fstest sheds some light on that:

"By closing the file descriptor before calling io_destroy, you pretty much guarantee that the last put on the ioctx will be done in interrupt context (during I/O completion).

Aha. AIO fgets a new struct file from the fd when it queues the ioctx. The completion of the FUSE_WRITE command from userspace causes the fuse server to call the AIO completion function. The completion puts the struct file, queuing a delayed fput to the fuse server task. When the fuse server task returns to userspace, it has to run the delayed fput, which in the case of a fuseblk server, it does synchronously.

Sending the FUSE_RELEASE command sychronously from fuse server threads is a bad idea because a client program can initiate enough simultaneous AIOs such that all the fuse server threads end up in delayed_fput, and now there aren't any threads left to handle the queued fuse commands.

Fix this by only using asynchronous fputs when closing files, and leave a comment explaining why.

index 54786f62a9d8..f1ef77a0be05 100644
@@ -356,8 +356,14 @@ void fuse_file_release(struct inode *inode, struct fuse_file *ff,
 	 * Make the release synchronous if this is a fuseblk mount,
 	 * synchronous RELEASE is allowed (and desirable) in this case
 	 * because the server can be trusted not to screw up.
+	 *
+	 * Always use the asynchronous file put because the current thread
+	 * might be the fuse server.  This can happen if a process starts some
+	 * aio and closes the fd before the aio completes.  Since aio takes its
+	 * own ref to the file, the IO completion has to drop the ref, which is
+	 * how the fuse server can end up closing its clients' files.
 	 */
-	fuse_file_put(ff, ff->fm->fc->destroy);
+	fuse_file_put(ff, false);
 }
 
 void fuse_release_common(struct file *file, bool isdir)

From: Amir Goldstein <[email protected]>

[ Upstream commit e26ee4efbc79610b20e7abe9d96c87f33dacc1ff ]

This removed the need to pass isdir argument to fuse_put_file().

Stable-dep-of: 26e5c67deb2e ("fuse: fix livelock in synchronous file put from fuseblk workers")

---
 fs/fuse/dir.c    |  2 +-
 fs/fuse/file.c   | 69 +++++++++++++++++++++++++++---------------------
 fs/fuse/fuse_i.h |  2 +-
 3 files changed, 41 insertions(+), 32 deletions(-)

index 0dbacdd7bb0d8..e54d70a42519b 100644
@@ -584,7 +584,7 @@ static int fuse_create_open(struct inode *dir, struct dentry *entry,
 		goto out_err;
 
 	err = -ENOMEM;
-	ff = fuse_file_alloc(fm);
+	ff = fuse_file_alloc(fm, true);
 	if (!ff)
 		goto out_put_forget_req;
 

index 6c3ec70c1b70d..d280ef6e346b4 100644
@@ -1022,7 +1022,7 @@ void fuse_read_args_fill(struct fuse_io_args *ia, struct file *file, loff_t pos,
  */
 int fuse_open_common(struct inode *inode, struct file *file, bool isdir);
 
-struct fuse_file *fuse_file_alloc(struct fuse_mount *fm);
+struct fuse_file *fuse_file_alloc(struct fuse_mount *fm, bool release);
 void fuse_file_free(struct fuse_file *ff);
 void fuse_finish_open(struct inode *inode, struct file *file);
 

2.51.0

From: "Darrick J. Wong" <[email protected]>

[ Upstream commit 26e5c67deb2e1f42a951f022fdf5b9f7eb747b01 ]

I observed a hang when running generic/323 against a fuseblk server. This test opens a file, initiates a lot of AIO writes to that file descriptor, and closes the file descriptor before the writes complete. Unsurprisingly, the AIO exerciser threads are mostly stuck waiting for responses from the fuseblk server:

# cat /proc/372265/task/372313/stack
[<0>] request_wait_answer+0x1fe/0x2a0 [fuse]
[<0>] __fuse_simple_request+0xd3/0x2b0 [fuse]
[<0>] fuse_do_getattr+0xfc/0x1f0 [fuse]
[<0>] fuse_file_read_iter+0xbe/0x1c0 [fuse]
[<0>] aio_read+0x130/0x1e0
[<0>] io_submit_one+0x542/0x860
[<0>] __x64_sys_io_submit+0x98/0x1a0
[<0>] do_syscall_64+0x37/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x4b/0x53

But the /weird/ part is that the fuseblk server threads are waiting for responses from itself:

# cat /proc/372210/task/372232/stack
[<0>] request_wait_answer+0x1fe/0x2a0 [fuse]
[<0>] __fuse_simple_request+0xd3/0x2b0 [fuse]
[<0>] fuse_file_put+0x9a/0xd0 [fuse]
[<0>] fuse_release+0x36/0x50 [fuse]
[<0>] __fput+0xec/0x2b0
[<0>] task_work_run+0x55/0x90
[<0>] syscall_exit_to_user_mode+0xe9/0x100
[<0>] do_syscall_64+0x43/0xf0
[<0>] entry_SYSCALL_64_after_hwframe+0x4b/0x53

The fuseblk server is fuse2fs so there's nothing all that exciting in the server itself. So why is the fuse server calling fuse_file_put? The commit message for the fstest sheds some light on that:

"By closing the file descriptor before calling io_destroy, you pretty much guarantee that the last put on the ioctx will be done in interrupt context (during I/O completion).

Aha. AIO fgets a new struct file from the fd when it queues the ioctx. The completion of the FUSE_WRITE command from userspace causes the fuse server to call the AIO completion function. The completion puts the struct file, queuing a delayed fput to the fuse server task. When the fuse server task returns to userspace, it has to run the delayed fput, which in the case of a fuseblk server, it does synchronously.

Sending the FUSE_RELEASE command sychronously from fuse server threads is a bad idea because a client program can initiate enough simultaneous AIOs such that all the fuse server threads end up in delayed_fput, and now there aren't any threads left to handle the queued fuse commands.

Fix this by only using asynchronous fputs when closing files, and leave a comment explaining why.

---
 fs/fuse/file.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

index 627ad8fadbec0..dd1864c95d074 100644
@@ -338,8 +338,14 @@ void fuse_file_release(struct inode *inode, struct fuse_file *ff,
 	 * Make the release synchronous if this is a fuseblk mount,
 	 * synchronous RELEASE is allowed (and desirable) in this case
 	 * because the server can be trusted not to screw up.
+	 *
+	 * Always use the asynchronous file put because the current thread
+	 * might be the fuse server.  This can happen if a process starts some
+	 * aio and closes the fd before the aio completes.  Since aio takes its
+	 * own ref to the file, the IO completion has to drop the ref, which is
+	 * how the fuse server can end up closing its clients' files.
 	 */
-	fuse_file_put(ff, ff->fm->fc->destroy);
+	fuse_file_put(ff, false);
 }
 
 void fuse_release_common(struct file *file, bool isdir)

2.51.0