From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6AF5D26B2B0 for ; Mon, 20 Oct 2025 16:03:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760976180; cv=none; b=CGtyM56oDWBXvtDKT+pv0I/ZoWJVQ78nLQCAQllBn6F7zD63Nb5BxYQdk59CDmyKnpg7Na4o4YRe/up4YIJ+jIVAEt7QfnU4+mXAdZkrOWjTOWVpM358oCIuhutLDG8ZsuHo+wO9JeHoBJ9cNYBluiZQNKrIoqaMnH32ubKKNl0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760976180; c=relaxed/simple; bh=jubJTDVj5zmj4Ax+kQUuvlUA0Dc2l5U4k3gTPI3Eagg=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=VVmpAz41T5q+oecp5bQduuLgXEC3AnF5qLEZ6AMfsziTx1yRspAS7A7vCxzHaoYZhcRpK+xIaSE9f1o4I69Zawt5JNLytmV6dFZjZ901YIeNIQinfTdDErsL9nJcVvfA31oBou6cB14axKYWrGqC8tOT2yGxzbm1arN/+abYrIA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=WGjGhz5W; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="WGjGhz5W" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7FE62C113D0; Mon, 20 Oct 2025 16:02:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1760976180; bh=jubJTDVj5zmj4Ax+kQUuvlUA0Dc2l5U4k3gTPI3Eagg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=WGjGhz5WuE8IF1Og9orhiTToMhmz9S2roT3qszjRIwBRgXk8zYaP2sZHJK+I9COzs m4vIxSmr2bUF9QbKFQ7+X2sllov8SKKVMQ+PD0BisCAwkf03+GTU89tCudd8ZmHzL7 524V6Fu7nTaZZVFeJXBbk+t/zU01SmUlAWYlsLkPt2sgLvci0EdHMw4BhAR1lfFrMU VyslSA6o7pMTQ2IoXUBbsHZx2/4v0ZK2rQQVAMpbrMrEEPXXGnQesP7ux1uiz0DekL 3tCA9jfbevyOFQfGaVtHZ7c51P1ykS3ydM/UhrfXEIPpbY2MqesEhPCU6+Hd1WqZCV ksXP1Y36H2b/w== From: Sasha Levin To: stable@vger.kernel.org Cc: "Darrick J. Wong" , Miklos Szeredi , Sasha Levin Subject: [PATCH 5.15.y 2/2] fuse: fix livelock in synchronous file put from fuseblk workers Date: Mon, 20 Oct 2025 12:02:56 -0400 Message-ID: <20251020160256.1828701-2-sashal@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20251020160256.1828701-1-sashal@kernel.org> References: <2025101629-privacy-morally-5172@gregkh> <20251020160256.1828701-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: "Darrick J. Wong" [ 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. Cc: stable@vger.kernel.org # v2.6.38 Fixes: 5a18ec176c934c ("fuse: fix hang of single threaded fuseblk filesystem") Signed-off-by: Darrick J. Wong Signed-off-by: Miklos Szeredi Signed-off-by: Sasha Levin --- fs/fuse/file.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/fs/fuse/file.c b/fs/fuse/file.c index d940bebe623e8..ebe49bf1155ad 100644 --- a/fs/fuse/file.c +++ b/fs/fuse/file.c @@ -340,8 +340,14 @@ 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