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 1604CFC1D; Sat, 8 Nov 2025 00:02:55 +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=1762560176; cv=none; b=dN1zZiNBzFy3QhVjC3Zdi1y6mUFKH68DYzBcs6C7NarkURMseskmXb7AZL/ktgPtEd4FpEGDe5aP3q+i/9L4PrsUI2n0rdkM2m4m9qgIwkTLACnlxrLn5wv7x3XHR4aR6IS+JjzgQWFVWPUttuPvWnxh938fc4DQ3Ztf08tyUXM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762560176; c=relaxed/simple; bh=viNt9afbNBdtQqi1Dc1Q/j7+Plf7H/XEwFpGMxTCOlo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=oFpXHDFgZdPplsH4C3pcdfLJ/vd5tzPypLzhZ0z6FxplY/FUJcIljJLGeG6hN7FaMaqJFu7xjN1CLedxi1JE6TZJEpQoNQzkFcv0YnacdKuej8vX2JoqguGg3T359nV0nHwiO3eLMX4Uqj4tilxr6Cbi5O0fGxNoA0cUag7+zjQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=h5DseiMz; 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="h5DseiMz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 849BFC116C6; Sat, 8 Nov 2025 00:02:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1762560175; bh=viNt9afbNBdtQqi1Dc1Q/j7+Plf7H/XEwFpGMxTCOlo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=h5DseiMzmvLusOH9gxk8iMToJaHV8GBC57kLneHq2mdC90oGQ9YT+Z9sk8VyBK252 iViivnBvYzJTWO/gojQ+2Vgwus0vyaRWdzqIcVV1OdhwBBQUACstwXLYvwwduikoT7 EQwD00ty3ghInmqVOFqy8BegCbwOXIx8CqNXXGQWpaQKbdt6aGFMQvIBuSYKQTFJ4d PE0onTIwzmK7m5VrGTZzBY7jx9Ra9KhMbrl7OGmqr9O23zCjdPd5c1qEwXdjQ/QJWb 8Mbng12xMCEwm4ERBgO4axkOFW+PYtFpg69/n4fXCoUy2IqwJd9xN17jv5E5MEjQPU bT05F1F21d+Qw== Date: Fri, 7 Nov 2025 16:02:54 -0800 From: "Darrick J. Wong" To: Bernd Schubert Cc: Joanne Koong , miklos@szeredi.hu, neal@gompa.dev, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 1/5] fuse: flush pending fuse events before aborting the connection Message-ID: <20251108000254.GK196391@frogsfrogsfrogs> References: <176169809222.1424347.16562281526870178424.stgit@frogsfrogsfrogs> <176169809274.1424347.4813085698864777783.stgit@frogsfrogsfrogs> <20251103221349.GE196370@frogsfrogsfrogs> <20251106001730.GH196358@frogsfrogsfrogs> <20251107042619.GK196358@frogsfrogsfrogs> Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 wrote: > >>> > >>> On Tue, Nov 04, 2025 at 11:22:26AM -0800, Joanne Koong wrote: > >>> > >>> > >>> > >>>>>>> + 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. --D > Thanks, > Bernd >