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 5F62442A80; Wed, 3 Jul 2024 09:22:49 +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=1719998569; cv=none; b=tsZOY4u8cNycf+x91st0pqKb2MnGBruWLRRAirLTaa47PaxB8XwQYu0Z0V76/49Kt8kV9GIJvPjJX0MtXTvPHvbOAEeLX3q9aoEz5UJginxkrZ9U4Twk6bykBsNab6Ydjm2ico7SxdsofjGNX7af3WBc7fsOvtVD1fMmg07T610= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1719998569; c=relaxed/simple; bh=VcMNyqxa/OyWVttyD2gJnZf2GezIMVfmTMgIXg2wyBg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=J9n6aNiWIhZVOgaukacnkvns6h2qoD0rknnWq1CVOI9hkLm8JmInrMYR3Wp5CGnaeLsGE35habqRlTIkUWUBmVywF4JJgVR3/g9Wt3YFQK3I8hINSW3sxsXA+frDPvFgBOQ4LzOMa74aRi4D8Gwg1WciF6U781U1VBkG9X50V9g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=vQyQgr6v; 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="vQyQgr6v" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2BEEFC32781; Wed, 3 Jul 2024 09:22:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1719998569; bh=VcMNyqxa/OyWVttyD2gJnZf2GezIMVfmTMgIXg2wyBg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=vQyQgr6vGRZYJ5CilNJD/htgWr0SkybCXRRAQkAxdx3pJaCx9l6tqy0zdS9W/lp5q 9h7qIrqIVHw4Tv82ptMGpb3ehq8vlb/Jc/9iKUxC+lAiK66z1P7CsB9h71UaWdEAYq c0jnCb3SHrSA3783TJs4rLLIZtewYR9kDw+9nwGluD+knloOc4jLLQK5U+h4qQQgLm eVzwEXzba7/SKtqtfKqf+k8OHAhd/fnPYmEM7d52YAXDS6/mN8oJ6B9FvvIKQ1zows niZ1oenDlYM8Ghr88JTX42sfSXtJnSg3y+4HBA4DKrZ6YX6gOtppsl+ApDBgJvxp1d 8I//4FZMA1q0Q== Date: Wed, 3 Jul 2024 11:22:43 +0200 From: Christian Brauner To: Alexander Larsson , Ian Kent Cc: Jan Kara , Matthew Wilcox , Lucas Karpinski , viro@zeniv.linux.org.uk, raven@themaw.net, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Eric Chanudet Subject: Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT_DETACH umount Message-ID: <20240703-mahnung-bauland-ffcacea4101e@brauner> References: <50512ec3-da6d-4140-9659-58e0514a4970@redhat.com> <20240627115418.lcnpctgailhlaffc@quack3> <20240627-abkassieren-grinsen-6ce528fe5526@brauner> <20240628-gelingen-erben-0f6e14049e68@brauner> <97cf3ef4-d2b4-4cb0-9e72-82ca42361b13@redhat.com> <20240701-zauber-holst-1ad7cadb02f9@brauner> <20240701-treue-irrtum-e695ee5efe83@brauner> Precedence: bulk X-Mailing-List: linux-kernel@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: <20240701-treue-irrtum-e695ee5efe83@brauner> On Mon, Jul 01, 2024 at 02:10:31PM GMT, Christian Brauner wrote: > On Mon, Jul 01, 2024 at 10:41:40AM GMT, Alexander Larsson wrote: > > On Mon, Jul 1, 2024 at 7:50 AM Christian Brauner wrote: > > > > > > > I always thought the rcu delay was to ensure concurrent path walks "see" the > > > > > > > > umount not to ensure correct operation of the following mntput()(s). > > > > > > > > > > > > Isn't the sequence of operations roughly, resolve path, lock, deatch, > > > > release > > > > > > > > lock, rcu wait, mntput() subordinate mounts, put path. > > > > > > The crucial bit is really that synchronize_rcu_expedited() ensures that > > > the final mntput() won't happen until path walk leaves RCU mode. > > > > > > This allows caller's like legitimize_mnt() which are called with only > > > the RCU read-lock during lazy path walk to simple check for > > > MNT_SYNC_UMOUNT and see that the mnt is about to be killed. If they see > > > that this mount is MNT_SYNC_UMOUNT then they know that the mount won't > > > be freed until an RCU grace period is up and so they know that they can > > > simply put the reference count they took _without having to actually > > > call mntput()_. > > > > > > Because if they did have to call mntput() they might end up shutting the > > > filesystem down instead of umount() and that will cause said EBUSY > > > errors I mentioned in my earlier mails. > > > > But such behaviour could be kept even without an expedited RCU sync. > > Such as in my alternative patch for this: > > https://www.spinics.net/lists/linux-fsdevel/msg270117.html > > > > I.e. we would still guarantee the final mput is called, but not block > > the return of the unmount call. > > That's fine but the patch as sent doesn't work is my point. It'll cause > exactly the issues described earlier, no? So I'm confused why this > version simply ended up removing synchronize_rcu_expedited() when > the proposed soluton seems to have been to use queue_rcu_work(). > > But anyway, my concern with this is still that this changes the way > MNT_DETACH behaves when you shut down a non-busy filesystem with > MNT_DETACH as outlined in my other mail. > > If you find a workable version I'm not entirely opposed to try this but > I wouldn't be surprised if this causes user visible issues for anyone > that uses MNT_DETACH on a non-used filesystem. Correction: I misremembered that umount_tree() is called with UMOUNT_SYNC only in the case that umount() isn't called with MNT_DETACH. I mentioned this yesterday in the thread but just in case you missed it I want to spell it out in detail as well. This is relevant because UMOUNT_SYNC will raise MNT_SYNC_UMOUNT on all mounts it unmounts. And that ends up being checked in legitimize_mnt() to ensure that legitimize_mnt() doesn't call mntput() during path lookup and risking EBUSY for a umount(..., 0) + mount() sequence for the same filesystem. But for umount(.., MNT_DETACH) UMOUNT_SYNC isn't passed and so MNT_SYNC_UMOUNT isn't raised on the mount and so legitimize_mnt() may end up doing the last mntput() and cleaning up the filesystem. In other words, a umount(..., MNT_DETACH) caller needs to be prepared to deal with EBUSY for a umount(..., MNT_DETACH) + mount() sequence. So I think we can certainly try this as long as we make it via queue_rcu_work() to handle the other mntput_no_expire() grace period dependency we discussed upthread. Thanks for making take a closer look.