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Message-ID: <79a32ab9308d6e63e066aa17c5c2492b51b55850.camel@kernel.org> Date: Mon, 30 Sep 2024 16:53:04 -0400 From: Jeff Layton <jlayton@...nel.org> To: Thomas Gleixner <tglx@...utronix.de>, John Stultz <jstultz@...gle.com>, Stephen Boyd <sboyd@...nel.org>, Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Jonathan Corbet <corbet@....net>, Chandan Babu R <chandan.babu@...cle.com>, "Darrick J. Wong" <djwong@...nel.org>, Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger.kernel@...ger.ca>, Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>, Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Chuck Lever <chuck.lever@...cle.com>, Vadim Fedorenko <vadim.fedorenko@...ux.dev> Cc: Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, linux-doc@...r.kernel.org, linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org, linux-btrfs@...r.kernel.org, linux-nfs@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper On Mon, 2024-09-30 at 22:19 +0200, Thomas Gleixner wrote: > On Mon, Sep 30 2024 at 15:37, Jeff Layton wrote: > > On Mon, 2024-09-30 at 21:16 +0200, Thomas Gleixner wrote: > > I have the following section in the multigrain-ts.rst file that gets > > added in patch 7 of this series. I'll also plan to add some extra > > wording about how backward realtime clock jumps can affect ordering: > > Please also add comments into the code / interface. > Will do. > > Inode Timestamp Ordering > > ======================== > > > > In addition to providing info about changes to individual files, file > > timestamps also serve an important purpose in applications like "make". These > > programs measure timestamps in order to determine whether source files might be > > newer than cached objects. > > > > Userland applications like make can only determine ordering based on > > operational boundaries. For a syscall those are the syscall entry and exit > > points. For io_uring or nfsd operations, that's the request submission and > > response. In the case of concurrent operations, userland can make no > > determination about the order in which things will occur. > > > > For instance, if a single thread modifies one file, and then another file in > > sequence, the second file must show an equal or later mtime than the first. The > > same is true if two threads are issuing similar operations that do not overlap > > in time. > > > > If however, two threads have racing syscalls that overlap in time, then there > > is no such guarantee, and the second file may appear to have been modified > > before, after or at the same time as the first, regardless of which one was > > submitted first. > > That makes me ask a question. Are the timestamps always taken in thread > (syscall) context or can they be taken in other contexts (worker, > [soft]interrupt, etc.) too? > That's a good question. The main place we do this is inode_set_ctime_current(). That is mostly called in the context of a syscall or similar sort of operation (io_uring, nfsd RPC request, etc.). I certainly wouldn't rule out a workqueue job calling that function, but this is something we do while dirtying an inode, and that's not typically done in interrupt context. -- Jeff Layton <jlayton@...nel.org>
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