linux-kernel - Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper

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Message-ID: <87plokzuy6.ffs@tglx>
Date: Mon, 30 Sep 2024 23:35:13 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jeff Layton <jlayton@...nel.org>, 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, Sep 30 2024 at 16:53, Jeff Layton wrote:
> On Mon, 2024-09-30 at 22:19 +0200, Thomas Gleixner wrote:
>> On Mon, Sep 30 2024 at 15:37, Jeff Layton wrote:
>> > 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.

The reason I'm asking is that if it's always syscall context,
i.e. write() or io_uring()/RPC request etc., then you can avoid the
whole global floor value dance and make it strictly per thread, which
simplifies the exercise significantly.

But even if it's not syscall/thread context then the worker or io_uring
state machine might just require to serialize against itself and not
coordinate with something else. But what do I know.

Thanks,

        tglx