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

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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>