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Date:   Tue, 19 Sep 2023 13:10:47 -0700
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To:     Jeff Layton <jlayton@kernel.org>, Bruno Haible <bruno@clisp.org>,
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References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org>
 <20230919110457.7fnmzo4nqsi43yqq@quack3>
 <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org>
 <4511209.uG2h0Jr0uP@nimes>
 <08b5c6fd3b08b87fa564bb562d89381dd4e05b6a.camel@kernel.org>
From:   Paul Eggert <eggert@cs.ucla.edu>
Organization: UCLA Computer Science Department
Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps
In-Reply-To: <08b5c6fd3b08b87fa564bb562d89381dd4e05b6a.camel@kernel.org>
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On 2023-09-19 09:31, Jeff Layton wrote:
> The typical case for make
> timestamp comparisons is comparing source files vs. a build target. If
> those are being written nearly simultaneously, then that could be an
> issue, but is that a typical behavior?

I vaguely remember running into problems with 'make' a while ago 
(perhaps with a BSDish system) when filesystem timestamps were 
arbitrarily truncated in some cases but not others. These files would 
look older than they really were, so 'make' would think they were 
up-to-date when they weren't, and 'make' would omit actions that it 
should have done, thus screwing up the build.

File timestamps can be close together with 'make -j' on fast hosts. 
Sometimes a shell script (or 'make' itself) will run 'make', then modify 
a file F, then immediately run 'make' again; the latter 'make' won't 
work if F's timestamp is mistakenly older than targets that depend on it.

Although 'make'-like apps are the biggest canaries in this coal mine, 
the issue also affects 'find -newer' (as Bruno mentioned), 'rsync -u', 
'mv -u', 'tar -u', Emacs file-newer-than-file-p, and surely many other 
places. For example, any app that creates a timestamp file, then backs 
up all files newer than that file, would be at risk.


> I wonder if it would be feasible to just advance the coarse-grained
> current_time whenever we end up updating a ctime with a fine-grained
> timestamp?

Wouldn't this need to be done globally, that is, not just on a per-file 
or per-filesystem basis? If so, I don't see how we'd avoid locking 
performance issues.


PS. Although I'm no expert in the Linux inode code I hope you don't mind 
my asking a question about this part of inode_set_ctime_current:

	/*
	 * If we've recently updated with a fine-grained timestamp,
	 * then the coarse-grained one may still be earlier than the
	 * existing ctime. Just keep the existing value if so.
	 */
	ctime.tv_sec = inode->__i_ctime.tv_sec;
	if (timespec64_compare(&ctime, &now) > 0)
		return ctime;

Suppose root used clock_settime to set the clock backwards. Won't this 
code incorrectly refuse to update the file's timestamp afterwards? That 
is, shouldn't the last line be "goto fine_grained;" rather than "return 
ctime;", with the comment changed from "keep the existing value" to "use 
a fine-grained value"?