Re: [PATCH 2/2] Make write(2) interruptible by a signal

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 23 Nov 2011 07:27:43 -0500
Theodore Tso <tytso@mit.edu> wrote:

> >> 
> >> Maybe this is not that big problem as SIGKILL is considered be to
> >> destructive already.
> > 
> > Yeah, I have dim dark memories that there are subtle problems with
> > interrupting write().  Linus might remember.

(err, you're sending 600-column emails)

> The big one is that you're lucky if application programmers check the
> return values of write(2), and if they do, it's likely they will only
> check for error returns and not necessarily for partial writes ---
> since no other Unix-like or Linux-like system has ever returned partial
> reads or writes for files except in error conditions.  We've barely
> gotten them trained to check for partial writes and reads with TCP
> connections and character devices, but I wouldn't bet on application
> programmers getting things right for files.
> 
> Still, if it's ***only*** for SIGKILL, we'll probably be OK, since
> for that one case there's no chance userspace can intercept the signal,
> so it can't do any recovery anyway.  (I could imagine some HPC program
> doing a massive 2GB write, and some user of that program depending on
> the fact that he can kill it at a predefined place by sending a SIGKILL
> and knowing that the file would be written up to that 2GB chunk --- but
> that's clearly an edge situation, as opposed to something that would
> effect most GNOME and KDE apps.) We just need to make sure we never try
> to do this for any other signal that could be caught, such as SIGINT or
> SIGQUIT or (worse yet) SIGTSTP.

That it is a fatal SIGKILL means that the *current* application doesn't
care.  But other processes will sometimes notice this change. 
Previously if an app did write(file, 128k) and was hit with SIGKILL, it
would write either 0 bytes or 128k bytes.  Now, it can write 36k bytes,
yes?  If the target file consisted of a stream of 128k records then the
user will claim, with some justification, that Linux corrupted it.

Dunno.  People do lots of weird and flakey things.  I have a suspicion
that we'll be hearing back from them about this change.