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SubjectRe: another subtle signals issue

On Tue, 11 Feb 2003, Roland McGrath wrote:
> I think sys_semop would be closer to right if it used ERESTARTSYS instead
> of EINTR.

You probably mean ERESTARTSYSNOHAND.

There are lots of system calls that simply are not restartable. So
TIF_SIGPENDING in general should be set only if required, and not "because
it's easier".

> The reason I am concerned about this is that I think any case that is
> broken by the lack of the optimization in the patch below must also be
> broken vis a vis the semantics of stop signals and SIGCONT (when SIG_DFL,
> SIG_IGN, or blocked). POSIX says that when a process is stopped by
> e.g. SIGSTOP, and then continued by SIGCONT, any functions that were in
> progress at the time of stop are unaffected unless SIGCONT runs a handler.
> That is, nobody returns EINTR because of the stop/continue.

This is what ERESTARTNOHAND does, but quite often if you get interrupted
you have to return _partial_ results, which is quite inefficient and
sometimes breaks programs (ie you get things like a read() from a pipe
that returns a partial result because you resized the window, and a
SIGWINCH happened - and that is _bad_).

The old code tried rather hard to make signals that were truly ignored
(SIGSTOP/SIGCONT is not of that kind) be total non-events because of
things like this.


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