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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.

    Linus

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