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    SubjectRe: [PATCH] RFC: futex: make futex_lock_pi interruptible
    Arnd Bergmann wrote:
    > On Friday 30 October 2009, Darren Hart wrote:
    >> Darren Hart wrote:
    >> This appears to work fine. Can anyone think of a reason why this is an unsafe
    >> thing to do? I'll have to create a much more elaborate test case and review
    >> the glibc code of course to make sure the glibc mutex state isn't compromised.
    > The only reason I can see against it is the need to use one of the
    > rt signal numbers from library code, which may conflict with other
    > users of the signal. Being able to avoid a signal altogether would
    > be really nice, as in the futex_cancel extension you mentioned.

    For the reason you mention, consumption of a signal number, the
    futex_cancel extension was how I originally set out to tackle this.
    However, Thomas and Peter both seemed to feel that the signal approach
    was a more standard way of interrupting a unix system call. One trick to
    the futex_cancel approach will be identifying which thread to cancel -
    since no other futex operation is thread specific. I suspect just
    overloading one of the argument to pass a TID would address that nicely.
    This would allow us to return ECANCELED from the kernel, which I think
    is a much more direct implementation.

    Peter and Thomas, could you comment on why the signal approach might be
    preferred over the futex_cancel extension?

    >> /* Need some kind of per-thread variable here */
    >> jmp_buf env;
    >> pthread_mutex_t mutex;
    > Maybe instead of per-thread variables (which should work
    > fine), you could do
    > typedef struct {
    > jmp_buf env;
    > pthread_mutex_t mutex;
    > } interruptible_mutex_t;

    I don't quite follow. There will be a 1:many relationship between
    mutex:threads, but there should be a 1:1 relationship between
    threads:env. Since multiple threads can block on one mutex, the above
    struct wouldn't provide the necessary number of env to set the jmp point
    for each one.... am I misunderstanding your suggestion?

    >> /* ensure the child has blocked on the lock */
    >> sleep(1);
    > In a real application, you might want to add some logic to avoid
    > this kind of race. For the test case, you probably need to do it
    > with the sleep.

    This would likely need to be handled within glibc, just as it manages
    the sequence counters for the condvars to deal with wake-up races.

    Darren Hart
    IBM Linux Technology Center
    Real-Time Linux Team

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      Last update: 2009-10-30 17:25    [W:0.026 / U:17.480 seconds]
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