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    SubjectRe: [PATCH -mm] swsusp: freeze user space processes first
    Date
    On Sunday 05 February 2006 12:18, Pavel Machek wrote:
    > On Ne 05-02-06 12:11:06, Rafael J. Wysocki wrote:
    > > On Sunday 05 February 2006 11:50, Ingo Molnar wrote:
    > > >
    > > > * Rafael J. Wysocki <rjw@sisk.pl> wrote:
    > > >
    > > > > > The logic in that loop makes my brain burst.
    > > > > >
    > > > > > What happens if a process does vfork();sleep(100000000)?
    > > > >
    > > > > The freezing of processes will fail due to the timeout.
    > > > >
    > > > > Without the if (!p->vfork_done) it would fail too, because the child
    > > > > would be frozen and the parent would wait for the vfork completion in
    > > > > the TASK_UNINTERRUPTIBLE state (ie. unfreezeable). But in that case
    > > > > we have a race between the "freezer" and the child process (ie. if the
    > > > > child gets frozen before it completes the vfork completion, the paret
    > > > > will be unfreezeable) which sometimes leads to a failure when it
    > > > > should not. [We have a test case showing this.]
    > > >
    > > > then i'd suggest to change the vfork implementation to make this code
    > > > freezable.
    > >
    > > I think you are right, but I don't know how to do this.
    > >
    > > > Nothing that userspace does should cause freezing to fail. If it does,
    > > > we've designed things incorrectly on the kernel side.
    > >
    > > I tend to agree.
    > >
    > > Generally, the problem is due to the use of completions where userland
    > > processes are waited for. The two places I know of are the vfork
    > > implementation and the usermode helper code.
    >
    > Can you produce userland testcase? If we have uninterruptible process for
    > days... that's a bug in kernel, suspend or not.

    Sure, no problem. [Pretty scary, no?]

    The test code:

    #include <sys/types.h>
    #include <unistd.h>

    int main(int argc, char *argv[])
    {
    vfork();
    sleep(300);

    return 0;
    }

    The result:

    rafael@albercik:~/programming/c> ./vfork_test &
    [1] 12288
    rafael@albercik:~/programming/c> ps l
    F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND
    0 500 6937 6931 17 0 10048 2368 read_c Ss+ pts/1 0:00 /bin/bash
    0 500 12139 12133 15 0 10052 2380 wait Ss pts/2 0:00 /bin/bash
    0 500 12288 12139 15 0 2420 304 fork D pts/2 0:00 ./vfork_tes
    1 500 12291 12288 18 0 2420 304 hrtime S pts/2 0:00 ./vfork_tes
    0 500 12372 12139 15 0 3596 820 - R+ pts/2 0:00 ps l
    rafael@albercik:~/programming/c> kill 12288
    rafael@albercik:~/programming/c> ps l
    F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND
    0 500 6937 6931 17 0 10048 2368 read_c Ss+ pts/1 0:00 /bin/bash
    0 500 12139 12133 15 0 10052 2380 wait Ss pts/2 0:00 /bin/bash
    0 500 12288 12139 15 0 2420 304 fork D pts/2 0:00 ./vfork_tes
    1 500 12291 12288 18 0 2420 304 hrtime S pts/2 0:00 ./vfork_tes
    0 500 12380 12139 17 0 3600 820 - R+ pts/2 0:00 ps l
    rafael@albercik:~/programming/c> kill -9 12288
    rafael@albercik:~/programming/c> ps l
    F UID PID PPID PRI NI VSZ RSS WCHAN STAT TTY TIME COMMAND
    0 500 6937 6931 17 0 10048 2368 read_c Ss+ pts/1 0:00 /bin/bash
    0 500 12139 12133 15 0 10052 2380 wait Ss pts/2 0:00 /bin/bash
    0 500 12288 12139 15 0 2420 304 fork D pts/2 0:00 ./vfork_tes
    1 500 12291 12288 18 0 2420 304 hrtime S pts/2 0:00 ./vfork_tes
    0 500 12387 12139 16 0 3596 816 - R+ pts/2 0:00 ps l
    rafael@albercik:~/programming/c>

    Greetings,
    Rafael
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