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    SubjectRe: [PATCH] sched: remove false-positive warning from wake_up_process()
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    On 11/30/2015 08:47 PM, Linus Torvalds wrote:
    > On Mon, Nov 30, 2015 at 5:34 PM, Sasha Levin <sasha.levin@oracle.com> wrote:
    >> Futex can have a spurious wake up before we actually wake it up on our own,
    >> which will trigger this warning if the task is still stopped.
    >
    > Actually, I think it would presumably be the other way around: a
    > spurious stale futex wakeup happens *after* the process has been woken
    > up for some other reason and moved to stopped state.
    >
    > (The "wake up and move to stopped state" could be for the same reason:
    > a SIGSTOP signal).
    >
    > So the setup is presumably something like this:
    >
    > - on cpu1: futex code is about to go to sleep, adds itself to the
    > futex hash chains, but then gets interrupted by a SIGSTOP
    >
    > - in the meantime, on cpu2, the futex is changed, and the wakup code
    > sees the process from cpu1 on the futex hash chains
    >
    > - on cpu1, the process has now removed itself from the hash chains,
    > and goes through the signal code that sets the state to STOPPED
    >
    > - in the meantime, on cpu2, the futex code now gets around to waking
    > things up, and sees that stopped state
    >
    > Roughly.

    What would the correct behaviour in that case be?

    Does waking up the task while it is being traced, and ptrace
    (or gdb) is not expecting a wakeup, break the tracing?

    --
    All rights reversed


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