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    SubjectRe: Clarification required about select vs wake_up race condition
    On 12/03/07, Ravinandan Arakali (rarakali) <rarakali@cisco.com> wrote:
    > Hi,
    > I am facing following problem and was wondering if somebody could help
    > me out.
    > Our char driver(pretty much like all other char drivers) does a
    > poll_wait()
    > and returns status depending on whether data is available to be read.
    > Even though some data is available to be read(verified using one of our
    > internal
    > commands), the select() never wakes up, inspite of any no. of messages
    > sent.
    >
    > To understand this, I was looking at the code of select vs
    > wake_up_interruptible().
    > I feel I am misunderstanding some part of the kernel code but will be
    > glad if
    > somebody can point it out.
    >
    > My understanding:
    > The do_select() sets the state of task to TASK_INTERRUPTIBLE and calls
    > the driver's
    > poll entry point. In our poll(), let's say immediately after we
    > determine that there's
    > nothing to be read, some data arrives causing a wake_up_interruptible()
    > on another CPU.
    > The wake up happens in the context of process sending the data. Since
    > the receiving
    > process was already added to the list of listeners, from looking at the
    > code of
    > try_to_wake_up(), it looks like it can set the state of the receiving
    > process to
    > TASK_RUNNING(I don't see any lock preventing this). After this happens,
    > the receiving
    > process goes to sleep (because of schedule_timeout called by do_select)
    > but
    > state is still set to TASK_RUNNING.

    No, it's not going to sleep then.

    The effect of schedule() being called with current->state ==
    TASK_RUNNING is a re-scheduling to another task with a higher prio (if
    any) or just getting back (iow, the task doesn't lose a cpu). For both
    cases, the task is on the runqueue.

    / Look how/when deactivate_task() is called in schedule() /

    Maybe there is a race in your code between (1) how you check "data is
    available" in poll and (2) a part that sets this fact (data is
    available)...


    --
    Best regards,
    Dmitry Adamushko
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