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    SubjectRe: [PATCH v3] wait: prevent waiter starvation in __wait_on_bit_lock
    On 01/20, Johannes Weiner wrote:
    >
    > > But, more importantly, I'm afraid we can also have the false negative,
    > > this "if (!test_bit())" test lacks the barriers. This can't happen with
    > > sync_page_killable() because it always calls schedule(). But let's
    > > suppose we modify it to check signal_pending() first:
    > >
    > > static int sync_page_killable(void *word)
    > > {
    > > if (fatal_signal_pending(current))
    > > return -EINTR;
    > > return sync_page(word);
    > > }
    > >
    > > It is still correct, but unless I missed something now __wait_on_bit_lock()
    > > has problems again.
    >
    > Hm, this would require the lock bit to be set without someone else
    > doing the wakeup. How could this happen?
    >
    > I could think of wake_up_page() happening BEFORE clear_bit_unlock()
    > and we have to be on the front of the waitqueue. Then we are already
    > running, the wake up is a nop, the !test_bit() is false and noone
    > wakes up the next real contender.
    >
    > But the wake up side uses a smp barrier after clearing the bit, so if
    > the bit is not cleared we can expect a wake up, no?

    Yes we have the barriers on the "wakeup", but this doesn't mean the
    woken task must see the result of clear_bit() (unless it was really
    unscheduled of course).

    > Or do we still need a read-side barrier before the test bit?

    Even this can't help afaics.

    Because the the whole clear_bit + wakeup sequence can happen after
    the "if (!test_bit()) check and before finish_wait(). Please note
    that from the waker's pov we are sleeping in TASK_KILLABLE state,
    it will wake up us if we are at the front of the waitqueue.

    (to clarify, I am talking about the imaginary sync_page_killable()
    above).

    Oleg.



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