On 02/10, Denys Vlasenko wrote:
>
> # define HEAD_MASK (TICKET_SLOWPATH_FLAG-1)
>
> ...
> unlock_again:
>
> val = xadd((&lock->ticket.head_tail, TICKET_LOCK_INC);
> if (unlikely(!(val & HEAD_MASK))) {
> /* overflow. we inadvertently incremented the tail word.
> * tail's lsb is TICKET_SLOWPATH_FLAG.
> * Increment inverted this bit, fix it up.
> * (inc _may_ have messed up tail counter too,
> * will deal with it after kick.)
> */
> val ^= TICKET_SLOWPATH_FLAG;
> }
>
> if (unlikely(val & TICKET_SLOWPATH_FLAG)) {
> ...kick the waiting task...
>
> val -= TICKET_SLOWPATH_FLAG;
> if (unlikely(!(val & HEAD_MASK))) {
> /* overflow. we inadvertently incremented tail word, *and*
> * TICKET_SLOWPATH_FLAG was set, increment overflowed
> * that bit too and incremented tail counter.
> * This means we (inadvertently) taking the lock again!
> * Oh well. Take it, and unlock it again...
> */
> while (1) {
> if (READ_ONCE(lock->tickets.head) != TICKET_TAIL(val))
> cpu_relax();
> }
> goto unlock_again;
> }
>
>
> Granted, this looks ugly.
complicated ;)
But "Take it, and unlock it again" simply can't work, this can deadlock.
Note that unlock() can be called after successful try_lock(). And other
problems with lock-ordering, like
lock(X);
lock(Y);
unlock(X);
Oleg.
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