Re: [Xen-devel] [PATCH] x86 spinlock: Fix memory corruption on completing completions

[Oleg Nesterov <oleg@xxxxxxxxxx>]

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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