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

[Oleg Nesterov <oleg@xxxxxxxxxx>]

On 02/11, Raghavendra K T wrote:
>
> On 02/10/2015 06:56 PM, Oleg Nesterov wrote:
>
>> In this case __ticket_check_and_clear_slowpath() really needs to cmpxchg
>> the whole .head_tail. Plus obviously more boring changes. This needs a
>> separate patch even _if_ this can work.
>
> Correct, but apart from this, before doing xadd in unlock,
> we would have to make sure lsb bit is cleared so that we can live with 1
> bit overflow to tail which is unused. now either or both of head,tail
> lsb bit may be set after unlock.

Sorry, can't understand... could you spell?

If TICKET_SLOWPATH_FLAG lives in .head arch_spin_unlock() could simply do

        head = xadd(&lock->tickets.head, TICKET_LOCK_INC);

        if (head & TICKET_SLOWPATH_FLAG)
                __ticket_unlock_kick(head);

so it can't overflow to .tail?

But probably I missed your concern.



And we we do this, probably it makes sense to add something like

        bool tickets_equal(__ticket_t one, __ticket_t two)
        {
                return (one ^ two) & ~TICKET_SLOWPATH_FLAG;
        }

and change kvm_lock_spinning() to use tickets_equal(tickets.head, want), plus
it can have more users in asm/spinlock.h.

Oleg.


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