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

On 02/11/2015 11:08 PM, Oleg Nesterov wrote:
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)

so it can't overflow to .tail?

You are right.
I totally forgot we can get rid of tail operations :)

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;

Very nice idea. I was tired of ~TICKET_SLOWPATH_FLAG usage all over in
the current (complex :)) implementation. These two suggestions helps

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

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