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

[Oleg Nesterov <oleg@xxxxxxxxxx>]

On 02/06, Sasha Levin wrote:
>
> Can we modify it slightly to avoid potentially accessing invalid memory:
> 
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index 5315887..cd22d73 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -144,13 +144,13 @@ static __always_inline void 
> arch_spin_unlock(arch_spinlock_t *lock
>         if (TICKET_SLOWPATH_FLAG &&
>                 static_key_false(&paravirt_ticketlocks_enabled)) {
>                 __ticket_t prev_head;
> -
> +               bool needs_kick = lock->tickets.tail & TICKET_SLOWPATH_FLAG;
>                 prev_head = lock->tickets.head;
>                 add_smp(&lock->tickets.head, TICKET_LOCK_INC);
> 
>                 /* add_smp() is a full mb() */
> 
> -               if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) {
> +               if (unlikely(needs_kick)) {

This doesn't look right too...

We need to guarantee that either unlock() sees TICKET_SLOWPATH_FLAG, or
the calller of __ticket_enter_slowpath() sees the result of add_smp().

Suppose that kvm_lock_spinning() is called right before add_smp() and it
sets SLOWPATH. It will block then because .head != want, and it needs
__ticket_unlock_kick().

Oleg.


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