Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention detection

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 04, 2015 at 09:15:31PM -0700, Tahsin Erdogan wrote:
> A spinlock is regarded as contended when there is at least one waiter.
> Currently, the code that checks whether there are any waiters rely on
> tail value being greater than head. However, this is not true if tail
> reaches the max value and wraps back to zero, so arch_spin_is_contended()
> incorrectly returns 0 (not contended) when tail is smaller than head.
> 
> The original code (before regression) handled this case by casting the
> (tail - head) to an unsigned value. This change simply restores that
> behavior.
> 
> Fixes: d6abfdb20223 ("x86/spinlocks/paravirt: Fix memory corruption on
> unlock")
> Signed-off-by: Tahsin Erdogan <tahsin@google.com>
> ---
>  arch/x86/include/asm/spinlock.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
> index cf87de3..64b6117 100644
> --- a/arch/x86/include/asm/spinlock.h
> +++ b/arch/x86/include/asm/spinlock.h
> @@ -169,7 +169,7 @@ static inline int arch_spin_is_contended(arch_spinlock_t *lock)
>  	struct __raw_tickets tmp = READ_ONCE(lock->tickets);
>  
>  	tmp.head &= ~TICKET_SLOWPATH_FLAG;
> -	return (tmp.tail - tmp.head) > TICKET_LOCK_INC;
> +	return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;

I'm not seeing it, everything in that expression is of __ticket_t type
(tail, head and TICKET_LOCK_INC), nothing should cause it to be cast to
another type due to conversion rules.

Or does - always cast to a signed type? Lemme go grab the C rules again.

I'm not seeing it.. Please explain better, iow. your changelog fails to
properly explain the problem.