Messages in this thread | | | Date | Tue, 5 May 2015 11:17:14 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention detection |
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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.
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