Messages in this thread Patch in this message | | | Date | Tue, 5 May 2015 16:30:07 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] x86/spinlocks: Fix regression in spinlock contention detection |
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On Tue, May 05, 2015 at 07:10:19AM -0700, Tahsin Erdogan wrote: > The conversion to signed happens with types shorter than int > (__ticket_t is either u8 or u16).
Argh yes, I see.
(ISO C99) 6.3.1.1 rule 2 states that: "If an int can represent all values of the original type, the value is converted to an int;... "
Originally I had only looked at: 6.3.1.8 Usual arithmetic conversions, which has the following bit: "If both operands have the same type, then no further conversion is needed."
Looking more closely, a sentence prior to that states: "Otherwise, the integer promotions are performed on both operands." which brings us back to that 6.3.1.1 rule 2.
I've slightly modified your Changelog to explicitly state this.
--- Subject: x86/spinlocks: Fix regression in spinlock contention detection From: Tahsin Erdogan <tahsin@google.com> Date: Mon, 4 May 2015 21:15:31 -0700
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.
This is because the normal C integer promotion rules will promote __ticket_t to int because its of lower rank (u8,u16 < int).
The original code (before regression) handled this case by casting the (tail - head) to an unsigned value. This change simply restores that behavior.
Cc: hpa@zytor.com Cc: x86@kernel.org Cc: Waiman.Long@hp.com Cc: borntraeger@de.ibm.com Cc: oleg@redhat.com Cc: raghavendra.kt@linux.vnet.ibm.com Cc: tglx@linutronix.de Cc: mingo@redhat.com Fixes: d6abfdb20223 ("x86/spinlocks/paravirt: Fix memory corruption on unlock") Signed-off-by: Tahsin Erdogan <tahsin@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: http://lkml.kernel.org/r/1430799331-20445-1-git-send-email-tahsin@google.com --- arch/x86/include/asm/spinlock.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/x86/include/asm/spinlock.h +++ b/arch/x86/include/asm/spinlock.h @@ -173,7 +173,7 @@ static inline int arch_spin_is_contended 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; } #define arch_spin_is_contended arch_spin_is_contended
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