On 11/03/2010 11:11 AM, Eric Dumazet wrote:
> Le mercredi 03 novembre 2010 Ã 10:59 -0400, Jeremy Fitzhardinge a
> Ãcrit :
>> From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
>>
>> The inner loop of __ticket_spin_lock isn't doing anything very special,
>> so reimplement it in C.
>>
>> For the 8 bit ticket lock variant, we use a register union to get direct
>> access to the lower and upper bytes in the tickets, but unfortunately gcc
>> won't generate a direct comparison between the two halves of the register,
>> so the generated asm isn't quite as pretty as the hand-coded version.
>> However benchmarking shows that this is actually a small improvement in
>> runtime performance on some benchmarks, and never a slowdown.
>>
>> We also need to make sure there's a barrier at the end of the lock loop
>> to make sure that the compiler doesn't move any instructions from within
>> the locked region into the region where we don't yet own the lock.
>>
>> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
>> ---
>> arch/x86/include/asm/spinlock.h | 58 +++++++++++++++++++-------------------
>> 1 files changed, 29 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
>> index d6d5784..6711d36 100644
>> --- a/arch/x86/include/asm/spinlock.h
>> +++ b/arch/x86/include/asm/spinlock.h
>> @@ -58,21 +58,21 @@
>> #if (NR_CPUS < 256)
>> static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>> {
>> - unsigned short inc = 1 << TICKET_SHIFT;
>> -
>> - asm volatile (
>> - LOCK_PREFIX "xaddw %w0, %1\n"
>> - "1:\t"
>> - "cmpb %h0, %b0\n\t"
>> - "je 2f\n\t"
>> - "rep ; nop\n\t"
>> - "movb %1, %b0\n\t"
>> - /* don't need lfence here, because loads are in-order */
>> - "jmp 1b\n"
>> - "2:"
>> - : "+Q" (inc), "+m" (lock->slock)
>> - :
>> - : "memory", "cc");
>> + register union {
>> + struct __raw_tickets tickets;
>> + unsigned short slock;
>> + } inc = { .slock = 1 << TICKET_SHIFT };
>> +
>> + asm volatile (LOCK_PREFIX "xaddw %w0, %1\n"
>> + : "+Q" (inc), "+m" (lock->slock) : : "memory", "cc");
>> +
>> + for (;;) {
>> + if (inc.tickets.head == inc.tickets.tail)
>> + return;
>> + cpu_relax();
>> + inc.tickets.head = ACCESS_ONCE(lock->tickets.head);
>> + }
>> + barrier(); /* make sure nothing creeps before the lock is taken */
> Isnt this barrier() never reached ?
Sorry, a later patch makes this clearer. I should have folded it in.
J
_______________________________________________
Virtualization mailing list
Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
https://lists.linux-foundation.org/mailman/listinfo/virtualization
