[Xen-devel] Re: [PATCH 02/20] x86/ticketlock: convert spin loop to C

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 11/03/2010 11:11 AM, Eric Dumazet wrote:
> Le mercredi 03 novembre 2010 à 10:59 -0400, Jeremy Fitzhardinge a
> écrit :
>> 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

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