Re: [PATCH 6/7] microblaze: Implement architecture spinlock

Michal Simek
2020-02-13T07:51:52Z

On 12. 02. 20 16:47, Peter Zijlstra wrote:
> On Wed, Feb 12, 2020 at 04:42:28PM +0100, Michal Simek wrote:
>> From: Stefan Asserhall <stefan.asserhall@xilinx.com>
>>
>> Using exclusive loads/stores to implement spinlocks which can be used on
>> SMP systems.
>>
>> Signed-off-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
>> Signed-off-by: Michal Simek <michal.simek@xilinx.com>
>> ---
>>
>> arch/microblaze/include/asm/spinlock.h | 240 +++++++++++++++++++
>> arch/microblaze/include/asm/spinlock_types.h | 25 ++
>> 2 files changed, 265 insertions(+)
>> create mode 100644 arch/microblaze/include/asm/spinlock.h
>> create mode 100644 arch/microblaze/include/asm/spinlock_types.h
>>
>> diff --git a/arch/microblaze/include/asm/spinlock.h b/arch/microblaze/include/asm/spinlock.h
>> new file mode 100644
>> index 000000000000..0199ea9f7f0f
>> --- /dev/null
>> +++ b/arch/microblaze/include/asm/spinlock.h
>> @@ -0,0 +1,240 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * Copyright (C) 2013-2020 Xilinx, Inc.
>> + */
>> +
>> +#ifndef _ASM_MICROBLAZE_SPINLOCK_H
>> +#define _ASM_MICROBLAZE_SPINLOCK_H
>> +
>> +/*
>> + * Unlocked value: 0
>> + * Locked value: 1
>> + */
>> +#define arch_spin_is_locked(x) (READ_ONCE((x)->lock) != 0)
>> +
>> +static inline void arch_spin_lock(arch_spinlock_t *lock)
>> +{
>> + unsigned long tmp;
>> +
>> + __asm__ __volatile__ (
>> + /* load conditional address in %1 to %0 */
>> + "1: lwx %0, %1, r0;\n"
>> + /* not zero? try again */
>> + " bnei %0, 1b;\n"
>> + /* increment lock by 1 */
>> + " addi %0, r0, 1;\n"
>> + /* attempt store */
>> + " swx %0, %1, r0;\n"
>> + /* checking msr carry flag */
>> + " addic %0, r0, 0;\n"
>> + /* store failed (MSR[C] set)? try again */
>> + " bnei %0, 1b;\n"
>> + /* Outputs: temp variable for load result */
>> + : "=&r" (tmp)
>> + /* Inputs: lock address */
>> + : "r" (&lock->lock)
>> + : "cc", "memory"
>> + );
>> +}
>
> That's a test-and-set spinlock if I read it correctly. Why? that's the
> worst possible spinlock implementation possible.

This was written by Stefan and it is aligned with recommended
implementation. What other options do we have?

Thanks,
Michal