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    SubjectRe: [PATCH 6/7] microblaze: Implement architecture spinlock
    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.

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