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SubjectRe: [PATCH 11/31] nds32: Atomic operations
Hi Greentime,

On Wed, Nov 08, 2017 at 01:54:59PM +0800, Greentime Hu wrote:
> From: Greentime Hu <greentime@andestech.com>
>
> Signed-off-by: Vincent Chen <vincentc@andestech.com>
> Signed-off-by: Greentime Hu <greentime@andestech.com>
> ---
> arch/nds32/include/asm/futex.h | 116 ++++++++++++++++++++++++
> arch/nds32/include/asm/spinlock.h | 178 +++++++++++++++++++++++++++++++++++++
> 2 files changed, 294 insertions(+)
> create mode 100644 arch/nds32/include/asm/futex.h
> create mode 100644 arch/nds32/include/asm/spinlock.h

[...]

> +static inline int
> +futex_atomic_cmpxchg_inatomic(u32 * uval, u32 __user * uaddr,
> + u32 oldval, u32 newval)
> +{
> + int ret = 0;
> + u32 val, tmp, flags;
> +
> + if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
> + return -EFAULT;
> +
> + smp_mb();
> + asm volatile (" movi $ta, #0\n"
> + "1: llw %1, [%6 + $ta]\n"
> + " sub %3, %1, %4\n"
> + " cmovz %2, %5, %3\n"
> + " cmovn %2, %1, %3\n"
> + "2: scw %2, [%6 + $ta]\n"
> + " beqz %2, 1b\n"
> + "3:\n " __futex_atomic_ex_table("%7")
> + :"+&r"(ret), "=&r"(val), "=&r"(tmp), "=&r"(flags)
> + :"r"(oldval), "r"(newval), "r"(uaddr), "i"(-EFAULT)
> + :"$ta", "memory");
> + smp_mb();
> +
> + *uval = val;
> + return ret;
> +}

I see you rely on asm-generic/barrier.h for your barrier definitions, which
suggests that you only need to prevent reordering by the compiler because
you're not SMP. Is that right? If so, using smp_mb() is a little weird.

What about DMA transactions? I imagine you might need some extra
instructions for the mandatory barriers there.

Also:

> +static inline void arch_spin_lock(arch_spinlock_t * lock)
> +{
> + unsigned long tmp;
> +
> + __asm__ __volatile__("1:\n"
> + "\tllw\t%0, [%1]\n"
> + "\tbnez\t%0, 1b\n"
> + "\tmovi\t%0, #0x1\n"
> + "\tscw\t%0, [%1]\n"
> + "\tbeqz\t%0, 1b\n"
> + :"=&r"(tmp)
> + :"r"(&lock->lock)
> + :"memory");
> +}

Here it looks like you're eliding an explicit barrier here because you
already have a "memory" clobber. Can't you do the same for the futex code
above?

Will

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