Re: [PATCH] powerpc/powernv: Fix concurrency issue with npu->mmio_atsd_usage

[Alistair Popple <alistair@popple.id.au>]

> There may be a long-term way to fix this at a larger scale, but for now
> resolve the immediate problem by gating our call to
> test_and_set_bit_lock() with one to test_bit(), which is obviously
> implemented without using a store.

I am less sure of this now but am continuing to investigate. However this patch
looks good.

Acked-by: Alistair Popple <alistair@popple.id.au>

> Signed-off-by: Reza Arbab <arbab@linux.ibm.com>
> ---
>  arch/powerpc/platforms/powernv/npu-dma.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/npu-dma.c b/arch/powerpc/platforms/powernv/npu-dma.c
> index 8cdf91f..c773465 100644
> --- a/arch/powerpc/platforms/powernv/npu-dma.c
> +++ b/arch/powerpc/platforms/powernv/npu-dma.c
> @@ -437,8 +437,9 @@ static int get_mmio_atsd_reg(struct npu *npu)
>  	int i;
>  
>  	for (i = 0; i < npu->mmio_atsd_count; i++) {
> -		if (!test_and_set_bit_lock(i, &npu->mmio_atsd_usage))
> -			return i;
> +		if (!test_bit(i, &npu->mmio_atsd_usage))
> +			if (!test_and_set_bit_lock(i, &npu->mmio_atsd_usage))
> +				return i;
>  	}
>  
>  	return -ENOSPC;
>