Re: [PATCH v4 3/4] arm64: alternatives: Remove READ_ONCE() usage during patch operation

[Mark Rutland]

On Tue, Nov 03, 2020 at 12:17:20PM +0000, Will Deacon wrote:
> In preparation for patching the internals of READ_ONCE() itself, replace
> its usage on the alternatives patching patch with a volatile variable
> instead.
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
>  arch/arm64/kernel/alternative.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index 73039949b5ce..a57cffb752e8 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -21,7 +21,8 @@
>  #define ALT_ORIG_PTR(a)		__ALT_PTR(a, orig_offset)
>  #define ALT_REPL_PTR(a)		__ALT_PTR(a, alt_offset)
>  
> -static int all_alternatives_applied;
> +/* Volatile, as we may be patching the guts of READ_ONCE() */
> +static volatile int all_alternatives_applied;
>  
>  static DECLARE_BITMAP(applied_alternatives, ARM64_NCAPS);
>  
> @@ -205,7 +206,7 @@ static int __apply_alternatives_multi_stop(void *unused)
>  
>  	/* We always have a CPU 0 at this point (__init) */
>  	if (smp_processor_id()) {
> -		while (!READ_ONCE(all_alternatives_applied))
> +		while (!all_alternatives_applied)
>  			cpu_relax();
>  		isb();
>  	} else {
> @@ -217,7 +218,7 @@ static int __apply_alternatives_multi_stop(void *unused)
>  		BUG_ON(all_alternatives_applied);
>  		__apply_alternatives(&region, false, remaining_capabilities);
>  		/* Barriers provided by the cache flushing */
> -		WRITE_ONCE(all_alternatives_applied, 1);
> +		all_alternatives_applied = 1;
>  	}
>  
>  	return 0;
> -- 
> 2.29.1.341.ge80a0c044ae-goog
>