Re: [PATCH] powerpc/syscall: Avoid atomic test_and_clear_thread_flag on fast path

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 9/29/26 12:47 PM, Mukesh Kumar Chaurasiya (IBM) wrote:
> In system_call_exception(), test_and_clear_thread_flag(TIF_SYSCALL_RET)
> performs an atomic bit test-and-clear operation on every system call entry.
> This introduces an unnecessary performance regression on the syscall fast
> path.
> 

Do you have numbers?

> Combine the TIF_SYSCALL_RET check with the unlikely error condition from
> syscall_enter_from_user_mode_randomize_stack() using non-atomic
> test_thread_flag(). If the flag is set or entry failed, only then clear the
> flag via clear_thread_flag() and return the error value.
> 

It probably needs a fixes tag.

> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> ---
>   arch/powerpc/kernel/syscall.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
> index fbefe1927b10..8529827d697f 100644
> --- a/arch/powerpc/kernel/syscall.c
> +++ b/arch/powerpc/kernel/syscall.c
> @@ -18,14 +18,12 @@ notrace long system_call_exception(struct pt_regs *regs, unsigned long r0)
>   	long ret;
>   	syscall_fn f;
>   
> -	if (unlikely(!syscall_enter_from_user_mode_randomize_stack(regs, &r0))) {
> +	if (unlikely(!syscall_enter_from_user_mode_randomize_stack(regs, &r0) ||
> +			test_thread_flag(TIF_SYSCALL_RET))) {
>   		clear_thread_flag(TIF_SYSCALL_RET);
>   		return syscall_get_error(current, regs);
>   	}
>   
> -	if (unlikely(test_and_clear_thread_flag(TIF_SYSCALL_RET)))

I am bit surprised that test_and_clear_bit does
atomic operations even when bit is not set.

> -		return syscall_get_error(current, regs);
> -
>   	if (unlikely(r0 >= NR_syscalls)) {
>   		if (unlikely(trap_is_unsupported_scv(regs))) {
>   			/* Unsupported scv vector */