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

[Shrikanth Hegde]

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) <mkchau...@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 */