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

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.

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.

Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <redacted>
---
 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)))
-		return syscall_get_error(current, regs);
-
 	if (unlikely(r0 >= NR_syscalls)) {
 		if (unlikely(trap_is_unsupported_scv(regs))) {
 			/* Unsupported scv vector */

-- 
2.55.0

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.

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

Le 29/09/2026 à 11:50, Shrikanth Hegde a écrit :

On 9/29/26 12:47 PM, Mukesh Kumar Chaurasiya (IBM) wrote:

quoted

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?

quoted

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.

It is not a bug, unless we can figure out a quantified regression.

quoted

Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <redacted>
---
  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.

It does (on ppc64 smp)

	sync
  1:	ldarx
	andc
	stdcx.
	bne 1b
	sync

It is optimised to clear a bit that is set. When the bit is already unset you still get the full sequence allthough it does nothing.

quoted

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

On Tue, 29 Sep 2026 12:47:44 +0530, 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.

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.

[...]

Applied to powerpc/next.

[1/1] powerpc/syscall: Avoid atomic test_and_clear_thread_flag on fast path
      https://git.kernel.org/powerpc/c/66236558dfac63d9bea5b0b6a66a4d717529c118

cheers