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

[Mukesh Kumar Chaurasiya (IBM)]

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) <mkchauras@xxxxxxxxx>
---
 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