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Tue, 29 Sep 2026 09:50:02 +0000 (GMT) Received: from [9.124.214.246] (unknown [9.124.214.246]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Tue, 29 Sep 2026 09:50:02 +0000 (GMT) Message-ID: <84ceaa9a-79fb-4585-a571-7701750df5d1@linux.ibm.com> Date: Tue, 29 Sep 2026 15:20:01 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] powerpc/syscall: Avoid atomic test_and_clear_thread_flag on fast path To: "Mukesh Kumar Chaurasiya (IBM)" References: <20260929071744.2965058-1-mkchauras@gmail.com> Content-Language: en-US From: Shrikanth Hegde Cc: maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com, chleroy@kernel.org, ritesh.list@gmail.com, ruanjinjie@huawei.com, tglx@kernel.org, mkchauras@linux.ibm.com, ryan.roberts@arm.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org In-Reply-To: <20260929071744.2965058-1-mkchauras@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-ORIG-GUID: ApzzVTlDQYCq2Y4V7Qkg3bkhsNJCRGJA X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwOTI5MDAzOCBTYWx0ZWRfX8FAb0KGCzep8 /eN1AM6bc65HWTWqsS2ucTj7hxu/C2l60Im4LSKrUi6bwQh/ublUCp1iLWm5Yn6PlAIN/JSjuwB xa0VHUmaEhxYKu9xvKKaT73PL8xDcmvDPm3DXGvPvZqn6hNNRzHgARpTCfars9FF8NhVchOOwdR v1U6ViBKfnLftZnr/oILHBDjcj6Bx+JDlRYLzNk55TAYvqEMgJNkzM7p6NC+GCNPepFQM4KVDh1 z0ZM0CE6mRM/u2w6HwT1azffjeroGHTSzwVn5xkWvmrdLopno7qcz5qJ/xTJNpH4+PqsazrQm+H kpJfYh2D9KubeMDtdC/XDSRhtPOoaD60k6ZxYbgnFK75V7yUnXNWsFX+MC18M3DdDfha08nba1a oJVscSeYMsMieCm3Q08dMkFFABREciSr4d/UZ+2AEQ24OnIvtlyFvE8F1MwHuwuheaNcH0VONJW XxuY9gitZno0xVSO50w== X-Proofpoint-Spam-Info: AW1haW4tMjYwOTI5MDAzOCBTYWx0ZWRfX+05Um9/R7ZxZ w9opcbTGycOOVIwHfpG7msuyZMI9kWxssNa74ExOK/EDfEGKBIXSQQvV/tSxH8HJwJpuSOPsHXb zM437IKf51xSsr4BECmUpL7F6KRpuXo= X-Authority-Analysis: v=2.4 cv=EY5d0/mC c=1 sm=1 tr=0 ts=6abb89d2 cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=IkcTkHD0fZMA:10 a=VdqzKS8jKosA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=RzCfie-kr_QcCd8fBx8p:22 a=pGLkceISAAAA:8 a=qzsYSUR3fVkzKN2XlXMA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: rkyr3gYrpHDPGkwfuuWFew15Jg18iSqY X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-09-29_03,2026-09-21_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 priorityscore=1501 suspectscore=0 adultscore=0 clxscore=1011 malwarescore=0 impostorscore=0 spamscore=0 lowpriorityscore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2609040000 definitions=main-2609290038 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) > --- > 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 */