linux-kernel - [PATCH AUTOSEL 4.14 18/29] s390/preempt: disable __preempt_count_add() optimization for PROFILE_ALL

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Message-Id: <20220530135057.1937286-18-sashal@kernel.org>
Date:   Mon, 30 May 2022 09:50:45 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Heiko Carstens <hca@...ux.ibm.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Sasha Levin <sashal@...nel.org>, gor@...ux.ibm.com,
        agordeev@...ux.ibm.com, vschneid@...hat.com,
        linux-s390@...r.kernel.org
Subject: [PATCH AUTOSEL 4.14 18/29] s390/preempt: disable __preempt_count_add() optimization for PROFILE_ALL_BRANCHES

From: Heiko Carstens <hca@...ux.ibm.com>

[ Upstream commit 63678eecec57fc51b778be3da35a397931287170 ]

gcc 12 does not (always) optimize away code that should only be generated
if parameters are constant and within in a certain range. This depends on
various obscure kernel config options, however in particular
PROFILE_ALL_BRANCHES can trigger this compile error:

In function ‘__atomic_add_const’,
    inlined from ‘__preempt_count_add.part.0’ at ./arch/s390/include/asm/preempt.h:50:3:
./arch/s390/include/asm/atomic_ops.h:80:9: error: impossible constraint in ‘asm’
   80 |         asm volatile(                                                   \
      |         ^~~

Workaround this by simply disabling the optimization for
PROFILE_ALL_BRANCHES, since the kernel will be so slow, that this
optimization won't matter at all.

Reported-by: Thomas Richter <tmricht@...ux.ibm.com>
Reviewed-by: Sven Schnelle <svens@...ux.ibm.com>
Signed-off-by: Heiko Carstens <hca@...ux.ibm.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/s390/include/asm/preempt.h | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
index 23a14d187fb1..1aebf09fbcd8 100644
--- a/arch/s390/include/asm/preempt.h
+++ b/arch/s390/include/asm/preempt.h
@@ -50,10 +50,17 @@ static inline bool test_preempt_need_resched(void)
 
 static inline void __preempt_count_add(int val)
 {
-	if (__builtin_constant_p(val) && (val >= -128) && (val <= 127))
-		__atomic_add_const(val, &S390_lowcore.preempt_count);
-	else
-		__atomic_add(val, &S390_lowcore.preempt_count);
+	/*
+	 * With some obscure config options and CONFIG_PROFILE_ALL_BRANCHES
+	 * enabled, gcc 12 fails to handle __builtin_constant_p().
+	 */
+	if (!IS_ENABLED(CONFIG_PROFILE_ALL_BRANCHES)) {
+		if (__builtin_constant_p(val) && (val >= -128) && (val <= 127)) {
+			__atomic_add_const(val, &S390_lowcore.preempt_count);
+			return;
+		}
+	}
+	__atomic_add(val, &S390_lowcore.preempt_count);
 }
 
 static inline void __preempt_count_sub(int val)
-- 
2.35.1