Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Will Deacon <will@kernel.org>
To:     linux-kernel@vger.kernel.org
Cc:     linux-arch@vger.kernel.org, kernel-team@android.com,
        Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Arnd Bergmann <arnd@arndb.de>
Subject: [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC prior to version 4.8
Date:   Fri, 10 Jan 2020 16:56:29 +0000
Message-Id: <20200110165636.28035-2-will@kernel.org>
In-Reply-To: <20200110165636.28035-1-will@kernel.org>
References: <20200110165636.28035-1-will@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Prior to version 4.8, GCC may miscompile READ_ONCE() by erroneously
discarding the 'volatile' qualifier:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

We've been working around this using some nasty hacks which make
READ_ONCE() both horribly complicated and also prevent us from enforcing
that it is only used on scalar types. Since GCC 4.8 is pretty old for
kernel builds now, emit a warning if we detect it during the build.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
 include/linux/compiler-gcc.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index d7ee4c6bad48..62afe874073e 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -14,6 +14,10 @@
 # error Sorry, your compiler is too old - please upgrade it.
 #endif
 
+#if GCC_VERSION < 40800
+# warning Your compiler is old and may miscompile the kernel due to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 - please upgrade it.
+#endif
+
 /* Optimization barrier */
 
 /* The "volatile" is due to gcc bugs */
-- 
2.25.0.rc1.283.g88dfdc4193-goog