linux-kernel - [PATCH] Documentation/atomic_t: Clarify signed vs unsigned

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Message-ID: <20190211170943.GP32477@hirez.programming.kicks-ass.net>
Date:   Mon, 11 Feb 2019 18:09:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] Documentation/atomic_t: Clarify signed vs unsigned


Clarify the whole signed vs unsigned issue for atomic_t.

There has been enough confusion on this topic to warrant a few explicit
words I feel.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 Documentation/atomic_t.txt | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
index 913396ac5824..dca3fb0554db 100644
--- a/Documentation/atomic_t.txt
+++ b/Documentation/atomic_t.txt
@@ -56,6 +56,23 @@ The 'full' API consists of (atomic64_ and atomic_long_ prefixes omitted for
   smp_mb__{before,after}_atomic()
 
 
+TYPES (signed vs unsigned)
+-----
+
+While atomic_t, atomic_long_t and atomic64_t use int, long and s64
+respectively (for hysterical raisins), the kernel uses -fno-strict-overflow
+(which implies -fwrapv) and defines signed overflow to behave like
+2s-complement.
+
+Therefore, an explicitly unsigned variant of the atomic ops is strictly
+unnecessary and we can simply cast, there is no UB.
+
+There was a bug in UBSAN prior to GCC-8 that would generate UB warnings for
+signed types.
+
+With this we also conform to the C/C++ _Atomic behaviour and things like
+P1236R1.
+
 
 SEMANTICS
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