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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) <>
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

+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
+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

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