path: root/Documentation/atomic_t.txt
diff options
authorPeter Zijlstra <>2019-02-11 18:09:43 +0100
committerPaul E. McKenney <>2019-03-18 10:27:52 -0700
commitf1887143f5984f23d2360f2efed6ef481bb41117 (patch)
tree81f6da1d80d8ba7e9472b5c346379b2bdf332851 /Documentation/atomic_t.txt
parentdb467147f131b8ab799066b9779addb8603d3baf (diff)
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) <> Acked-by: Will Deacon <> Acked-by: Boqun Feng <> Signed-off-by: Paul E. McKenney <>
Diffstat (limited to 'Documentation/atomic_t.txt')
1 files changed, 17 insertions, 0 deletions
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 @@ Barriers:
+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