Commit-ID: f1887143f5984f23d2360f2efed6ef481bb41117
Gitweb: https://git.kernel.org/tip/f1887143f5984f23d2360f2efed6ef481bb41117
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Mon, 11 Feb 2019 18:09:43 +0100
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
CommitDate: Mon, 18 Mar 2019 10:27:52 -0700
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@xxxxxxxxxxxxx>
Acked-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Boqun Feng <boqun.feng@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
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 @@ Barriers:
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
---------
|