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authorPeter Zijlstra <peterz@infradead.org>2019-02-11 20:09:43 +0300
committerPaul E. McKenney <paulmck@linux.ibm.com>2019-03-18 20:27:52 +0300
commitf1887143f5984f23d2360f2efed6ef481bb41117 (patch)
tree81f6da1d80d8ba7e9472b5c346379b2bdf332851 /Documentation/atomic_t.txt
parentdb467147f131b8ab799066b9779addb8603d3baf (diff)
downloadlinux-f1887143f5984f23d2360f2efed6ef481bb41117.tar.xz
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@infradead.org> Acked-by: Will Deacon <will.deacon@arm.com> Acked-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Diffstat (limited to 'Documentation/atomic_t.txt')
-rw-r--r--Documentation/atomic_t.txt17
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:
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
---------