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| author | Peter Zijlstra <peterz@infradead.org> | 2019-02-11 18:09:43 +0100 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.ibm.com> | 2019-03-18 18:27:52 +0100 |
| commit | f1887143f5984f23d2360f2efed6ef481bb41117 (patch) | |
| tree | 81f6da1d80d8ba7e9472b5c346379b2bdf332851 /Documentation/atomic_t.txt | |
| parent | locking/memory-barriers/kokr: Update Korean translation to replace smp_cond_a... (diff) | |
| download | linux-f1887143f5984f23d2360f2efed6ef481bb41117.tar.xz linux-f1887143f5984f23d2360f2efed6ef481bb41117.zip | |
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.txt | 17 |
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 --------- |