Adding upstream version 4.19.249.upstream/4.19.249

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 71 insertions, 0 deletions

diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt
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+++ b/Documentation/atomic_bitops.txt
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+
+On atomic bitops.
+
+
+While our bitmap_{}() functions are non-atomic, we have a number of operations
+operating on single bits in a bitmap that are atomic.
+
+
+API
+---
+
+The single bit operations are:
+
+Non-RMW ops:
+
+  test_bit()
+
+RMW atomic operations without return value:
+
+  {set,clear,change}_bit()
+  clear_bit_unlock()
+
+RMW atomic operations with return value:
+
+  test_and_{set,clear,change}_bit()
+  test_and_set_bit_lock()
+
+Barriers:
+
+  smp_mb__{before,after}_atomic()
+
+
+All RMW atomic operations have a '__' prefixed variant which is non-atomic.
+
+
+SEMANTICS
+---------
+
+Non-atomic ops:
+
+In particular __clear_bit_unlock() suffers the same issue as atomic_set(),
+which is why the generic version maps to clear_bit_unlock(), see atomic_t.txt.
+
+
+RMW ops:
+
+The test_and_{}_bit() operations return the original value of the bit.
+
+
+ORDERING
+--------
+
+Like with atomic_t, the rule of thumb is:
+
+ - non-RMW operations are unordered;
+
+ - RMW operations that have no return value are unordered;
+
+ - RMW operations that have a return value are fully ordered.
+
+ - RMW operations that are conditional are unordered on FAILURE,
+   otherwise the above rules apply. In the case of test_and_{}_bit() operations,
+   if the bit in memory is unchanged by the operation then it is deemed to have
+   failed.
+
+Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
+clear_bit_unlock() which has RELEASE semantics.
+
+Since a platform only has a single means of achieving atomic operations
+the same barriers as for atomic_t are used, see atomic_t.txt.
+