@@ -1053,6 +1053,33 @@ encoding.
.imm = BPF_ADD, .code = BPF_ATOMIC | BPF_W | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg
.imm = BPF_ADD, .code = BPF_ATOMIC | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg
+The basic atomic operations supported (from architecture v4 onwards) are:
+
+ BPF_ADD
+ BPF_SUB
+ BPF_AND
+ BPF_OR
+ BPF_XOR
+
+Each having isomorphic semantics with the ``BPF_ADD`` example, that is: the
+memory location addresed by ``dst_reg + off`` is atomically modified, with
+``src_reg`` as the other operand. If the ``BPF_FETCH`` flag is set in the
+immediate, then these operations also overwrite ``src_reg`` with the
+pre-modification value from memory.
+
+The more special operations are:
+
+ BPF_XCHG
+
+This atomically exchanges ``src_reg`` with the value addressed by ``dst_reg +
+off``.
+
+ BPF_CMPXCHG
+
+This atomically compares the value addressed by ``dst_reg + off`` with
+``R0``. If they match it is replaced with ``src_reg``, The pre-modification
+value is loaded back to ``R0``.
+
Note that 1 and 2 byte atomic operations are not supported.
You may encounter BPF_XADD - this is a legacy name for BPF_ATOMIC, referring to
Signed-off-by: Brendan Jackman <jackmanb@google.com> --- Documentation/networking/filter.rst | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+)