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docs: bpf: Clarify BPF_CMPXCHG wording

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Based on [1], BPF_CMPXCHG should always load the old value into R0. The
phrasing in bpf.rst is somewhat ambiguous in this regard, improve it to
make this aspect crystal clear.

  [1] https://lore.kernel.org/bpf/CAADnVQJFcFwxEz=wnV=hkie-EDwa8s5JGbBQeFt1TGux1OihJw@mail.gmail.com/

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210210142853.82203-1-iii@linux.ibm.com
This commit is contained in:
Ilya Leoshkevich 2021-02-10 15:28:52 +01:00 committed by Alexei Starovoitov
parent 1336c66247
commit 6a5df969ff
1 changed files with 2 additions and 2 deletions
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4
Documentation/networking/filter.rst
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@ -1076,8 +1076,8 @@ 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 value that was there
before is loaded back to ``R0``.
``R0``. If they match it is replaced with ``src_reg``. In either case, the
value that was there before is zero-extended and loaded back to ``R0``.
Note that 1 and 2 byte atomic operations are not supported.