[DPDK/core Bug 1385] rt_bitops.h fails to give implied atomicity guarantees

[DPDK/core Bug 1385] rt_bitops.h fails to give implied atomicity guarantees

[bugzilla]

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https://bugs.dpdk.org/show_bug.cgi?id=1385

            Bug ID: 1385
           Summary: rt_bitops.h fails to give implied atomicity guarantees
           Product: DPDK
           Version: 23.11
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: core
          Assignee: dev@dpdk.org
          Reporter: mattias.ronnblom@ericsson.com
  Target Milestone: ---

The documentation (and the naming) for the rte_bit_relaxed_*() functions in
rte_bitops.h makes clear that all such functions have a relaxed memory order.

The use of the term "relaxed", which most C programmers likely are familiar
with from the C11 memory model specification, itself implies that the
operations are supposed to be atomic. Why otherwise mention the memory
operations are relaxed? Relaxed is the default for non-atomic loads and stores.
In addition, why otherwise declare the address as volatile?

An even stronger indication are the test-and-set family of "relaxed"
rte_bitops.h functions. "Test-and-set" in the low-level C programming context
always implies an atomic operation. A non-atomic test-and-set does not make
sense.

In summary, a perfectly valid interpretation of the API contract is that the
functions are atomic.

However, their implementation is not, which may not be obvious to the causal
API user.

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