Messages in this thread | | | Date | Thu, 29 Dec 2022 10:54:50 +0100 | Subject | Re: [PATCH 00/19] Introduce __xchg, non-atomic xchg | From | Andrzej Hajda <> |
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Forgive me late response - Holidays,
On 22.12.2022 18:21, Andrew Morton wrote: > On Thu, 22 Dec 2022 12:46:16 +0100 Andrzej Hajda <andrzej.hajda@intel.com> wrote: > >> Hi all, >> >> I hope there will be place for such tiny helper in kernel. >> Quick cocci analyze shows there is probably few thousands places >> where it could be useful. > So to clarify, the intent here is a simple readability cleanup for > existing open-coded exchange operations.
And replace private helpers with common one, see the last patch - the ultimate goal would be to replace all occurrences of fetch_and_zero with __xchg.
> The intent is *not* to > identify existing xchg() sites which are unnecessarily atomic and to > optimize them by using the non-atomic version. > > Have you considered the latter?
If you mean some way of (semi-)automatic detection of such cases, then no. Anyway this could be quite interesting challenge.
> >> I am not sure who is good person to review/ack such patches, > I can take 'em. > >> so I've used my intuition to construct to/cc lists, sorry for mistakes. >> This is the 2nd approach of the same idea, with comments addressed[0]. >> >> The helper is tiny and there are advices we can leave without it, so >> I want to present few arguments why it would be good to have it: >> >> 1. Code readability/simplification/number of lines: >> >> Real example from drivers/net/ethernet/mellanox/mlx5/core/esw/qos.c: >> - previous_min_rate = evport->qos.min_rate; >> - evport->qos.min_rate = min_rate; >> + previous_min_rate = __xchg(evport->qos.min_rate, min_rate); >> >> For sure the code is more compact, and IMHO more readable. >> >> 2. Presence of similar helpers in other somehow related languages/libs: >> >> a) Rust[1]: 'replace' from std::mem module, there is also 'take' >> helper (__xchg(&x, 0)), which is the same as private helper in >> i915 - fetch_and_zero, see latest patch. >> b) C++ [2]: 'exchange' from utility header. >> >> If the idea is OK there are still 2 qestions to answer: >> >> 1. Name of the helper, __xchg follows kernel conventions, >> but for me Rust names are also OK. > I like replace(), or, shockingly, exchange(). > > But... Can we simply make swap() return the previous value? > > previous_min_rate = swap(&evport->qos.min_rate, min_rate);
As Alexander already pointed out, swap requires 'references' to two variables, in contrast to xchg which requires reference to variable and value. So we cannot use swap for cases: old_value = __xchg(&x, new_value);
Regards Andrzej
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