wg: Remove some atomics for kp pointers.
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Authored by jhb on Jul 29 2022, 4:57 PM.

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markj

Summary

Atomics are not needed for accesses synchronized by a mutex.

There is one unlocked read of the pointer for which the atomic
load doesn't do anything (in terms of limiting races).

Atomics remain for accesses under the NET_EPOCH to provide
READ_ONCE semantics.

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rG FreeBSD src repository

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Buildable 47978
Build 44865: arc lint + arc unit

jhb requested review of this revision.Jul 29 2022, 4:57 PM

There is one unlocked read of the pointer for which the atomic load doesn't do anything (in terms of limiting races).

Does it make sense for this case to have a comment (i.e., explaining why the race is not important)?

jhb added inline comments.Aug 2 2022, 6:28 PM

sys/dev/wg/wg_noise.c
727	This is the unlocked read and it's just an optimization to avoid the lock below (and not an uncommon pattern, so I think it is obvious in context)

sys/dev/wg/wg_noise.c
727	Fair enough, I did not look at each case to find the unlocked read and when pointed out in context it's clear enough to me. [That said I don't think a comment wouldn't be out of place]

sys/dev/wg/wg_noise.c
586	Possibly these two should remain as atomics as they are used to provide READ_ONCE semantics.
702–703	Another READ_ONCE
860–861	Another READ_ONCE
889–890	Another READ_ONCE