[Bug 284057] vmxnet3/iflib: crash in vmxnet3_isc_txd_credits_update

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=284057

--- Comment #7 from Andriy Gapon <[email protected]> ---
(In reply to Kristof Provost from comment #6)
My thinking on purely defensive code is that maybe we can stash vxcr_next into
a local variable, check and increment it (with the wraparound) and then update
back vxcr_next, maybe using atomic compare and set.

This won't eliminate the race and won't improve correctness of
vmxnet3_isc_txd_credits_update result, but it should make the code even safer
comparing to your proposed patch where the initial array access indexed by
vxcr_next may still be out of bounds.

In the crash that I reported here, vxcr_next didn't run wild, it got wrapped
around.
But for a moment it was equal to vxcr_ndesc and that was enough to cause the
crash.
Which is very rare, admittedly.

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