R. Christian McDonald
2b911d947b
Readers check rms->writers without a fence, and rms_wunlock() drops
it with a plain store. rms_wlock() interrupts every CPU before the
first writer blocks readers, but nothing orders them when the last
writer leaves: on a weakly ordered CPU a reader can see the count
reach zero and still read what the writer replaced, such as callbacks
of a module unloaded under the lock.
Fence every CPU before the last writer drops the count: rename
rms_wlock_switch() to rms_block_readers() and add
rms_unblock_readers(), a wrapper for cpus_fence_seq_cst(). Readers
stay fence-free; the cost lands on the rare unlock.
Reviewed by: mjg
Discussed with: kib
Fixes: 1f162fef76 ("Add read-mostly sleepable locks")
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D60658
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