rms: Order a writer's stores before its readers see it leaveHEADmain

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:			1f162fef76a0 ("Add read-mostly sleepable locks")
MFC after:		2 weeks
Sponsored by:		Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D60658

0 files changed, 0 insertions, 0 deletions