SOLARIS: Avoid vcache locks when flushing pages for RO vnodes

SOLARIS: Avoid vcache locks when flushing pages for RO vnodes

We have multiple code paths that hold the following locks at the same
time:

    - avc->lock for a vcache
    - The page lock for a page in 'avc'

In order to avoid deadlocks, we need a consistent ordering for obtaining
these two locks. The code in afs_putpage() currently obtains avc->lock
before the page lock (Obtain*Lock is called before pvn_vplist_dirty).

The code in afs_getpages() also obtains avc->lock before the page lock,
but it does so in a loop for all requested pages (via pvn_getpages()).
On the second iteration of that loop, it obtains avc->lock, and the page
from the first iteration of the loop is still locked. Thus, it obtains a
page lock before locking avc->lock in some cases.

Since we have two code paths that obtain those two locks in a different
order, a deadlock can occur. Fixing this properly requires changing at
least one of those code paths, so the locks are taken in a consistent
order. However, doing so is complex and will be done in a separate
future commit.

For this commit, we can avoid the deadlock for RO volumes by simply
avoiding taking avc->lock in afs_putpages() at all while the pages are
locked. Normally, we lock avc->lock because pvn_vplist_dirty() will call
afs_putapage() for each dirty page (and afs_putapage() requires
avc->lock held). But for RO volumes, we will have no dirty pages
(because RO volumes cannot be written to from a client), and so
afs_putapage() will never be called.

So to avoid this deadlock issue for RO volumes, avoid taking avc->lock
across the pvn_vplist_dirty() call in afs_putpage(). We now pass a dummy
pageout callback function to pvn_vplist_dirty() instead, which should
never be called, and which panics if it ever is.

We still need to hold avc->lock a few other times during afs_putpage()
for other minor reasons, but none of these hold page locks at the same
time, so the deadlock issue is still avoided.

[mmeffie: comments, and fix missing write lock, fix lock releases]
[adeason: revised commit message]

Change-Id: Iec11101147220828f319dae4027e7ab1f08483a6
Reviewed-on: https://gerrit.openafs.org/12247
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@dson.org>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>