mds: document EXCL -> (MIX or SYNC) transition decision

Previously (in w26f6a8e48ae575f17c850e28e969d55bceefbc0f), for reasons that
are somewhat obscured by passage of time, we did

+      if ((other_wanted & (CEPH_CAP_GRD|CEPH_CAP_GWR)) ||

But then we noticed that the loner may want to RD/WR and we are losing the
loner status for some other reason.  So just recently in
b48dfeba3f99451815a5e2a538bea15cd87220d2 we changed it to

+      if (((other_wanted|loner_wanted) & (CEPH_CAP_GRD|CEPH_CAP_GWR)) ||

Then we noticed that a non-loner wanting to read and a loner wanting to
read (i.e., no writers!) would lead to MIX, even when we want SYNC.
So in 07b36992da35e8b54acf76af6c893a0d86f048fb we changed to

+      if (((other_wanted|loner_wanted) & CEPH_CAP_GWR) ||

This appears to be correct.  The possible choices (wrt caps wanted):

loner  other   want
R      R       SYNC
R      R|W     MIX
R      W       MIX
R|W    R       MIX
R|W    R|W     MIX
R|W    W       MIX
W      R       MIX
W      R|W     MIX
W      W       MIX

Which means any writer -> we want MIX.  We only want SYNC when there is
nobody who wants to write.  Because you can't write in SYNC.  Which in
retrospect seems obvious.

Signed-off-by: Sage Weil <sage@inktank.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/mds/Locker.cc b/src/mds/Locker.cc
index a8ec19f765e..d5d5e6b4c37 100644
--- a/src/mds/Locker.cc
+++ b/src/mds/Locker.cc
@@ -4036,6 +4036,17 @@ void Locker::file_eval(ScatterLock *lock, bool *need_issue)
 	(in->inode.is_dir() && in->multiple_nonstale_caps())) {  // FIXME.. :/
       dout(20) << " should lose it" << dendl;
       // we should lose it.
+      //  loner  other   want
+      //  R      R       SYNC
+      //  R      R|W     MIX
+      //  R      W       MIX
+      //  R|W    R       MIX
+      //  R|W    R|W     MIX
+      //  R|W    W       MIX
+      //  W      R       MIX
+      //  W      R|W     MIX
+      //  W      W       MIX
+      // -> any writer means MIX; RD doesn't matter.
       if (((other_wanted|loner_wanted) & CEPH_CAP_GWR) ||
 	  lock->is_waiter_for(SimpleLock::WAIT_WR))
 	scatter_mix(lock, need_issue);