mds: allow try_eval to eval unstable locks in freezing object

Unstable locks hold auth_pins on the object, it prevents the freezing
object become frozen and then unfreeze. So try_eval() should not wait
for freezing object

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: Don't acquire replica object's versionlock

Both CInode and CDentry's versionlocks are of type LocalLock.
Acquiring LocalLock in replica object is useless and problematic.
For example, if two requests try acquiring a replica object's
versionlock, the first request succeeds, the second request
is added to wait queue. Later when the first request finishes,
MDCache::request_drop_foreign_locks() finds the lock's parent is
non-auth, it skips waking requests in the wait queue. So the
second request hangs.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: clear lock flushed if replica is waiting for AC_LOCKFLUSHED

So eval_gather() will not skip calling scatter_writebehind(),
otherwise the replica lock may be in flushing state forever.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: call eval() after caps are exported

For an inode just changed authority, if the new auth MDS want to
change a lock in the inode from 'sync' to 'lock' state before caps
are exported. The lock in replica can be in 'sync->lock' state
because client caps prevent it from transitting to 'lock' state.
So we should call eval() after clearing client caps.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: don't forward client request from MDS

Forwarding client request that was from MDS will trigger assertion
in MDS::forward_message_mds(). MDS only send client requests for
stray migration/reintegration, so it's safe to drop them.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: drop locks if requiring auth pinning new objects.

Locker::acquire_locks() skip auth pinning replica object if we only
request a rdlock and the lock is read-lockable. To get all locks,
we may call Locker::acquire_locks() several times, locks in replca
objects may become not read-lockable between calls. So it is
possible we need auth pin new objects after already take some locks.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: consider revoking caps in imported caps as issued

The clients may already send caps release message to the exporting
MDS, so the importing MDS waits for the release message forever.
consider revoking caps as issued can avoid this issue.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: fix open_remote_inode race

discover_ino() may return -ENOENT if it races with other FS activities.
so use C_MDC_RetryOpenRemoteIno instead of C_MDC_OpenRemoteIno as
onfinish callback.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: fix assertion in handle_cache_expire

During export, it's possible to get cache expire messages in
DISCOVERING, FREEZING and PREPPING state.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: allow open_remote_ino() to open xlocked dentry

discover_ino() has a parameter want_xlocked. The parameter indicates
if remote discover handler can proceed when xlocked dentry is
encountered. open_remote_ino() uses discover_ino() to find non-auth
inode, but always set 'want_xlocked' to false. This may cause
dead lock in some corner cases. For example:

we rename a inode's primary dentry to one of its remote dentry and
send slave request to one witness MDS. but before the slave request
reaches the witness MDS, the inode is trimmed from the witness MDS'
cache. Then when the slave request arrives, open_remote_ino() will
be called during traversing the destpath. open_remote_ino() calls
discover_ino() with 'want_xlocled=false' to find the inode.
discover_ino() sends MDiscover message to the inode's authority MDS.
The handler of MDiscover message finds the inode's primary dentry
is xlocked and it sleeps.

The fix is add a parameter 'want_xlocked' to open_remote_ino() and
make open_remote_ino() pass the parameter to discover_ino().

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: use rdlock_try() when checking NULL dentry

Use rdlock_try() instead can_read() when path_traverse encounters
a NULL dentry. This can partly avoid infinitely waiting for the
dentry to become readable when the dentry is replica.

Strictly speaking, use rdlock_try() is still enough because auth
MDS may drop the REQRDLOCK message in some cases.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: fix freeze inode deadlock

CInode::freeze_inode() is used in the case of cross authority rename.
Server::handle_slave_rename_prep() calls it to wait for all other
operations on source inode to complete. This happens after all locks
for the rename operation are acquired. But to acquire locks, we need
auth pin locks' parent objects first. So there is an ABBA deadlock
if someone auth pins the source inode after locks for rename are
acquired and before Server::handle_slave_rename_prep() is called.
The fix is freeze and auth pin the source inode at the same time.

This patch introduces CInode::freeze_auth_pin(), it waits for all
other MDRequests to release auth pins, then change the inode to
FROZENAUTHPIN state, this state prevents other MDRequests from
getting new auth pins.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>