__d_lookup_done() wakes waiters on dentry->d_wait.  On PREEMPT_RT we are
not allowed to do that with preemption disabled, since the wakeup
acquired wait_queue_head::lock, which is a "sleeping" spinlock on RT.

Calling it under dentry->d_lock is not a problem, since that is also a
"sleeping" spinlock on the same configs.  Unfortunately, two of its
callers (__d_add() and __d_move()) are holding more than just ->d_lock
and that needs to be dealt with.

The key observation is that wakeup can be moved to any point before
dropping ->d_lock.

As a first step to solve this, move the wake up outside of the
hlist_bl_lock() held section.

This is safe because:

Waiters get inserted into ->d_wait only after they'd taken ->d_lock
and observed DCACHE_PAR_LOOKUP in flags.  As long as they are
woken up (and evicted from the queue) between the moment __d_lookup_done()
has removed DCACHE_PAR_LOOKUP and dropping ->d_lock, we are safe,
since the waitqueue ->d_wait points to won't get destroyed without
having __d_lookup_done(dentry) called (under ->d_lock).

->d_wait is set only by d_alloc_parallel() and only in case when
it returns a freshly allocated in-lookup dentry.  Whenever that happens,
we are guaranteed that __d_lookup_done() will be called for resulting
dentry (under ->d_lock) before the wq in question gets destroyed.

With two exceptions wq lives in call frame of the caller of
d_alloc_parallel() and we have an explicit d_lookup_done() on the
resulting in-lookup dentry before we leave that frame.

One of those exceptions is nfs_call_unlink(), where wq is embedded into
(dynamically allocated) struct nfs_unlinkdata.  It is destroyed in
nfs_async_unlink_release() after an explicit d_lookup_done() on the
dentry wq went into.

Remaining exception is d_add_ci(). There wq is what we'd found in
->d_wait of d_add_ci() argument. Callers of d_add_ci() are two
instances of ->d_lookup() and they must have been given an in-lookup
dentry.  Which means that they'd been called by __lookup_slow() or
lookup_open(), with wq in the call frame of one of those.

Result of d_alloc_parallel() in d_add_ci() is fed to
d_splice_alias(), which either returns non-NULL (and d_add_ci() does
d_lookup_done()) or feeds dentry to __d_add() that will do
__d_lookup_done() under ->d_lock.  That concludes the analysis.

Let __d_lookup_unhash():

  1) Lock the lookup hash and clear DCACHE_PAR_LOOKUP
  2) Unhash the dentry
  3) Retrieve and clear dentry::d_wait
  4) Unlock the hash and return the retrieved waitqueue head pointer
  5) Let the caller handle the wake up.
  6) Rename __d_lookup_done() to __d_lookup_unhash_wake() to enforce
     build failures for OOT code that used __d_lookup_done() and is not
     aware of the new return value.

This does not yet solve the PREEMPT_RT problem completely because
preemption is still disabled due to i_dir_seq being held for write. This
will be addressed in subsequent steps.

An alternative solution would be to switch the waitqueue to a simple
waitqueue, but aside of Linus not being a fan of them, moving the wake up
closer to the place where dentry::lock is unlocked reduces lock contention
time for the woken up waiter.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/20220613140712.77932-3-bigeasy@linutronix.deSigned-off-by: Al Viro <viro@zeniv.linux.org.uk>