[PATCH 0/4 v2] fs/dcache: Resolve the last RT woes.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

This is v2 of the series, v1 is available at
   https://https://lkml.kernel.org/r/.kernel.org/all/20220613140712.77932-1-bigeasy@linutronix.de/

v1…v2:
   - Make patch around Al's description of a bug in d_add_ci(). I took
     the liberty to make him Author and added his signed-off-by since I
     sinmply added a patch-body around his words.

   - The reasoning of why delaying the wakeup is reasonable has been
     replaced with Al's analysis of the code.

   - The split of wake up has been done differently (and I hope this is
     what Al meant). First the wake up has been pushed to the caller and
     then delayed to end_dir_add() after preemption has been enabled.

   - There is still __d_lookup_unhash(), __d_lookup_unhash_wake() and
     __d_lookup_done() is removed.
     __d_lookup_done() is removed because it is exported and the return
     value changes which will affect OOT users which are not aware of
     it.
     There is still d_lookup_done() which invokes
     __d_lookup_unhash_wake(). This can't remain in the header file due
     to cyclic depencies which in turn can't resolve wake_up_all()
     within the inline function.

The original cover letter:

PREEMPT_RT has two issues with the dcache code:

   1) i_dir_seq is a special cased sequence counter which uses the lowest
      bit as writer lock. This requires that preemption is disabled.

      On !RT kernels preemption is implictly disabled by spin_lock(), but
      that's not the case on RT kernels.

      Replacing i_dir_seq on RT with a seqlock_t comes with its own
      problems due to arbitrary lock nesting. Using a seqcount with an
      associated lock is not possible either because the locks held by the
      callers are not necessarily related.

      Explicitly disabling preemption on RT kernels across the i_dir_seq
      write held critical section is the obvious and simplest solution. The
      critical section is small, so latency is not really a concern.

   2) The wake up of dentry::d_wait waiters is in a preemption disabled
      section, which violates the RT constraints as wake_up_all() has
      to acquire the wait queue head lock which is a 'sleeping' spinlock
      on RT.

      There are two reasons for the non-preemtible section:

         A) The wake up happens under the hash list bit lock

         B) The wake up happens inside the i_dir_seq write side
            critical section

       #A is solvable by moving it outside of the hash list bit lock held                                                                                                                      
       section.

       Making #B preemptible on RT is hard or even impossible due to lock
       nesting constraints.

       A possible solution would be to replace the waitqueue by a simple
       waitqueue which can be woken up inside atomic sections on RT.

       But aside of Linus not being a fan of simple waitqueues, there is
       another observation vs. this wake up. It's likely for the woken up
       waiter to immediately contend on dentry::lock.

       It turns out that there is no reason to do the wake up within the
       i_dir_seq write held region. The only requirement is to do the wake
       up within the dentry::lock held region. Further details in the
       individual patches.

       That allows to move the wake up out of the non-preemptible section
       on RT, which also reduces the dentry::lock held time after wake up.

Thanks,

Sebastian