gfs2: Fix mmap + page fault deadlocks (part 2)

gfs2: Fix mmap + page fault deadlocks (part 2)

Now that we handle self-recursion on the inode glock in gfs2_fault and
gfs2_page_mkwrite, we need to take care of more complex deadlock
scenarios like the following (example by Jan Kara):

Two independent processes P1, P2. Two files F1, F2, and two mappings M1,
M2 where M1 is a mapping of F1, M2 is a mapping of F2. Now P1 does DIO
to F1 with M2 as a buffer, P2 does DIO to F2 with M1 as a buffer. They
can race like:

P1                                      P2
read()                                  read()
  gfs2_file_read_iter()                   gfs2_file_read_iter()
    gfs2_file_direct_read()                 gfs2_file_direct_read()
      locks glock of F1                       locks glock of F2
      iomap_dio_rw()                          iomap_dio_rw()
        bio_iov_iter_get_pages()                bio_iov_iter_get_pages()
          <fault in M2>                           <fault in M1>
            gfs2_fault()                            gfs2_fault()
              tries to grab glock of F2               tries to grab glock of F1

Those kinds of scenarios are much harder to reproduce than
self-recursion.

We deal with such situations by using the LM_FLAG_OUTER flag to mark
"outer" glock taking.  Then, when taking an "inner" glock, we use the
LM_FLAG_TRY flag so that locking attempts that don't immediately succeed
will be aborted.  In case of a failed locking attempt, we "unroll" to
where the "outer" glock was taken, drop the "outer" glock, and fault in
the first offending user page.  This will re-trigger the "inner" locking
attempt but without the LM_FLAG_TRY flag.  Once that has happened, we
re-acquire the "outer" glock and retry the original operation.

Reported-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>