powerpc/spufs: Rework fcheck() usage

powerpc/spufs: Rework fcheck() usage

Currently the spu coredump code triggers an RCU warning:

  =============================
  WARNING: suspicious RCU usage
  5.7.0-rc3-01755-g7cd49f0b7ec7 #1 Not tainted
  -----------------------------
  include/linux/fdtable.h:95 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 2, debug_locks = 1
  1 lock held by spu-coredump/1343:
   #0: c0000007fa22f430 (sb_writers#2){.+.+}-{0:0}, at: .do_coredump+0x1010/0x13c8

  stack backtrace:
  CPU: 0 PID: 1343 Comm: spu-coredump Not tainted 5.7.0-rc3-01755-g7cd49f0b7ec7 #1
  Call Trace:
    .dump_stack+0xec/0x15c (unreliable)
    .lockdep_rcu_suspicious+0x120/0x144
    .coredump_next_context+0x148/0x158
    .spufs_coredump_extra_notes_size+0x54/0x190
    .elf_coredump_extra_notes_size+0x34/0x50
    .elf_core_dump+0xe48/0x19d0
    .do_coredump+0xe50/0x13c8
    .get_signal+0x864/0xd88
    .do_notify_resume+0x158/0x3c8
    .interrupt_exit_user_prepare+0x19c/0x208
    interrupt_return+0x14/0x1c0

This comes from fcheck_files() via fcheck().

It's pretty clearly documented that fcheck() must be wrapped with
rcu_read_lock(), adding that fixes the RCU warning.

hch points out that once we've released the RCU read lock the file may
be closed and freed, which would leave us with a pointer to a freed
spu_context.

To avoid that, take a reference to the spu_context while we hold the
RCU read lock, and drop that reference later once we're done with the
context.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200508130633.2532759-1-mpe@ellerman.id.au