[Dle-develop] [PATCH v4 0/2] pstore, efi_pstore: Avoid deadlock in non-blocking paths

[Seiji A. <sei...@hd...>]

Changelog 
v3 -> v4
 - Add EXPORT_SYMBOL_GPL(pstore_cannot_block_path) to build successfully
   with CONFIG_EFI_VARS=m. (PATCH 1/2)

v2 -> v3
 - Merge modifications of pstore part in 2/2 to 1/2.
 - Rename pstore_is_non_blocking_path() to pstore_cannot_block_path().

v1 -> v2
 - Erase a logic checking the number of online cpus.
 - Create a patchset to fix deadlocking issue in both pstore filesystem and
   efi_pstore driver.
   - Introduce a function, is_non_blocking_path(), to check if pstore 
     is in panic and emergency-restart paths (PATCH 1/2)
   - Avoid efi_pstore_driver is blocked in non-blocking paths
     such as nmi, panic and emergency-restart paths (PATCH 2/2)

[Issue]

There are some paths in kernel which shouldn't be blocked, 
like NMI, panic case after stopping cpus, emergency-restart.

On the other hand, current pstore avoids blocking in a NMI path 
but it may be blocked in other paths.
Also, an efi_pstore driver may be blocked in all of those paths 
because it simply takes a spin lock at writing time.

If they are blocked in those paths, the system will hang up and
it has a big impact for users.

Here is an example scenario which pstore is blocked in panic path.

 - cpuA grabs psinfo->buf_lock
 - cpuB panics and calls smp_send_stop
 - smp_send_stop sends IRQ to cpuA
   after 1 second, cpuB gives up on cpuA and sends an NMI instead
 - cpuA is now in an NMI handler while still holding buf_lock.
   And then, cpuB is deadlocked by taking efi_pstore->lock again.

This case may happen if a firmware has a bug and cpuA is stuck in it.

[Solution]

This patchset avoids that pstore and efi_pstore driver are blocked 
in the non-blocking paths like NMI, panic, and emrgency-restart
by introducing a function checking if they are in those paths.
Please see each patch for detailed explanations.

Seiji Aguchi (2):
  [PATCH v4 1/2] pstore: Avoid deadlock in panic and emergency-restart path
  [PATCH v4 2/2] efi_pstore: Avoid deadlock in non-blocking paths

 drivers/firmware/efivars.c |   11 ++++++++++-
 fs/pstore/platform.c       |   35 +++++++++++++++++++++++++++++------
 include/linux/pstore.h     |    6 ++++++
 3 files changed, 45 insertions(+), 7 deletions(-)