[SRU][D][PATCH v2 0/3] bcache: fix hung task timeout in bch_bucket_alloc()

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/1784665

[Impact]

bcache_allocator can call the following:

 bch_allocator_thread()
  -> bch_prio_write()
     -> bch_bucket_alloc()
        -> wait on &ca->set->bucket_wait

But the wake up event on bucket_wait is supposed to come from
bch_allocator_thread() itself causing a deadlock.

[Test Case]

This is a simple script that can easily trigger the deadlock condition:
https://launchpadlibrarian.net/381282009/bcache-basic-repro.sh

A better test case has been also provided in LP: #1796292:
https://bugs.launchpad.net/curtin/+bug/1796292/+attachment/5280353/+files/curtin-nvme.sh

[Fix]

Fix by making the call to bch_prio_write() non-blocking, so that
bch_allocator_thread() never waits on itself. Moreover, make sure to
wake up the garbage collector thread when bch_prio_write() is failing to
allocate buckets to increase the chance of freeing up more buckets.

In addition to that it would be safer to also import other upstream
bcache fixes (all clean cherry picks):

eb8cbb6df38f6e5124a3d5f1f8a3dbf519537c60 bcache: improve bcache_reboot()
9951379b0ca88c95876ad9778b9099e19a95d566 bcache: never writeback a discard operation

[Regression Potential]

The upstream fixes are all clean cherry picks from stable (most of them
are small cleanups), so regression potential is minimal.

The only special patch is "UBUNTU: SAUCE: bcache: fix deadlock in
bcache_allocator" that is addressing the main deadlock bug (that seems
to be a mainline bug - not fixed yet). We should spend more time trying
to reproduce this deadlock with a mainline kernel and post the patch to
the LKML for review / feedback.

However, considering that this patch seems to fix/prevent the specific
deadlock problem reported in this bug (tested on the affected platform),
it should be considered safe to apply it as it is for now, to prevent
potential hung task timeout conditions.

Changes in v2:
 - fix potential buckets leak in "UBUNTU: SAUCE: bcache: fix deadlock in
   bcache_allocator"

----------------------------------------------------------------
Andrea Righi (1):
      UBUNTU: SAUCE: bcache: fix deadlock in bcache_allocator

Coly Li (1):
      bcache: improve bcache_reboot()

Daniel Axtens (1):
      bcache: never writeback a discard operation

 drivers/md/bcache/alloc.c     |  5 ++++-
 drivers/md/bcache/bcache.h    |  2 +-
 drivers/md/bcache/super.c     | 39 +++++++++++++++++++++++++++++++--------
 drivers/md/bcache/writeback.h |  3 +++
 4 files changed, 39 insertions(+), 10 deletions(-)