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

[Kleber Souza]

On 8/7/19 2:46 PM, Andrea Righi wrote:
> 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(-)
> 
> 


Acked-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>