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  1. 26 Sep, 2022 3 commits
    • Ioannis Angelakopoulos's avatar
      btrfs: add lockdep annotations for num_writers wait event · e1489b4f
      Ioannis Angelakopoulos authored 
      Annotate the num_writers wait event in fs/btrfs/transaction.c with
lockdep in order to catch deadlocks involving this wait event.
Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarIoannis Angelakopoulos <iangelak@fb.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      e1489b4f
    • Ioannis Angelakopoulos's avatar
      btrfs: add macros for annotating wait events with lockdep · ab9a323f
      Ioannis Angelakopoulos authored 
      Introduce four macros that are used to annotate wait events in btrfs code
with lockdep;

  1) the btrfs_lockdep_init_map
  2) the btrfs_lockdep_acquire,
  3) the btrfs_lockdep_release
  4) the btrfs_might_wait_for_event macros.

The btrfs_lockdep_init_map macro is used to initialize a lockdep map.

The btrfs_lockdep_<acquire,release> macros are used by threads to take
the lockdep map as readers (shared lock) and release it, respectively.

The btrfs_might_wait_for_event macro is used by threads to take the
lockdep map as writers (exclusive lock) and release it.

In general, the lockdep annotation for wait events work as follows:

The condition for a wait event can be modified and signaled at the same
time by multiple threads. These threads hold the lockdep map as readers
when they enter a context in which blocking would prevent signaling the
condition. Frequently, this occurs when a thread violates a condition
(lockdep map acquire), before restoring it and signaling it at a later
point (lockdep map release).

The threads that block on the wait event take the lockdep map as writers
(exclusive lock). These threads have to block until all the threads that
hold the lockdep map as readers signal the condition for the wait event
and release the lockdep map.

The lockdep annotation is used to warn about potential deadlock scenarios
that involve the threads that modify and signal the wait event condition
and threads that block on the wait event. A simple example is illustrated
below:

Without lockdep:

TA                                        TB
cond = false
                                          lock(A)
                                          wait_event(w, cond)
                                          unlock(A)
lock(A)
cond = true
signal(w)
unlock(A)

With lockdep:

TA                                        TB
rwsem_acquire_read(lockdep_map)
cond = false
                                          lock(A)
                                          rwsem_acquire(lockdep_map)
                                          rwsem_release(lockdep_map)
                                          wait_event(w, cond)
                                          unlock(A)
lock(A)
cond = true
signal(w)
unlock(A)
rwsem_release(lockdep_map)

In the second case, with the lockdep annotation, lockdep would warn about
an ABBA deadlock, while the first case would just deadlock at some point.
Reviewed-by: default avatarJosef Bacik <josef@toxicpanda.com>
Signed-off-by: default avatarIoannis Angelakopoulos <iangelak@fb.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      ab9a323f
    • Qu Wenruo's avatar
      btrfs: dump extra info if one free space cache has more bitmaps than it should · 62cd9d44
      Qu Wenruo authored 
      There is an internal report on hitting the following ASSERT() in
recalculate_thresholds():

 	ASSERT(ctl->total_bitmaps <= max_bitmaps);

Above @max_bitmaps is calculated using the following variables:

- bytes_per_bg
  8 * 4096 * 4096 (128M) for x86_64/x86.

- block_group->length
  The length of the block group.

@max_bitmaps is the rounded up value of block_group->length / 128M.

Normally one free space cache should not have more bitmaps than above
value, but when it happens the ASSERT() can be triggered if
CONFIG_BTRFS_ASSERT is also enabled.

But the ASSERT() itself won't provide enough info to know which is going
wrong.
Is the bg too small thus it only allows one bitmap?
Or is there something else wrong?

So although I haven't found extra reports or crash dump to do further
investigation, add the extra info to make it more helpful to debug.
Reviewed-by: default avatarAnand Jain <anand.jain@oracle.com>
Signed-off-by: default avatarQu Wenruo <wqu@suse.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
      62cd9d44
  3. 25 Sep, 2022 8 commits
  5. 24 Sep, 2022 10 commits
  7. 23 Sep, 2022 18 commits
  9. 22 Sep, 2022 1 commit
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