Btrfs deadlock fix

Hi Linus,

My for-linus branch:

git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs.git for-linus

Has a fix for a long standing deadlock that we've been trying to nail down for a while. It ended up being a bad interaction with the fair reader/writer locks and the order btrfs reacquires locks in the btree.

Chris Mason (1) commits (+25/-15):
btrfs: fix lockups from btrfs_clear_path_blocking

Total: (1) commits (+25/-15)

 fs/btrfs/ctree.c   | 14 ++------------
 fs/btrfs/locking.c | 24 +++++++++++++++++++++---
 fs/btrfs/locking.h |  2 ++
 3 files changed, 25 insertions(+), 15 deletions(-)

Not sure if this is related, but with -rc4, on this ppc64el box, when grub updates (it starts trying to mount everything under the sun with every filesystem), the box hangs with:

[  177.344522] INFO: rcu_sched detected stalls on CPUs/tasks: { 8}

(detected by 9, t=25883 jiffies, g=3642, c=3641, q=5206)

With xmon I can catch it with this backtrace:

[link register ] d00000000ecec3f8 test_check_exists+0x1f8/0x280 [btrfs]
[c000000f0cc63930] d00000000ecec2b0 test_check_exists+0xb0/0x280 [btrfs] (unreliable) [c000000f0cc63990] d00000000ed2a4b4 btrfs_test_free_space_cache+0x774/0x1150 [btrfs] [c000000f0cc63a20] d00000000ed30f04 init_btrfs_fs+0x11c/0x210 [btrfs] [c000000f0cc63a90] c00000000000b4dc do_one_initcall+0x12c/0x280 [c000000f0cc63b60] c0000000001628b8 load_module+0x2088/0x2970 [c000000f0cc63d50] c0000000001633d0 SyS_finit_module+0xc0/0x120 [c000000f0cc63e30] c00000000000927c syscall_exit+0x0/0x7c
--- Exception: c01 (System Call) at 00003fff88697184

Note that afaik there isn't any btrfs partition on any disk on that machine... well not that I know of :)

Cheers,
Ben.

This should be from CONFIG_BTRFS_FS_RUN_SANITY_TESTS=y. I do test with this on here, but I'll look it over for ppc related fun.

The deadlock fix from this pull shouldn't be related. Just to make sure I read your email right, this stall isn't new with rc6 is it?

-chris

No, I observed it with -rc4 which is the first 3.18 I tried on this specific machine. I can try to bisect but it will take a while, it didn't happen afaik with 3.17

Cheers,
Ben.

Hi Chris,

I thought the fix for the scrub/replace deadlock would be included in this pull, I can reproduce it on each run of xfstests with 3.18-rc.

btrfs: fix dead lock while running replace and defrag concurrently
https://patchwork.kernel.org/patch/5264531/

I've retested it again including this pull and still deadlocks reliably at btrfs/070.

This wasn't a new problem, so I had it queued for the merge window.

-chris

Well, I don't remember seeing this problem with anything 3.17 based but I can check. I know this does not mean that it was introduced in 3.18, only that other changes exposed the problem, but this still IMHO qualifies as a bug to be fixed within 3.18. Besides, it makes testing 3.18-rc slightly more annoying (either skip the test or manually pick the patch after each pull).