This looks like a deadlock in FIEMAP:
# cat /proc/version
Linux version 4.1.8-zb64+ (root@buildhost) (gcc version 4.9.2 (Debian
4.9.2-10) ) #1 SMP PREEMPT Tue Sep 22 00:54:04 EDT 2015
# cat /proc/6943/stack
[<ffffffff8142b93d>] lock_extent_bits+0x1ad/0x200
[<ffffffff81431e04>] extent_fiemap+0x1a4/0x580
[<ffffffff8140f695>] btrfs_fiemap+0x45/0x50
[<ffffffff81277367>] do_vfs_ioctl+0x157/0x560
[<ffffffff812777f1>] SyS_ioctl+0x81/0xa0
[<ffffffff81b037b2>] system_call_fastpath+0x16/0x7a
[<ffffffffffffffff>] 0xffffffffffffffff
# cat /proc/8914/stack
[<ffffffff8142b93d>] lock_extent_bits+0x1ad/0x200
[<ffffffff81431e04>] extent_fiemap+0x1a4/0x580
[<ffffffff8140f695>] btrfs_fiemap+0x45/0x50
[<ffffffff81277367>] do_vfs_ioctl+0x157/0x560
[<ffffffff812777f1>] SyS_ioctl+0x81/0xa0
[<ffffffff81b037b2>] system_call_fastpath+0x16/0x7a
[<ffffffffffffffff>] 0xffffffffffffffff
# ls -l /proc/6943/fd /proc/8914/fd
/proc/6943/fd:
total 0
lrwx------ 1 root root 64 Oct 23 03:58 0 -> /dev/pts/47
lrwx------ 1 root root 64 Oct 23 03:58 1 -> /dev/pts/47
lrwx------ 1 root root 64 Oct 23 03:57 2 -> /dev/pts/47
lr-x------ 1 root root 64 Oct 23 03:58 3 -> /dedup-arena/current
lr-x------ 1 root root 64 Oct 23 03:58 4 -> /dedup-arena/current
lr-x------ 1 root root 64 Oct 23 03:58 5 ->
/dedup-arena/current/home/builder/workdir/android/nex1/LINUX/android/out/target/product/mxp1010_qsb/symbols/system/lib/libwebcore.so
lrwx------ 1 root root 64 Oct 23 03:58 6 ->
/dedup-arena/current/home/builder/backup/home/builder/workdir/android/nex1/LINUX/android/out/target/product/mxp1010_qsb/symbols/system/lib/libwebcore.so
/proc/8914/fd:
total 0
lrwx------ 1 root root 64 Oct 23 03:58 0 -> /dev/pts/52
lrwx------ 1 root root 64 Oct 23 03:58 1 -> /dev/pts/52
lrwx------ 1 root root 64 Oct 23 03:57 2 -> /dev/pts/52
lr-x------ 1 root root 64 Oct 23 03:58 3 ->
/dedup-arena/snapshots/2015-08-31-00-19-42
lr-x------ 1 root root 64 Oct 23 03:58 4 ->
/dedup-arena/snapshots/2015-08-31-00-19-42
lr-x------ 1 root root 64 Oct 23 03:58 5 ->
/dedup-arena/snapshots/2015-08-31-00-19-42/home/builder/workdir/android/nex1/LINUX/android/out/target/product/mxp1010_qsb/symbols/system/lib/libwebcore.so
lrwx------ 1 root root 64 Oct 23 03:58 6 ->
/dedup-arena/current/home/builder/workdir/android/nex1/LINUX/android/out/target/product/mxp1010_qsb/symbols/system/lib/libwebcore.so
Note that "snapshots/2015-08-31-00-19-42" is a snapshot of the "current"
subvol, and these particular files most likely share extents within the
"current" subvol due to dedup (I can't compare their FIEMAPs so I don't
know for sure ;).
I am able to get a FIEMAP from /proc/8914/fd/5, but when I try to FIEMAP
the other two files my FIEMAP process gets stuck in lock_extent_bits.
# ls -liL /proc/6943/fd /proc/8914/fd
/proc/6943/fd/:
total 528088
50 crw--w---- 1 root tty 136, 47 Oct 18 20:13 0
50 crw--w---- 1 root tty 136, 47 Oct 18 20:13 1
50 crw--w---- 1 root tty 136, 47 Oct 18 20:13 2
102531369 drwx------ 1 root root 126 Oct 8 21:37 3
102531369 drwx------ 1 root root 126 Oct 8 21:37 4
25449434 -rw-r--r-- 1 root root 270378576 Aug 28 2009 5
104189066 -rw-r--r-- 1 root root 270378576 Aug 28 2009 6
/proc/8914/fd/:
total 528088
55 crw--w---- 1 root tty 136, 52 Oct 18 23:06 0
55 crw--w---- 1 root tty 136, 52 Oct 18 23:06 1
55 crw--w---- 1 root tty 136, 52 Oct 18 23:06 2
256 drwxr-xr-x 1 root root 534 Aug 31 00:19 3
256 drwxr-xr-x 1 root root 534 Aug 31 00:19 4
25449434 -rw-r--r-- 1 root root 270378576 Aug 28 2009 5
25449434 -rw-r--r-- 1 root root 270378576 Aug 28 2009 6
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