Possible FIEMAP deadlock?

Possible FIEMAP deadlock?

[Zygo Blaxell]

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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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