processes stuck in llseek

Dan Merillat
2011-08-16T03:59:52Z

I noticed a series of hung_task notifications in dmesg, so I went
poking at it. Process is 'dropbox', and it's stuck trying to llseek
it's library.zip file.

strace of dropbox:
...
stat("/home/x/.dropbox-dist/library.zip", {st_mode=S_IFREG|0755,
st_size=11575179, ...}) = 0
open("/home/x/.dropbox-dist/library.zip", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=11575179, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x7fa766ea8000
fstat(3, {st_mode=S_IFREG|0755, st_size=11575179, ...}) = 0
lseek(3, 11571200, SEEK_SET

SEEK_SET is less than st_size

strace of dd if=library.zip of=/dev/null bs=1 seek=11571200:

open("library.zip", O_RDONLY) = 3
dup2(3, 0) = 0
close(3) = 0
lseek(0, 0, SEEK_CUR

[72960.716080] INFO: task dropbox:1348 blocked for more than 120 seconds.
[72960.716084] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[72960.716087] dropbox D ffff8800762d8a78 0 1348 1 0x00000004
[72960.716092] ffff880069cede18 0000000000000086 0000000000096000
0000000000000000
[72960.716097] ffff880069cec000 00000000000119c0 ffff8800762d8700
00000000000119c0
[72960.716101] ffff880069cedfd8 0000000000004000 ffff880069cedfd8
00000000000119c0
[72960.716106] Call Trace:
[72960.716113] [<ffffffff81331f9e>] ? trace_hardirqs_on_thunk+0x3a/0x3c
[72960.716119] [<ffffffff8155710a>] ? retint_restore_args+0xe/0xe
[72960.716124] [<ffffffff810e8ef1>] ? noop_llseek+0xa/0xa
[72960.716129] [<ffffffff810340ff>] ? mutex_spin_on_owner+0x1c/0x45
[72960.716133] [<ffffffff81555be0>] __mutex_lock_slowpath+0xd2/0x116
[72960.716137] [<ffffffff81559edd>] ? do_page_fault+0x374/0x3e6
[72960.716140] [<ffffffff81555a87>] mutex_lock+0x16/0x27
[72960.716146] [<ffffffff812c8ad7>] btrfs_file_llseek+0x38/0x297
[72960.716150] [<ffffffff81331fda>] ? trace_hardirqs_off_thunk+0x3a/0x6c
[72960.716153] [<ffffffff810e8f2c>] vfs_llseek+0x2e/0x30
[72960.716155] [<ffffffff810e9311>] sys_lseek+0x3e/0x5d
[72960.716159] [<ffffffff8155d4fb>] system_call_fastpath+0x16/0x1b

Oddly enough, it's just llseek. I can copy/read the file sequentially
just fine, but llseek fails on a copy as well. Once I get physically
to the system I'll recompile to an unmodified kernel and try again.
Other files of the same approximate age llseek correctly.

Kernel is 3.1-rc1 with the fglrx module from AMD and the following
patch: (I was playing with cross-subvolume reflinking, but not on
this file)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 7cf0133..a13b5e2 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2182,7 +2182,7 @@ static noinline long btrfs_ioctl_clone(struct
file *file, unsigned long srcfd,
goto out_fput;

ret = -EXDEV;
- if (src->i_sb != inode->i_sb || BTRFS_I(src)->root != root)
+ if (src->i_sb != inode->i_sb)
goto out_fput;

ret = -ENOMEM;
@@ -2246,13 +2246,13 @@ static noinline long btrfs_ioctl_clone(struct
file *file, unsigned long srcfd,
* note the key will change type as we walk through the
* tree.
*/
- ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+ ret = btrfs_search_slot(NULL, BTRFS_I(src)->root, &key, path, 0, 0);
if (ret < 0)
goto out;

nritems = btrfs_header_nritems(path->nodes[0]);
if (path->slots[0] >= nritems) {
- ret = btrfs_next_leaf(root, path);
+ ret = btrfs_next_leaf(BTRFS_I(src)->root, path);
if (ret < 0)
goto out;
if (ret > 0)
@@ -2313,7 +2313,7 @@ static noinline long btrfs_ioctl_clone(struct
file *file, unsigned long srcfd,
else
new_key.offset = destoff;

- trans = btrfs_start_transaction(root, 1);
+ trans = btrfs_start_transaction(root, 3);
if (IS_ERR(trans)) {
ret = PTR_ERR(trans);
goto out;