8556395.925003] XFS (sdi): xfs_log_force: error - returned.
[8556407.425047] INFO: task umount: blocked for more than seconds.----这个默认是120，该环境上是被人为设置1200
[8556407.425653] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[8556407.426050] umount D ffff881fffdd47c0 0x00000080------------------------19286是pid，19283是ppid
[8556407.426056] ffff883e39587d80 ffff883e39587fd8 ffff883e39587fd8
[8556407.426066] ffff883e39587fd8 00000000000147c0 ffff881fd3260000 ffff881f860fec80
[8556407.426072] ffff883ed79e1a20 ffff881f860fecc0 ffff881f860fece8 ffff881f860fec90------------这12个64位的值为栈里的值
[8556407.426078] Call Trace:
[8556407.426092] [<ffffffff8163e879>] schedule+0x29/0x70
[8556407.426137] [<ffffffffa07c6cd1>] xfs_ail_push_all_sync+0xc1/0x110 [xfs]
[8556407.426143] [<ffffffff810a7610>] ? wake_up_atomic_t+0x30/0x30
[8556407.426165] [<ffffffffa07b1b98>] xfs_unmountfs+0x68/0x190 [xfs]
[8556407.426186] [<ffffffffa07b267b>] ? xfs_mru_cache_destroy+0x6b/0x90 [xfs]
[8556407.426204] [<ffffffffa07b4a52>] xfs_fs_put_super+0x32/0x90 [xfs]
[8556407.426211] [<ffffffff811e3026>] generic_shutdown_super+0x56/0xe0-------已经执行完fsnotify_umount_inodes
[8556407.426215] [<ffffffff811e3477>] kill_block_super+0x27/0x70
[8556407.426218] [<ffffffff811e37b9>] deactivate_locked_super+0x49/0x60
[8556407.426222] [<ffffffff811e3db6>] deactivate_super+0x46/0x60
[8556407.426229] [<ffffffff81200d25>] mntput_no_expire+0xc5/0x120
[8556407.426234] [<ffffffff812025cb>] SyS_umount+0x15b/0x5b0
[8556407.426239] [<ffffffff81649909>] system_call_fastpath+0x16/0x1b
[8556407.426242] sending NMI to all CPUs:
对应的函数为：
/*
* Push out all items in the AIL immediately and wait until the AIL is empty.
*/
void
xfs_ail_push_all_sync(
struct xfs_ail *ailp)
{
struct xfs_log_item *lip;
DEFINE_WAIT(wait); spin_lock(&ailp->xa_lock);
while ((lip = xfs_ail_max(ailp)) != NULL) {
prepare_to_wait(&ailp->xa_empty, &wait, TASK_UNINTERRUPTIBLE);
ailp->xa_target = lip->li_lsn;
wake_up_process(ailp->xa_task);
spin_unlock(&ailp->xa_lock);
schedule();
spin_lock(&ailp->xa_lock);
}
spin_unlock(&ailp->xa_lock); finish_wait(&ailp->xa_empty, &wait);
}
排查的时候走了弯路，因为看的是：
crash> dis -l xfs_ail_push_all_sync+0xc1
/usr/src/debug/kernel-3.10.-327.22..el7/linux-3.10.-327.22..el7.x86_64/include/linux/spinlock.h:
0xffffffffa07c6cd1 <xfs_ail_push_all_sync+>: mov %r13,%rdi
所以想当然地认为在第一个 获取spinlock的地方，
spin_lock(&ailp->xa_lock);---------以为在这hung住
while ((lip = xfs_ail_max(ailp)) != NULL) {
然后查找这把锁：
crash> struct super_block.s_fs_info ffff883f7fe67800
s_fs_info = 0xffff883f7fe67000
crash> mod -s xfs
MODULE NAME SIZE OBJECT FILE
ffffffffa0803d40 xfs /lib/modules/3.10.-327.22..el7.x86_64/kernel/fs/xfs/xfs.ko
crash> struct -xo xfs_mount.m_ail 0xffff883f7fe67000
struct xfs_mount {
[ffff883f7fe67010] struct xfs_ail *m_ail;
} struct xfs_mount.m_ail 0xffff883f7fe67000
m_ail = 0xffff881f860fec80 crash> struct -xo xfs_ail.xa_lock 0xffff881f860fec80
struct xfs_ail {
[ffff881f860fecc0] spinlock_t xa_lock;
}
而且这把锁的值是：
crash> struct xfs_ail.xa_lock 0xffff881f860fec80
xa_lock = {
{
rlock = {
raw_lock = {
{
head_tail = ,
tickets = {
head = ,
tail =
}
}
}
}
}
}
我一看到这个是非0值，想当然认为是被持有了，为了验证我的想法，我甚至随便找了一把锁查看它的值，点背的时候，喝凉水都塞牙，我随手选择的锁是：
crash> p gc_lock
gc_lock = $ = {
{
rlock = {
raw_lock = {
{
head_tail = ,
tickets = {
head = ,
tail =
}
}
}
}
}
}
后来仔细看了在没有开启 CONFIG_QUEUED_SPINLOCKS 的情况下，锁的定义才是如此，否则的话，是如下：
#ifndef __GENKSYMS__
typedef struct qspinlock {
atomic_t val;
} arch_spinlock_t;
#else
typedef u32 __ticketpair_t; typedef struct arch_spinlock {
union {
__ticketpair_t head_tail;
struct __raw_tickets {
__ticket_t head, tail;
} tickets;
};
} arch_spinlock_t;
#endif
所以要么0，要么1的情况只适合于第一种定义，以前因为业务默认配置的config问题，查的锁相关的crash，大多是第一种定义，但是当时没注意到这个细节，只是简单扫了一眼，浪费我两个小时，所以下盘不稳总会摔。
查看running状态的都没有持有这把锁，（其实按照第二种定义，是正常的，因为此时本就没人持有这把锁，因为head和tail相等）比较奇怪，后来找同事讨论，同事认为是while循环没跳出，所以获取锁的那行是schedule下面那行，还没有执行。
想起来自己曾经写过的博客《https://www.cnblogs.com/10087622blog/p/9558024.html》,应该是如此
void
xfs_ail_push_all_sync(
struct xfs_ail *ailp)
{
struct xfs_log_item *lip;
DEFINE_WAIT(wait); spin_lock(&ailp->xa_lock);
while ((lip = xfs_ail_max(ailp)) != NULL) {
prepare_to_wait(&ailp->xa_empty, &wait, TASK_UNINTERRUPTIBLE);
ailp->xa_target = lip->li_lsn;
wake_up_process(ailp->xa_task);
spin_unlock(&ailp->xa_lock);
schedule();--------------------这个和while循环一直跳不出去，
spin_lock(&ailp->xa_lock);
}
spin_unlock(&ailp->xa_lock); finish_wait(&ailp->xa_empty, &wait);
}
跳不出去的原因跟io相关，也就是wake_up 相关的内核线程之后，内核线程还是没有完成对应的任务：
crash> struct xfs_ail.xa_task 0xffff881f860fec80
xa_task = 0xffff883f8901dc00 crash> task_struct.pid 0xffff883f8901dc00
pid = crash> set
PID:
COMMAND: "xfsaild/sdi"
TASK: ffff883f8901dc00 [THREAD_INFO: ffff881f3d828000]
CPU:
STATE: TASK_INTERRUPTIBLE
crash> bt
PID: TASK: ffff883f8901dc00 CPU: COMMAND: "xfsaild/sdi"
# [ffff881f3d82bcd0] __schedule at ffffffff8163df9b
# [ffff881f3d82bd38] schedule at ffffffff8163e879
# [ffff881f3d82bd48] schedule_timeout at ffffffff8163c295
# [ffff881f3d82bdf8] xfsaild at ffffffffa07c6a4f [xfs]
# [ffff881f3d82bec8] kthread at ffffffff810a661f
# [ffff881f3d82bf50] ret_from_fork at ffffffff81649858
xfsaild 无法完成的原因也是因为IO异常:
附部分相关io异常的message日志：
kernel: sd :::: [sdi] CDB: Read() 4c b0
kernel: sd :::: [sdi] CDB: Read() 8a
kernel: blk_update_request: I/O error, dev sdi, sector
kernel: sd :::: rejecting I/O to offline device
kernel: XFS (sdi): metadata I/O error: block 0xee173de0 ("xfs_trans_read_buf_map") error numblks
kernel: XFS (sdi): metadata I/O error: block 0x15da29d90 ("xfs_trans_read_buf_map") error numblks
kernel: sd :::: rejecting I/O to offline device
kernel: XFS (sdi): xfs_imap_to_bp: xfs_trans_read_buf() returned error -.
kernel: XFS (sdi): metadata I/O error: block 0xeca8a480 ("xfs_trans_read_buf_map") error numblks
kernel: sd :::: rejecting I/O to offline device
kernel: sd :::: rejecting I/O to offline device
对应的bug解决可以参照：https://bugzilla.redhat.com/show_bug.cgi?id=1267042