crash工具分析大型Linux服务器死锁实战

Linux服务器背景：

CPUS: 40

MEMORY: 127.6 GB

MACHINE: x86_64 (2199 Mhz)

Linux Kernel: 4.4.121

TASKS: 19411

其实这不算大型服务器，我见过大型的一般内存4T起步，300多个cpu.

故障背景：

客户发现系统卡死，手动触发了kdump.

使用下面命令发现有3092个D状态进程。

crash> ps | grep UN | wc -l

3092

小编头一回碰到这么多进程变成D状态，第一感觉应该就是死锁导致。

于是:

crash> ps | grep UN > UN (导入UN文件慢慢玩）

发现UN里面有大量的ps、sudo这样的进程。

于是挑了一个ps进程查看它的堆栈：

crash> bt 50486PID: 50486  TASK: ffff881171480c80  CPU: 18  COMMAND: "ps" #0 [ffff8810d1d9bce0] schedule at ffffffff815f353d #1 [ffff8810d1d9bd40] rwsem_down_read_failed at ffffffff815f61ea #2 [ffff8810d1d9bd98] call_rwsem_down_read_failed at ffffffff813271d4 #3 [ffff8810d1d9bde0] down_read at ffffffff815f58b3 #4 [ffff8810d1d9bde8] proc_pid_cmdline_read at ffffffff8126c5e8 #5 [ffff8810d1d9be70] __vfs_read at ffffffff81202fd6 #6 [ffff8810d1d9bee8] vfs_read at ffffffff8120360a #7 [ffff8810d1d9bf18] sys_read at ffffffff81204382

先看下proc_pid_cmdline_read 函数源码：

static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,                                     size_t _count, loff_t *pos){        ...        tsk = get_proc_task(file_inode(file));        if (!tsk)                return -ESRCH;        mm = get_task_mm(tsk);        down_read(&mm->mmap_sem);        ...}
void __sched down_read(struct rw_semaphore *sem)

从源码中知道 ps命令是在查询某个进程的状态时，执行down_read()没有获取到要查询进程struct mm_struct上的sem(读写锁)

所以下面开始汇编几个函数，先找到sem的值：

crash> dis proc_pid_cmdline_read....0xffffffff8126c5d7 <proc_pid_cmdline_read+135>: lea    0x68(%r15),%rax0xffffffff8126c5db <proc_pid_cmdline_read+139>: mov    %rax,%rdi0xffffffff8126c5de <proc_pid_cmdline_read+142>: mov    %rax,0x18(%rsp)0xffffffff8126c5e3 <proc_pid_cmdline_read+147>: callq  0xffffffff815f58a0 <down_read>
crash> dis down_read0xffffffff815f58a0 <down_read>: nopl   0x0(%rax,%rax,1) [FTRACE NOP]0xffffffff815f58a5 <down_read+5>:       mov    %rdi,%rax0xffffffff815f58a8 <down_read+8>:       lock incq (%rax)0xffffffff815f58ac <down_read+12>:      jns    0xffffffff815f58b3 <down_read+19>0xffffffff815f58ae <down_read+14>:      callq  0xffffffff813271c0 <call_rwsem_down_read_failed>0xffffffff815f58b3 <down_read+19>:      retq   
crash> dis call_rwsem_down_read_failed0xffffffff813271c0 <call_rwsem_down_read_failed>:       push   %rdi0xffffffff813271c1 <call_rwsem_down_read_failed+1>:     push   %rsi0xffffffff813271c2 <call_rwsem_down_read_failed+2>:     push   %rcx0xffffffff813271c3 <call_rwsem_down_read_failed+3>:     push   %r80xffffffff813271c5 <call_rwsem_down_read_failed+5>:     push   %r90xffffffff813271c7 <call_rwsem_down_read_failed+7>:     push   %r100xffffffff813271c9 <call_rwsem_down_read_failed+9>:     push   %r110xffffffff813271cb <call_rwsem_down_read_failed+11>:    push   %rdx0xffffffff813271cc <call_rwsem_down_read_failed+12>:    mov    %rax,%rdi0xffffffff813271cf <call_rwsem_down_read_failed+15>:    callq  0xffffffff815f6100 <rwsem_down_read_failed>

发现call_rwsem_down_read_failed() 中push %rdi ,%rdi就是 down_read()的参数，也就是sem的值。

下面把call_rwsem_down_read_failed()的堆栈读出来找参数：

crash> rd  ffff8810d1d9bd98 -e ffff8810d1d9bde0ffff8810d1d9bd98:  ffffffff813271d4 ffff880000000000   .q2.............ffff8810d1d9bda8:  0000000000024200 ffff88203f0226f0   .B.......&.? ...ffff8810d1d9bdb8:  00000000004037ae 0000000000000078   .7@.....x.......ffff8810d1d9bdc8:  0000000000000000 0000000000000020   ........ .......ffff8810d1d9bdd8:  ffff88203c8fc0a8

发现ffff88203c8fc0a8 就是sem的值，

crash> whatis -o mm_struct...   [100] spinlock_t page_table_lock;   [104] struct rw_semaphore mmap_sem; (offset 0x68)

所以

mm_struct-> ffff88203c8fc040 (0xffff88203c8fc0a8 - 0x64)

所以马上就能知道ps命令在查询哪个进程时不能获取到sem锁。

crash> struct mm_struct.owner,mm_users  ffff88203c8fc040  owner = 0xffff881f17b044c0  mm_users = {    counter = 3952
crash> struct task_struct.comm,pid 0xffff881f17b044c0  comm = "hmsserver\000st\000\000\000"  pid = 21909

发现那个进程是21909,知道了是这个进程，有没有用呢，看天意了，先查询它的堆栈再说：

crash> bt 21909PID: 21909  TASK: ffff881f17b044c0  CPU: 15  COMMAND: "hmsserver" #0 [ffff881f2ab43dd0] schedule at ffffffff815f353d #1 [ffff881f2ab43e30] rwsem_down_read_failed at ffffffff815f61ea #2 [ffff881f2ab43e80] call_rwsem_down_read_failed at ffffffff813271d4 #3 [ffff881f2ab43ec8] down_read at ffffffff815f58b3 #4 [ffff881f2ab43ed0] __do_page_fault at ffffffff81066d31 #5 [ffff881f2ab43f28] do_page_fault at ffffffff81066dcb #6 [ffff881f2ab43f50] page_fault at ffffffff815fa822

发现并没有什么用，它自己都没能获取到sem锁.

这样就放弃吗? 生活还得继续，再说客户也不答应啊:)

从读写锁rw_semaphore 开始柯南侦探模式：

crash> struct rw_semaphore -x ffff88203c8fc0a8struct rw_semaphore {  count = 0xffffffff00000001,   wait_list = {    next = 0xffff880ee85bfe10,     prev = 0xffff88117aa67d50  },   ...  owner = 0x0 }

发现 owner = 0x0, 说明是一个读者持有了这把锁，这又增加了难度，

crash> struct -o rw_semaphorestruct rw_semaphore {   [0] long count;   [8] struct list_head wait_list;  [24] raw_spinlock_t wait_lock;  [28] struct optimistic_spin_queue osq;  [32] struct task_struct *owner;}

这样，然后我就把所有等待这个锁的进程全部找出来，把持有sem锁地址

ffff88203c8fc0a8(rw_semaphore)的进程也全部找出来，第一个输出结果有2947个进程，第二个的结果有3032个进程。一般持有这个sem锁的进程在3032-2947的进程上，我找了好几个小时都没看出端倪，好吧，下班吧，难搞哦。

crash> list rwsem_waiter.list -s rwsem_waiter.task,type -h 0xffff880ee85bfe10

crash> search -t  ffff88203c8fc0a8

从上面的分析来看，完全找不到线索，而且进程特别特别多，但是小编也没放弃，吃这碗饭，也没办法:)

于是看下sudo进程，找出一个sudo进程(sudo和ps看起来功能就完全没交叉)：

PID: 22216  TASK: ffff8810ec908e80  CPU: 15  COMMAND: "sudo" #0 [ffff880f1d09fc78] schedule at ffffffff815f353d #1 [ffff880f1d09fcd8] schedule_preempt_disabled at ffffffff815f3e2e #2 [ffff880f1d09fce8] __mutex_lock_slowpath at ffffffff815f5665 #3 [ffff880f1d09fd48] mutex_lock at ffffffff815f56f3 #4 [ffff880f1d09fd58] rtnetlink_rcv at ffffffff81512135 #5 [ffff880f1d09fd68] netlink_unicast at ffffffff815331c3 #6 [ffff880f1d09fda0] netlink_sendmsg at ffffffff815335b3 #7 [ffff880f1d09fe10] sock_sendmsg at ffffffff814e6cc6 #8 [ffff880f1d09fe28] SYSC_sendto at ffffffff814e70f7 #9 [ffff880f1d09ff50] entry_SYSCALL_64_fastpath at ffffffff815f7683

这里又弄出mutex_lock，真是一波未平，一波又起。

用同样的方法找mutex_waiter的地址

void __sched mutex_lock(struct mutex *lock)

crash> rd -SS ffff880f1d09fce8 -e ffff880f1d09fd48ffff880f1d09fce8:  __mutex_lock_slowpath+149 ffff880126b63cf0 ffff880f1d09fcf8:  ffff880fd53f7cf0 [ffff8810ec908e80:task_struct] ffff880f1d09fd08:  00000000024000c0 0000000000000180 ffff880f1d09fd18:  rtnl_mutex       [ffff8810d2971800:kmalloc-256] ffff880f1d09fd28:  0000000000000014 0000000000000000 ffff880f1d09fd38:  0000000000000000 [ffff8810a00ba000:kmalloc-2048]

从堆栈来看ffff880fd53f7cf0就是mutex_waiter，rtnl_mutex 就是mutex_lock的参数。

这样有些眉目了，在内核源码中找到 rtnl_mutex是在下面文件中调用。

net/core/rtnetlink.c:
void rtnl_lock(void){        mutex_lock(&rtnl_mutex);}

所以看下mutex的owner是谁？

crash> struct mutex 0xffffffff81f0a180struct mutex {  count = {    counter = -143  }, ...  },   wait_list = {    next = 0xffff880f077d3dd0,     prev = 0xffff88020545fcf0  },   owner = 0xffff88103e4dc300, ...}

crash> struct task_struct.pid,comm 0xffff88103e4dc300  pid = 26299  comm = "kworker/7:2\000\000\000\000"

owner找到了，是kworker/7:2进程，看下它的堆栈，发现是i40e内核驱动模块持有了mutex锁。

crash> bt 26299PID: 26299  TASK: ffff88103e4dc300  CPU: 7   COMMAND: "kworker/7:2" #0 [ffff880f4235fba8] schedule at ffffffff815f353d #1 [ffff880f4235fc08] schedule_timeout at ffffffff815f63e1 #2 [ffff880f4235fcb0] msleep at ffffffff810ee599 #3 [ffff880f4235fcc0] napi_disable at ffffffff814fd4be #4 [ffff880f4235fcd8] i40e_down at ffffffffa0628ab9 [i40e] #5 [ffff880f4235fd10] i40e_vsi_close at ffffffffa0628b73 [i40e] #6 [ffff880f4235fd30] i40e_close at ffffffffa0628ef1 [i40e] #7 [ffff880f4235fd38] i40e_pf_quiesce_all_vsi at ffffffffa0628c8a [i40e] #8 [ffff880f4235fd50] i40e_prep_for_reset at ffffffffa0628d81 [i40e] #9 [ffff880f4235fd70] i40e_do_reset at ffffffffa062ccf3 [i40e]#10 [ffff880f4235fd90] i40e_service_task at ffffffffa062ead2 [i40e]#11 [ffff880f4235fe38] process_one_work at ffffffff81097984#12 [ffff880f4235fe78] worker_thread at ffffffff81098566#13 [ffff880f4235fed0] kthread at ffffffff8109da59#14 [ffff880f4235ff50] ret_from_fork at ffffffff815f7aaf

接下来查看内核i40e驱动代码:

i40e_service_task  i40e_reset_subtask    (rtnl_lock) i40e_do_reseti40e_quiesce_vsi  vsi->netdev->netdev_ops->ndo_stop(vsi->netdev);

发现在 i40e_reset_subtask确实持有了锁.

list mutex_waiter.list -s mutex_waiter.task -h 0xffff880f077d3dd0 > mutex_list-0xffff880f077d3dd0

因为在上面发现i40e_reset_subtask函数持有了rtnl_lock（mutex锁），

所以看下这部分源码：

static void i40e_reset_subtask(struct i40e_pf *pf){        u32 reset_flags = 0;
        rtnl_lock();        ...if (reset_flags && !test_bit(__I40E_DOWN, &pf->state) && !test_bit(__I40E_CONFIG_BUSY, &pf->state))   i40e_do_reset(pf, reset_flags);
unlock:        rtnl_unlock();}

发现确实调用i40e_do_reset之后 schedule()切换走了。

void __rtnl_unlock(void){        mutex_unlock(&rtnl_mutex);}
void rtnl_unlock(void){        /* This fellow will unlock it for us. */        netdev_run_todo();}