linux-kernel - Query: Crash is coming during /prod/PID/stat and do

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Date:   Tue, 9 Jan 2018 19:03:05 +0530
From:   "Kohli, Gaurav" <gkohli@...eaurora.org>
To:     peterz@...radead.org, john.ogness@...utronix.de, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Query: Crash is coming during /prod/PID/stat and do_exit of same task

HI ,

We are seeing crash in do_task_stat while accessing stack pointer, It 
seems same task has already completed do_exit call.
So it seems a race between them:

Below is the crash trace:
49750.534377] Kernel BUG at ffffff8e7a4c53a8 [verbose debug info 
unavailable]
[49750.534394] task: ffffffe7b4475580 task.stack: ffffffe7a5f0c000
[49750.534400] PC is at do_task_stat+0x740/0x908
[49750.534402] LR is at do_task_stat+0xa4/0x908
[49750.534403] pc : [<ffffff8e7a4c53a8>] lr : [<ffffff8e7a4c4d0c>] 
pstate: 80400145
[49750.534404] sp : ffffffe7a5f0fbd0

and here is stack trace on that core:

-000|user_stack_pointer(inline)
-000|do_task_stat(
     |    m = 0xFFFFFFE7A5CD7380,
     |    ns = 0xFFFFFF8E7C43C748,
     |  ?,
     |    task = 0xFFFFFFE80D8C2280,
     |  ?)
     |  tty_pgrp = 0
     |  ppid = 2084696064
     |  sid = 0
     |  mm = 0xFFFFFFE7B4424140
     |  tcomm = (84, 9, 71, 122, 142, 255, 255, 255, 48, 253, 240, 165, 
231, 255, 255, 255)
     |  flags = 18446743969119403392
-001|proc_tgid_stat(
     |    m = 0xFFFFFFE7A5CD7380,
     |  ?,

Below are task stats which shows , process completed the do_exit call:
struct task_struct.flags -x 0xFFFFFFE80D8C2280
   flags = 0x40870c

crash_64> struct task_struct.exit_code -x 0xFFFFFFE80D8C2280
   exit_code = 0x6

    struct task_struct.state -x 0xFFFFFFE80D8C2280
   state = 0x40

In our build both patches are there ,
fs/proc: report eip/esp in /prod/PID/stat for coredumping

and also  task.state has already set PF_DUMPCORE as it got the sigabrt 
signal.

Regards
Gaurav


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