Blocked I/O in read() and mmap()

[Rajat Sharma]

It seems this task "landscape-sysin" is trying to peek into virtual memory
of your processes and the process within mmap call is holding its
mm->mmap_sem semaphore which grants access to its address space.
landscape-sysin is trying to grab this semaphore to poke into address space
of your mmap process address space. As from your description, it might be
invoked everytime you are opening a new shell. Not sure why this process
bother's about other process address space. Little googling shows this as
relevant to your case:

http://www.techques.com/question/2-66765/Disable-usage-of-console-kit-daemon-in-Ubuntu

Your read process is innocent and not involved in this deadlock.

-Rajat


On Wed, Feb 26, 2014 at 4:13 AM, Le Tan <tamlokveer at gmail.com> wrote:

> Hi, I am writing a driver module. Now I have some questions about blocked
> I/O.
> my_read() is the read function in the file_operations struct in my
> module. my_read() is just as simple as this:
> ssize_t my_read(....)
> {
>     if(wait_event_interruptible(dev->queue, a == b))
>         return -ERESTARTSYS;
>     return count;
> }
> Then I write a simple program to open and read the device. Obviously
> the program will be blocked. Now I still can open a new shell window
> and log in ( I use xshell).
>
> However, then I implement my_mmap(), the mmap function in the
> file_operations struct in my module, like this:
> int my_mmap(....)
> {
>     if(wait_event_interruptible(dev->queue, a == b))
>         return -ERESTARTSYS;
>     return 0;
> }
> Then I write a simple program to open and mmap() the device. Obviously
> the program will be blocked again. However, when I open a new shell
> window in xshell and try to connect to the linux, it displays like
> this:
>
> Connecting to 192.168.146.118:22...
> Connection established.
> To escape to local shell, press 'Ctrl+Alt+]'.
>
> And I can't log in! Then after a while, in the syslog, there is one
> message like this:
> [38306.614103] INFO: task landscape-sysin:17616 blocked for more than
> 120 seconds.
> [38306.614114] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> disables this message.
> [38306.614120] landscape-sysin D ffffffff8180fb60     0 17616  17609
> 0x00000000
> [38306.614125]  ffff88031d609c90 0000000000000082 ffff88032fffdb08
> 0000000000000000
> [38306.614130]  ffff8803130bdc40 ffff88031d609fd8 ffff88031d609fd8
> ffff88031d609fd8
> [38306.614133]  ffff88062150c530 ffff8803130bdc40 0000004100000000
> ffff8803130bdc40
> [38306.614137] Call Trace:
> [38306.614147]  [<ffffffff816b2c49>] schedule+0x29/0x70
> [38306.614151]  [<ffffffff816b3acd>] rwsem_down_read_failed+0x9d/0xf0
> [38306.614157]  [<ffffffff81341824>] call_rwsem_down_read_failed+0x14/0x30
> [38306.614160]  [<ffffffff816b1644>] ? down_read+0x24/0x2b
> [38306.614166]  [<ffffffff81153661>] __access_remote_vm+0x41/0x1f0
> [38306.614170]  [<ffffffff81153ddb>] access_process_vm+0x5b/0x80
> [38306.614175]  [<ffffffff811ea423>] proc_pid_cmdline+0x93/0x120
> [38306.614178]  [<ffffffff811eb425>] proc_info_read+0xa5/0xf0
> [38306.614182]  [<ffffffff81186e84>] vfs_read+0xb4/0x180
> [38306.614185]  [<ffffffff81187102>] SyS_read+0x52/0xa0
> [38306.614189]  [<ffffffff816bc8c2>] system_call_fastpath+0x16/0x1b
>
> If I terminate the program by force, then I can log in right now.
> So, are there any differences between the read and the mmap function
> to the wait_event_interruptible()? Why? If I want to block mmap() just
> like blocking read(), what should I do? Or it is impossible?
> Thanks!
>
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