Why do you need to block in mmap()? mmap is supposed to create a mapping area in virtual address space for the process. Actual transfer happens later through page fault handlers on demand basis. look at vm_operations fault/readpage etc methods, these might be the places you want to wait for the data.

On Wed, Feb 26, 2014 at 4:14 PM, Le Tan <tamlokveer@gmail.com> wrote:

So what should I do if I want the mmap() not to return right now? Is
it strange to block in mmap() and few people will do this? Thanks for
your help!

2014-02-27 4:45 GMT+08:00 Rajat Sharma <fs.rajat@gmail.com>:

> 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@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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>
>