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

[Rajat Sharma]

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 at 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 at 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 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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> >
> >
>
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