Re: Sleeping thread not receive signal until it wakes up

[Luong Ngo]

On 3/7/07, linux-os (Dick Johnson) <linux-os@xxxxxxxxxxxx> wrote:
> >
> > On 3/7/07, linux-os (Dick Johnson) <linux-os@xxxxxxxxxxxx> wrote:
> >>
> >> On Wed, 7 Mar 2007, Luong Ngo wrote:
> >>
> >>> Hi all,
> >>>
> >>> I am having this problem. I have a process with 2 threads created. One
> >>> of the thread will keep calling IOCTL  to get information from the
> >>> kernel and will be blocked if there is no new information. If there is
> >>> information retured, the thread will be checked to see if any error
> >>> happens and trigger an action. Since we have no way to know if the
> >>> error is gone (Hardware provides no signal), so what we do is when
> >>> trigger an action for the error, we will set an timer using alarm()
> >>> and register a SIGALRM handler in the thread by using sigaction. After
> >>> setting the alarm, the thread will loop back and call IOCTL, which
> >>> could cause it to be put to sleep. The problem is the SIGALRM handler
> >>> does not receive the SIGALRM while the thread is being blocked by
> >>> IOCTL. And if we generated some event so that the IOCTL is returned
> >>> with new information, the SIGALRM handler is invoked right away.
> >>> However, as I read the manual, which says a thread/process should be
> >>> waken up even when it sleeps if there is a signal delivered to it. Am
> >>> I right?
> >>> One thing I don't know it mattters or not is that I am not using
> >>> sigwait to block the process and wait for signal because the thread
> >>> need to go back to the IOCTL call and be slept on that. So I used
> >>> sigaction to register the signal handler in hope that this handler wil
> >>> be invoked by the kernel when there is an SIGALRM delivered to the
> >>> thread.
> >>> Could anyone tell me if I did something wrong and what is the correct
> >>> way to achieve this task? I tried to avoid creating another thread
> >>> which will call sigwait and block until the IOCTL thread send it
> >>> explicitly a signal because I want to use timer.
> >>>
> >>>
> >>> Thank you in advance,
> >>> LNgo
> >>> -
> >>
> >> Later versions of the kernel lock the kernel when an ioctl() is
> >> entered. This means that if you sleep in the ioctl(), nothing
> >> will get scheduled.
> >>
> >> You can do the following (possibly unsafe) in your ioctl():
> >>
> >>     int locked = kernel_locked();
> >>
> >>     ......... code
> >>     ......... code
> >>
> >>     if(locked)  // Before sleeping section
> >>        unlock_kernel();
> >>     .......... sleeping code
> >>     if(locked)  // After sleeping section
> >>         lock_kernel();
> >>
> >>
> >>
> >> Cheers,
> >> Dick Johnson
> >> Penguin : Linux version 2.6.16.24 on an i686 machine (5592.71 BogoMips).
> >> New book: http://www.AbominableFirebug.com/
> >> _
> >>
> >> Thank you.
> >>
> On Wed, 7 Mar 2007, Luong Ngo wrote:
>
> > Hi Dick,
> > Thanks for your response. In my ioctl in the kernel, I use
> > interruptible_sleep_on to sleep on a queue and will be wake up by the
> > the ISR routine when interrupt happens, so  isn't
> > interruptible_sleep_on supposed to be interruptable, from its name? I
> > am using kernel 2.6.14.
> >
> > Thanks again,
> > LNgo
>
> Please don't "top post," you need to put answers at the bottom.
>
> Interruptible_sleep_on is interruptible, but for your task to
> actually be awakened and your alarm handler to get some CPU,
> it needs to be scheduled. If the BKL (big kernel lock) is
> held, it won't be scheduled until it is released.
>
> So, even though the semaphore that the "wake_up_interruptible()"
> function called, has been enabled, not a lot will happen until the
> kernel lock is released. The ISR code that executed
> wake_up_interruptible() doesn't schedule. It just returns to your
> interrupt handler.
>
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.16.24 on an i686 machine (5592.71 BogoMips).
> New book: http://www.AbominableFirebug.com/
> _
> 
>
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>
> Thank you.

I added the check if kernel_locked and then unlock_kernel if it is but
the signal handler is still not invoked.
Here is how my code is

static int my_ioctl(...)
{
     case GET_STH:



             spin_lock_irq(dev->lock);
              ...................
              locked = kernel_locked();
             if(locked)
                 unlock_kernel();

             spin_unlock_irq(dev->lock);
             interruptible_sleep_on(&qu);

            if(locked)
                lock_kernel();
            spin_lock_irq(dev->lock);

            break;
}


Anything kernel configuration I need to be aware of to enable
preemption in kernel?


Thank you,
LNgo
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