Re: Sleeping thread not receive signal until it wakes up

[Luong Ngo]

On 3/8/07, Parav K Pandit <parav_pandit@xxxxxxxxxxxx> wrote:
> -----Original Message-----
> From: linux-kernel-owner@xxxxxxxxxxxxxxx
> [mailto:linux-kernel-owner@xxxxxxxxxxxxxxx] On Behalf Of Luong Ngo
> Sent: Friday, March 09, 2007 8:54 AM
> To: Robert Hancock
> Cc: linux-kernel; tglx@xxxxxxxxxxxxx
> Subject: Re: Sleeping thread not receive signal until it wakes up
>
> On 3/8/07, Robert Hancock <hancockr@xxxxxxx> wrote:
> > Luong Ngo wrote:
> > > Hi Thomas and Dick,
> > > I appreciate all the responses. They are very good information to me.
> > > Actually, it wasn't me working on the driver but it's been there long
> > > time. I thought I just need to add the signal and signal handling
> > > part, not expecting it would lead me to the driver space.
> > > Here is what I have in the driver. Maybe racing condition could happen
> > > in scenario that the ioctl realease the lock but befor going to sleep,
> > > the ISR is invoked and call waking up on the queue, hence the ioctl
> > > will not be waken up since the wak up cal already executed. But I
> > > believe in our system, this could be tolerant since the hardware would
> > > keep raising interrupt if the abnormal condition still exists (Due to
> > > the ioctl being blocked so user app nevers get a chance to service the
> > > device). But is this the reason why my signal handler not get executed
> > > at all? Theoretically, according to the Richard Stevens book, I think
> > > the process should be waken up and received the signal even if it gets
> > > blocked in the IOCTL call, am i right?
> >
> > ..
> >
> > > static int ats89_ioctl(struct inode *inode, struct file *file, u_int
> > > cmd, u_long arg)
> > > {
> > >
> > >          switch(cmd){
> > >           case GET_IRQ_CMD: {
> > >            u32  regMask32;
> > >
> > >           spin_lock_irq(dev->lock);
> > >           while ((dev->irqMask & dev->irqEvent) == 0) {
> > >                 // Sleep until board interrupt happens
> > >                 spin_unlock_irq(dev->lock);
> > >                 interruptible_sleep_on(&(dev->boardIRQWaitQueue));
> > >                 if (uncond_wakeup) {
> > >                     /* don't go back to loop */
> > >                     break;
> > >                 }
> > >                 spin_lock_irq(dev->lock);
> > >             }
> >
> > Kernel code does not get pre-empted by signals. If the code needs to be
> > interruptible by signals this has to be handled explicitly.
> > interruptible_sleep on will stop waiting if your task gets a signal, but
> > your code doesn't check the signal_pending flag to know whether it
> > should exit the loop. If signal_pending(current) is set after the sleep
> > you should likely be returning -ERESTARTSYS to allow the task to handle
> > the signal. Then after the signal handler from the task returns, the
> > ioctl will get called again.
> >
> > Also, as was pointed out, you should not use the sleep_on family of
> > functions, use the wait_event functions intead. sleep_on is racy, if the
> > interrupt happened just before you do the sleep, you'll sit there
> > waiting for something that already occurred.
> >
> > --
> > Robert Hancock      Saskatoon, SK, Canada
> > To email, remove "nospam" from hancockr@xxxxxxxxxxxxx
> > Home Page: http://www.roberthancock.com/
> >
> >
> > Robert, thanks a lot for your suggestion
> > But I have added the signal_pending(current) check and signal handler
> > is not invoked
>
> >           spin_lock_irq(dev->lock);
> >           while ((dev->irqMask & dev->irqEvent) == 0) {
> >                 // Sleep until board interrupt happens
> >                 spin_unlock_irq(dev->lock);
> >                 interruptible_sleep_on(&(dev->boardIRQWaitQueue));
> >
> >                 if(signal_pending(current) {
> >                           return -ERESTARTSYS;
> >                 }
> >
> >                 if (uncond_wakeup) {
> >                     /* don't go back to loop */
> >                     break;
> >                 }
> >                 spin_lock_irq(dev->lock);
> >             }
> >Still no luck yet.
> >LNgo
> >-
>
> I guess you need to call allow_signal(xxx) before you go for sleep.
> Parav
>
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Thanks Parav, adding singal_allow(SIGALRM) wakeup the blocking
interruptible_sleep_on and checking the signal_pending would return
true now. But a quick question is when detecting signal_pending, I
return -ERESTARTSYS, but the user process receive the return value of
ioctl call is -1, shouldn't it be -4 for EINTR?

Thanks all for the responses,
-LNgo
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