Re: interruptible_sleep_on, interrupts and device drivers

From: Tushar Adeshara
Date: Sat Oct 15 2005 - 08:10:57 EST

On 10/15/05, Gabriele Brugnoni <news@xxxxxxxxxxxxxxx> wrote:
> Hello,
> I'm writing a device driver for a UART to be used to drive a RS485 line.
> I would use the interruptible_sleep_on function to wait for receiver ready,
> or for the transmitter to finish.
> I've take a look at how other device driver use this function, and a big
> question is born on my mind:
> In several device driver I've seen that:
> The code check for the availability of data.
> If not available, a call the interruptible_sleep_on is made.
> In the interrupt handler, when data becomes ready, a wakeup_interruptible
> call is made.
> Often the variables that indicates if data is available or not are tested
> between spin_lock and unlock (or save_irq and restore_irq) so that the
> interrupt handler cannot change the status during check.
> But:
> If the test is made with IRQ closed, and IRQ are then enabled after the test
> but before the call to interruptible_sleep_on, what happen if the handler
> break the procedure immediately before entering the interruptible_sleep_on
> function ?
> I beleave that the interrupt handler, calling the wakeup function, will not
> wake our process, because is not in the waiting list. But at return from
> IRQ handler, the process will continue execution calling the sleep
> function, and nobody will wake it because the data is now already available.
> I try to explain better with an example:
> This may be a function that should wait that the UART send all chars.
> When the interrupt for shift empty will be serviced, the
> wakeup_interruptible function will wake all waiting processes in wait list.
> 10: void waitEndTX ( void )
> 11: {
> 12: unsigned long flags;
> 13:
> 14: //....
> 15:
> 16: spin_lock_irqsave(&mylock, flags);
> 17: if( tx_done ) {
> 18: spin_lock_irqrestore(&mylock, flags);
> 19: return;
> 20: }
> 21: spin_lock_irqrestore(&mylock, flags);
> 22: interruptible_sleep_on ( &uartwait );
> 23:}
> This type of code is very used in a lot of device drivers, in kernel 2.6.x
> (the char/specialix, for example).
> OK, Now we suppose that the UART finish to send the character exactely when
> the process has closed the IRQ and is about to the the tx_done variable.
> (At line 17 for example).
> The hardware will asserts the IRQ line, but the processor has the IRQ
> closed, so the execution will continue in our process.
> Now at line 21 our process enable the IRQ.
> The CPU will immediately jumps to irq handler, breaking the execution at
> line 22. Our handler will set the variable tx_done to one.
> When the cpu control returns to our calling process, at line 21, the tx_done
> has already tested and it's value was zero. The execution continue with the
> call to interruptible_sleep_on function, that will put our process in the
> uartwait queue, but nobody will wakeup it.
> If this is true, a lot of device driver in linux kernel may have this
> problem. This sound me very strange, so where I'm wrong ?

That is the reason why wait_event and friends are introduced.
See LDD3 , Ch 10. Interrupt Handling. It contains a good example
of simple char driver which also takes care of missed Interrupt.

> Thanks in advance.
> Gabriele
> (Note: I've tried to post this message with another program but I think
> without success. I apologize if this message will appears twice.)
> -
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It's not a problem, it's an opportunity for improvement. Lets improve.
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