On Tue, Nov 25, 2003 at 04:52:20PM -0500, Kapoor, Pankaj wrote:
> While studying the implementation of tasklets and softirq processing I came
> across certain issues which I have outlined below.
> The function mipsIRQ in the file mipsIRQ.s is the registered interrupt
> handler for all general purpose interrupts.
> The first thing that the function does is that it saves all registers. It
> then checks the CAUSE register to check the source of the interrupt.
> all we are interested in is INT5 (Timer) and INT0 (i.e. all other devices)
> Consider a timer interrupt which would cause the code to jump to 0x8000:0180
> and cause all the registers to be saved (SAVE_ALL). It would then jump to
> mips_timer_interrupt function in the file time.c
> The function services the timer interrupt. At the end of the function there
> is an irq_exit and a check to see if there are any SOFT IRQ pending.
> If there are any the function jumps to the do_softirq function defined in
> the softirq.c. The function gets the softirq pending list, enables
> and cycles through all pending soft irq's calling the appropriate handlers.
> Remember that the interrupts are enabled while executing the various bottom
> half handlers.
> Now there are 2 cases that can happen
> 1. Since we have not exited the ISR and the exception level has still not
> been restored there can be no more interrupts that are generated in the
> system. In such a case does that mean that the all bottom half handlers
> pending execution will run with interrupts disabled.
> NOTE: This does not seem likely because the local_irq_enable routine
> _sti which clears the exception level in the status register and also
> sets the IE bit.
Refer to your own note. It is more correct. :)
> 2. If we have large number of tasklets or if the bottom half handlers take
> to execute, then we could get another timer interrupt or other device
> interrupts causing context saves which would cause the stack to grow and
> CRASH the system.
> Context is restored only when the code returns from do_softirq and uses the
The nested interrupt call, do_IRQ(), may still try to call do_softirq() but
that it will return immediately as it discovers another instance of do_softirq()
is running. No further nesting occurs as a result.