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
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.
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
Is there anything that I am missing in this whole picture ?