On Tue, Nov 25, 2003 at 04:52:20PM -0500, Kapoor, Pankaj wrote: > All, > > 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. > Currently > 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 > the > 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 > interrupts > 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 > calls > _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 > time > 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 > ret_from_irq. > 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. Jun
