Sunday, November 25, 2007

INTR_FAST and selwakeup

I stumbled over this problem a couple of years ago, ignored it, was punished for my carelessness, came out with hackerish solution and now I know "The Right Way" to solve this issue. So, the headache starts when you're trying to wakeup userland application polling on a descriptor from INTR_FAST IRQ handler (for 7.X and later that would be interrupt filter). INTR_FAST/interrupt filter routines are usually used for timing-critical tasks and run in IRQ dispatcher context, so no operations that may cause context switch are allowed in this code. Unfortunately selwakeup(9) tries to acquire sellock that leads to a possible context switch and leaves us in a total mess. My solution was handmade kernel thread that has been running through list of channels and performed all dirty work, not the cleanest and easiest to maintain code. "Zaptel-bsd take 2" utilizes taskqueue(9) interface to work around IRQ handler limitations. There is handler routine:

static void
handle_selwakeup(void *context, int pending)
struct selinfo *sel = context;

then selinfo/task structures for every channel:

/* thingy for select stuff */
struct selinfo sel;
struct task selwakup_task;

which should be initialized during channel allocation:

TASK_INIT(&chan->selwakup_task, 0,
handle_selwakeup, &chan->sel);

and used anytime we'd like to call selwakeup:


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