Date: Fri, 03 Jun 2005 10:18:22 -0700 From: Maksim Yevmenkin <maksim.yevmenkin@savvis.net> To: Norbert Koch <NKoch@demig.de> Cc: "Freebsd-Hackers@Freebsd. Org" <freebsd-hackers@freebsd.org> Subject: Re: synchronization question about /sys/dev/vkbd/vkbd.c Message-ID: <42A090DE.8060002@savvis.net> In-Reply-To: <000001c5683f$8c7c1e60$4801a8c0@ws-ew-3.W2KDEMIG> References: <000001c5683f$8c7c1e60$4801a8c0@ws-ew-3.W2KDEMIG>
index | next in thread | previous in thread | raw e-mail
Norbert,
> When looking at /sys/dev/vkbd/vkbd.c I found
> one thing, that I do not understand.
>
> There are three places, where a flag TASK is used:
>
> 1. in vkbd_dev_close():
>
> while(state->ks_flag & TASK) VKBD_SLEEP (...);
>
> 2. in vkbd_dev_write()
>
> VKBD_LOCK ();
> ...
> if (!(state->ks_flags & TASK) && task_enqueue(...))
> state->ks_flags |= TASK;
> ...
> VKBD_UNLOCK ();
>
> 3. in vkbd_dev_intr()
>
> ...
> /* call intr */
> ...
> VKBD_LOCK();
> state->ks_flags &= ~TASK;
> wakeup(...);
> VKBD_UNLOCK();
>
> As I understand:
> vkbd_dev_write() writes data into its queue
> and wants vkbd_dev_intr() to process the queue.
vkbd_dev_intr() is a "interrupt handler". the real keyboard would
generate interrupt when keys are pressed/released. vkbd(4) does not have
real keyboard. instead, as soon as vkbd_dev_write() puts scancodes into
the queue it schedules vkbd_dev_intr() task (to emulate keyboard
interrupt). the TASK flag is used to indicate the fact that "intrrupt"
is pending and vkbd(4) does not need to schedule one.
> My question is:
> Is it not possible, that vkbd_dev_intr() could be
> interrupted at any position before the VKBD_LOCK()
> and then vkbd_dev_write() called?
in theory it is possible.
> If yes, how should vkbd_dev_write() know, that it should
> call task_enqueue(), as TASK is still set?
well, i guess it is possible to miss interrupt in this case. also, the
scancodes are not lost, they will be processed on next write.
i suspect that the vkbd_dev_intr() should be interrupted exactly in between
(*kbdsw[kbd->kb_index]->intr)(kbd, NULL);
and
VKBD_LOCK(state);
this is because most/all of intr() keyboard methods have something like
while (check_char) {
read_char()
...
}
> Why not always call task_enqueue() unconditionally here
> and leave TASK only to synchronize the close call?
yes, that could be done. it is also possible to have a callout going few
times a second to check if there is a scancodes in the queue and
schedule vkbd_dev_intr(). funny that atkbd(4) and ukbd(4) have just this.
thanks,
max
help
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?42A090DE.8060002>