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>
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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
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