Messages in this thread | | | Date | Mon, 19 Mar 2007 22:40:39 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] vt: fix a potential race in the VT_WAITACTIVE handler |
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On Thu, 15 Mar 2007 15:10:23 +0100 Michal Januszewski <spock@gentoo.org> wrote:
> On a multiprocessor machine the VT_WAITACTIVE ioctl call may return 0 > if fg_console has already been updated in redraw_screen(), but the > console switch itself hasn't been completed. Fix this by checking > fg_console in vt_waitactive() with the console sem held. > > Signed-off-by: Michal Januszewski <spock@gentoo.org> > > --- > diff --git a/drivers/char/vt_ioctl.c b/drivers/char/vt_ioctl.c > index 3a5d301..00b5b34 100644 > --- a/drivers/char/vt_ioctl.c > +++ b/drivers/char/vt_ioctl.c > @@ -1041,8 +1041,12 @@ int vt_waitactive(int vt) > for (;;) { > set_current_state(TASK_INTERRUPTIBLE); > retval = 0; > - if (vt == fg_console) > + acquire_console_sem(); > + if (vt == fg_console) { > + release_console_sem(); > break; > + } > + release_console_sem(); > retval = -EINTR; > if (signal_pending(current)) > break; >
OK. I think. It's hard to tell. I assume that the acquire_console_sem() in here is to synchronise against some other function which also takes acquire_console_sem(), but it is not clear which.
So could you please redo this with a comment which tells the reader exactly what's being protected against what, and why?
Also, I always feel a bit worried by:
set_current_state(TASK_INTERRUPTIBLE); down(...);
because if it hits contention, the down() will undo the set_curremt_state(). Now that's normally OK because we loop, and because the semaphore won't normally be 100% contended all the time. Unless someone reimplements down() so it happens to return in state TASK_RUNNING all the time, which they could legitimately do (although this would probably break stuff such as the above).
But still, it is nicer to do
down(...); set_current_state(TASK_INTERRUPTIBLE);
if possible, and I think it is possible here.
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