On Thu, 15 Mar 2007 15:10:23 +0100 Michal Januszewski <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]>
>
> ---
> 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.
Thanks.
-
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