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SubjectRe: [PATCH] Avoid a race in complete_change_console()
Chris, THANK YOU!  I've been having a problem on my SMP system for months
now, where if I started X, switched to a text console, and then back to X,
the system would lock up. Your patch actually fixed it!

Alan, could you please make sure this patch makes it into the kernel?


> Currently drivers/char/vt.c:complete_change_console() calls
> do_blank_screen() when switching from text to graphics mode after
> sending acqsig to the controlling process of the new console.
>
> On an SMP system, acqsig can wake the controlling process up and get it
> working before do_blank_screen completes. At least on my ia64 systems,
> this causes accesses to the VGA range from the kernel and accesses to
> framebuffer from X to overlap with one another causing system lockups.
>
> Appended is a patch which ensures that do_blank_screen() is called
> before delivering acqsig instead of after. do_blank_screen() is still
> called if signal delivery fails (as far as I could tell, handling this
> case was the reason screen blanking was done after signal delivery in
> the first place).
>
> Can someone familiar with this code comment on the correctness of this
> patch? The patch is against vanilla 2.4.13. Thanks,
>
> Chris
>
>
> --- linux-2.4.13-ia64-011024-pristine/drivers/char/vt.c Mon Sep 17 22:52:35 2001
> +++ linux-2.4.13-ia64-011024-dev/drivers/char/vt.c Thu Oct 25 20:46:08 2001
> @@ -1184,6 +1184,24 @@
> switch_screen(new_console);
>
> /*
> + * This can't appear below a successful kill_proc(). If it did,
> + * then the *blank_screen operation could occur while X, having
> + * received acqsig, is waking up on another processor. This
> + * condition can lead to overlapping accesses to the VGA range
> + * and the framebuffer (causing system lockups).
> + *
> + * To account for this we duplicate this code below only if the
> + * controlling process is gone and we've called reset_vc.
> + */
> + if (old_vc_mode != vt_cons[new_console]->vc_mode)
> + {
> + if (vt_cons[new_console]->vc_mode == KD_TEXT)
> + unblank_screen();
> + else
> + do_blank_screen(1);
> + }
> +
> + /*
> * If this new console is under process control, send it a signal
> * telling it that it has acquired. Also check if it has died and
> * clean up (similar to logic employed in change_console())
> @@ -1209,19 +1227,15 @@
> * to account for and tracking tty count may be undesirable.
> */
> reset_vc(new_console);
> - }
> - }
>
> - /*
> - * We do this here because the controlling process above may have
> - * gone, and so there is now a new vc_mode
> - */
> - if (old_vc_mode != vt_cons[new_console]->vc_mode)
> - {
> - if (vt_cons[new_console]->vc_mode == KD_TEXT)
> - unblank_screen();
> - else
> - do_blank_screen(1);
> + if (old_vc_mode != vt_cons[new_console]->vc_mode)
> + {
> + if (vt_cons[new_console]->vc_mode == KD_TEXT)
> + unblank_screen();
> + else
> + do_blank_screen(1);
> + }
> + }
> }
>
> /*




Mike Edwards

Brainbench certified Master Linux Administrator
http://www.brainbench.com/transcript.jsp?pid=158188
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