Re: [PATCH] console close race fix resend

From: Gordon Oliver (
Date: Tue Dec 11 2001 - 03:54:11 EST

On 2001.12.10 22:05 Robert Love wrote:
> Ehh, I don't think so. Here is the whole patched function:
> static void con_flush_chars(struct tty_struct *tty)
> {
> struct vt_struct *vt = (struct vt_struct *)tty->driver_data;
> if (in_interrupt()) /* from flush_to_ldisc */
> return;
> pm_access(pm_con);
> acquire_console_sem();
> if (vt)
> set_cursor(vt->vc_num);
> release_console_sem();
> }
> When we check vt, it isn't stale. vt is a _pointer_ to the data so that
> first reference against it is guaranteed to grab the correct value. The
> only possible race is between the if and the set_cursor, but that isn't
> an issue because we acquired the console semaphore. There is no race
> here.

I like the patch that Andrew Morton sent in reply to this better.
Note that in the event that the above code does the following sequence
it will cause a stale pointer to be used:

        con_flush_chars con_close
        vt = <>
                           tty->driver_data = NULL

Now it _might_ be ok to act on a stale vt pointer, but it sure
feels like thin ice. I'm not sure that there is any danger of
the vt data being modified in a way that would break this, but
since the tty no longer has a reference it is bad practice.

What the earlier patch did is created some very subtle semantics
for a small window of a race. It fixed the blaring bug (the OOPS)
but left a possible one that would be harder to find....
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This archive was generated by hypermail 2b29 : Sat Dec 15 2001 - 21:00:19 EST