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Subject: Re: [PATCH] tty race on con_close and con_flush_chars
From: Robert Love <rml@tech9.net>
To: Robert Love <rml@tech9.net>
Cc: linus@transmeta.com, laughing@shared-source.org,
        linux-kernel@vger.kernel.org, tytso@thunk.org, andrewm@uow.edu.au
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Date: 29 Oct 2001 22:13:55 -0500
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Someone pointed out (via private email) that a race can still exist
between checking that vt is non-zero and acquiring the semaphore,
especially since we can sleep doing that.

I agree; the following patch should alleviate that race.

diff -u linux-2.4.13-ac5/drivers/char/console.c
linux/drivers/char/console.c
--- linux-2.4.13-ac5/drivers/char/console.c	Mon Oct 29 17:27:19 2001
+++ linux/drivers/char/console.c	Mon Oct 29 22:11:27 2001
@@ -2387,8 +2387,14 @@
 		return;
 
 	pm_access(pm_con);
+
+	/*
+	 * If we raced with con_close(), `vt' may be null. 
+	 * Hence this bandaid.   - akpm
+	 */
 	acquire_console_sem();
-	set_cursor(vt->vc_num);
+	if (vt)
+		set_cursor(vt->vc_num);
 	release_console_sem();
 }
 

	Robert Love

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