linux-kernel - i386 doublefault handler is broken with CONFIG_DEBUG

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Message-ID: <46BB4599.2020900@redhat.com>
Date:	Thu, 09 Aug 2007 12:49:29 -0400
From:	Chuck Ebbert <cebbert@...hat.com>
To:	linux-kernel <linux-kernel@...r.kernel.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, Andi Kleen <ak@....de>
Subject: i386 doublefault handler is broken with CONFIG_DEBUG_SPINLOCK

The new percpu code has apparently broken the doublefault handler
when CONFIG_DEBUG_SPINLOCK is set. Doublefault is handled by
a hardware task, making the check

        SPIN_BUG_ON(lock->owner == current, lock, "recursion");

fault because it uses the FS register to access the percpu data
for current, and that register is zero in the new TSS. (The trace
I saw was on 2.6.20 where it was GS, but it looks like this will
still happen with FS on 2.6.22.)

Initializing FS in the doublefault_tss should fix it.

Signed-off-by: Chuck Ebbert <cebbert@...hat.com>

---

 NOTE: not even compile tested.

 arch/i386/kernel/doublefault.c |    1 +
 1 file changed, 1 insertion(+)

--- linux-2.6.22.noarch.orig/arch/i386/kernel/doublefault.c
+++ linux-2.6.22.noarch/arch/i386/kernel/doublefault.c
@@ -63,6 +63,7 @@ struct tss_struct doublefault_tss __cach
 		.cs		= __KERNEL_CS,
 		.ss		= __KERNEL_DS,
 		.ds		= __USER_DS,
+		.fs		= __KERNEL_PERCPU,

 		.__cr3		= __pa(swapper_pg_dir)
 	}
-
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