[PATCH] 2.4 preemption bug in bh_kmap_irq

[Joe Korty <joe.korty@ccur.com>]

  [I submitted this bug to Alan some time ago, he agreed it was
   a problem but felt that it should be fixed in the 2.4 preemption
   patch.]

Hi Robert, Everyone,

The bh_kmap_irq/bh_kunmap_irq functions are broken in 2.4.21-pre5.
However no symptoms occur unless the preemption patch is applied.

The bug is that bh_map_irq *conditionally* calls kmap_atomic (which
disables preemption as one of its functions), while bh_unmap_irq
*unconditionally* calls kunmap_atomic (which enables it).  This
imbalance results in a occasional off-by-one preempt_count, which in
turn causes IDE PIO mode interrupt code (specifically, read_intr) to
erronously invoke preempt_schedule while at interrupt level.

The below patch compiles and boots ide=nodma on my preempt 2.4 kernel
on the one motherboard that had the problem.  Before this patch, the
kernel would not even boot for that motherboard.  I also applied and
test booted a pure 2.4.21-pre5 kernel with this patch.

The patch implements my preference for simplicity, so you may want to
take some other approach if maximal performance is what you want.

Joe




--- include/linux/highmem.h.orig	2003-03-12 05:01:56.000000000 -0500
+++ include/linux/highmem.h	2003-03-12 16:07:04.000000000 -0500
@@ -33,22 +33,10 @@
 {
 	unsigned long addr;
 
-	__save_flags(*flags);
-
-	/*
-	 * could be low
-	 */
-	if (!PageHighMem(bh->b_page))
-		return bh->b_data;
-
-	/*
-	 * it's a highmem page
-	 */
-	__cli();
+	local_irq_save(*flags);
 	addr = (unsigned long) kmap_atomic(bh->b_page, KM_BH_IRQ);
 
-	if (addr & ~PAGE_MASK)
-		BUG();
+	BUG_ON (addr & ~PAGE_MASK);
 
 	return (char *) addr + bh_offset(bh);
 }
@@ -58,7 +46,7 @@
 	unsigned long ptr = (unsigned long) buffer & PAGE_MASK;
 
 	kunmap_atomic((void *) ptr, KM_BH_IRQ);
-	__restore_flags(*flags);
+	local_irq_restore(*flags);
 }
 
 #else /* CONFIG_HIGHMEM */

>From alan@lxorguk.ukuu.org.uk  Wed Mar 12 22:19:43 2003
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Subject: Re: [PATCH] bug in 2.4 bh_kmap_irq() breaks IDE under preempt patch
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Joe Korty <joe.korty@ccur.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, 2003-03-12 at 22:13, Joe Korty wrote:
> The bug is that bh_map_irq *conditionally* calls kmap_atomic (which
> disables preemption as one of its functions), while bh_unmap_irq
> *unconditionally* calls kunmap_atomic (which enables it).  This

Thats a pre-empt bug ont a bh_map_irq bug. I'm glad you've found it
however. It explains a few things and will be useful for people wanting
pre-empt 2.4 .