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Subject[PATCH] bug in 2.4 bh_kmap_irq() breaks IDE under preempt patch
Date
Hi Alan, Everyone,
The bh_map_irq/bh_unmap_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 */
-
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