Re: [PATCH] bug in 2.4 bh_kmap_irq() breaks IDE under preempt patch

[Jens Axboe]

On Wed, Mar 12 2003, Joe Korty wrote:
> 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.

I fixed this in 2.5 ages ago, just didn't get it in 2.4 block-highmem...
There's a tiny bit missing from your patch:

> --- 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);
	local_irq_disable();

>  	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);
>  }

other than that it's fine. See 2.5 for reference.

-- 
Jens Axboe