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 */
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 .
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
On Thu, Mar 13 2003, Marc-Christian Petersen wrote:
> On Thursday 13 March 2003 10:26, Jens Axboe wrote:
>
> Hi Jens,
>
> > There's a tiny bit missing from your patch:
> > > - * 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);
> local_irq_enable();
>
> ^ isn't this missing too with your suggested one-liner?
no, the local_irq_restore() brings back the irq flags from before we did
the irq disable. if interrupts were disabled before bh_kmap_irq() was
called, we must not enable them. basically, maintain the same flags.
--
Jens Axboe
On Thursday 13 March 2003 10:26, Jens Axboe wrote:
Hi Jens,
> There's a tiny bit missing from your patch:
> > - * 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);
local_irq_enable();
^ isn't this missing too with your suggested one-liner?
ciao, Marc
On Thursday 13 March 2003 11:28, Jens Axboe wrote:
Hi Jens,
> > local_irq_enable();
> > ^ isn't this missing too with your suggested one-liner?
> no, the local_irq_restore() brings back the irq flags from before we did
> the irq disable. if interrupts were disabled before bh_kmap_irq() was
> called, we must not enable them. basically, maintain the same flags.
hmm, then I am blind 8-) ... Thanks.
ciao, Marc
> 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:
>
> > + local_irq_save(*flags);
> local_irq_disable();
>
> other than that it's fine. See 2.5 for reference.
local_irq_save() does a local_irq_disable() as part of its function.
Joe
On Thu, Mar 13 2003, Joe Korty wrote:
> > 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:
> >
> > > + local_irq_save(*flags);
> > local_irq_disable();
> >
> > other than that it's fine. See 2.5 for reference.
>
> local_irq_save() does a local_irq_disable() as part of its function.
indeed, you are right.
--
Jens Axboe