[RFC] arm/mm: add a protection when flushing anonymous pages

[yanpai.chen@gmail.com (Andrew Yan-Pai Chen)]

On Mon, Oct 1, 2012 at 11:50 AM, Jason Lin <kernel.jason@gmail.com> wrote:
>
> Dear all:
> By the way, I used the CPU with VIPT cache.
> Thanks.
>
> 2012/10/1 Jason Lin <kernel.jason@gmail.com>:
> > Dear all:
> > When I do LTP direct I/O tests, it produced a cache coherence issue
> > caused by flush_pfn_alias() (arch/arm/mm/flush.c).
> > I think we should to disable preemption or lock before setting page
> > table and enable preemption or unlock after flushing pages
> >
> > Any comments are appreciated.
> > Thanks.
>
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[RFC PATCH] make flush_pfn_alias() nonpreemptible

Since flush_pfn_alias() is preemptible, it is possible to be
preempted just after set_top_pte() is done. If the process
which preempts the previous happened to invoke flush_pfn_alias()
with the same colour vaddr as that of the previous, the same
top pte will be overwritten. When switching back to the previous,
it attempts to flush cache lines with incorrect mapping. Then
no lines (or wrong lines) will be flushed because of the nature
of vipt caches.

diff --git a/arch/arm/mm/flush.c b/arch/arm/mm/flush.c
index 40ca11e..bd07918 100644
--- a/arch/arm/mm/flush.c
+++ b/arch/arm/mm/flush.c
@@ -27,6 +27,8 @@ static void flush_pfn_alias(unsigned long pfn,
unsigned long vaddr)
        unsigned long to = FLUSH_ALIAS_START + (CACHE_COLOUR(vaddr) <<
PAGE_SHIFT);
        const int zero = 0;

+       preempt_disable();
+
        set_top_pte(to, pfn_pte(pfn, PAGE_KERNEL));

        asm(    "mcrr   p15, 0, %1, %0, c14\n"
@@ -34,6 +36,8 @@ static void flush_pfn_alias(unsigned long pfn,
unsigned long vaddr)
            :
            : "r" (to), "r" (to + PAGE_SIZE - L1_CACHE_BYTES), "r" (zero)
            : "cc");
+
+       preempt_enable();
 }

--
Regards,
Andrew