Subject: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

From: [email-masked] (Stephen Boyd)
Date: Mon, 13 Feb 2012 10:13:27 -0800

On 02/13/12 10:09, Nicolas Pitre wrote:
> On Mon, 13 Feb 2012, Rabin Vincent wrote:
>
>> On Fri, Feb 3, 2012 at 07:33, Stephen Boyd <[email-masked]> wrote:
>>> armv7's flush_cache_all() flushes caches via set/way. To
>>> determine the cache attributes (line size, number of sets,
>>> etc.) the assembly first writes the CSSELR register to select a
>>> cache level and then reads the CCSIDR register. The CSSELR register
>>> is banked per-cpu and is used to determine which cache level CCSIDR
>>> reads. If the task is migrated between when the CSSELR is written and
>>> the CCSIDR is read the CCSIDR value may be for an unexpected cache
>>> level (for example L1 instead of L2) and incorrect cache flushing
>>> could occur.
>>>
>>> Disable interrupts across the write and read so that the correct
>>> cache attributes are read and used for the cache flushing
>>> routine. We disable interrupts instead of disabling preemption
>>> because the critical section is only 3 instructions and we want
>>> to call v7_dcache_flush_all from __v7_setup which doesn't have a
>>> full kernel stack with a struct thread_info.
>>>
>>> This fixes a problem we see in scm_call() when flush_cache_all()
>>> is called from preemptible context and sometimes the L2 cache is
>>> not properly flushed out.
>>>
>>> Signed-off-by: Stephen Boyd <[email-masked]>
>>> Cc: Catalin Marinas <[email-masked]>
>>> Cc: Nicolas Pitre <[email-masked]>
>>> ---
>>> diff --git a/arch/arm/mm/cache-v7.S b/arch/arm/mm/cache-v7.S
>>> index 07c4bc8..654a5fc 100644
>>> --- a/arch/arm/mm/cache-v7.S
>>> +++ b/arch/arm/mm/cache-v7.S
>>> @@ -54,9 +54,15 @@ loop1:
>>>        and     r1, r1, #7                      @ mask of the bits for current cache only
>>>        cmp     r1, #2                          @ see what cache we have at this level
>>>        blt     skip                            @ skip if no cache, or just i-cache
>>> +#ifdef CONFIG_PREEMPT
>>> +       save_and_disable_irqs r9                @ make cssr&csidr read atomic
>>> +#endif
>>>        mcr     p15, 2, r10, c0, c0, 0          @ select current cache level in cssr
>>>        isb                                     @ isb to sych the new cssr&csidr
>>>        mrc     p15, 1, r1, c0, c0, 0           @ read the new csidr
>>> +#ifdef CONFIG_PREEMPT
>>> +       restore_irqs r9
>>> +#endif
>>>        and     r2, r1, #7                      @ extract the length of the cache lines
>>>        add     r2, r2, #4                      @ add 4 (line length offset)
>>>        ldr     r4, =0x3ff
>> This patch breaks the kernel boot when lockdep is enabled.
>>
>> v7_setup (called before the MMU is enabled) calls v7_flush_dcache_all,
>> and the save_and_disable_irqs added by this patch ends up calling
>> into lockdep C code (trace_hardirqs_off()) when we are in no position
>> to execute it (no stack, no MMU).
>>
>> The following fixes it.  Perhaps it can be folded in?
> Absolutely.  
>
> No tracing what so ever should be involved here.
>

Thanks. Russell has already merged the original patch to the fixes
branch. Hopefully he can fold this one in.

-- 
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