linux-kernel - Re: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

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Date:	Mon, 13 Feb 2012 23:24:53 +0530
From:	Rabin Vincent <rabin@....in>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Catalin Marinas <catalin.marinas@....com>,
	Nicolas Pitre <nico@...xnic.net>
Subject: Re: [PATCH] ARM: cache-v7: Disable preemption when reading CCSIDR

On Fri, Feb 3, 2012 at 07:33, Stephen Boyd <sboyd@...eaurora.org> 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 <sboyd@...eaurora.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Nicolas Pitre <nico@...xnic.net>
> ---
> 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?

diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
index 62f8095..23371b1 100644
--- a/arch/arm/include/asm/assembler.h
+++ b/arch/arm/include/asm/assembler.h
@@ -137,6 +137,11 @@
 	disable_irq
 	.endm

+	.macro	save_and_disable_irqs_notrace, oldcpsr
+	mrs	\oldcpsr, cpsr
+	disable_irq_notrace
+	.endm
+
 /*
  * Restore interrupt state previously stored in a register.  We don't
  * guarantee that this will preserve the flags.
diff --git a/arch/arm/mm/cache-v7.S b/arch/arm/mm/cache-v7.S
index 7a24d396..a655d3d 100644
--- a/arch/arm/mm/cache-v7.S
+++ b/arch/arm/mm/cache-v7.S
@@ -55,7 +55,7 @@ loop1:
 	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
+	save_and_disable_irqs_notrace 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
--
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