On Mon, 13 Feb 2012, Rabin Vincent wrote:

On Fri, Feb 3, 2012 at 07:33, Stephen Boyd [off-list ref] wrote:

quoted

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 <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Nicolas Pitre <nico@fluxnic.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?

Absolutely.  

No tracing what so ever should be involved here.