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

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Feb 02, 2012 at 11:24:46AM -0800, Stephen Boyd 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 preemption across the write and read so that the correct
> cache attributes are read and used for the cache flushing
> routine. 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.

This isn't going to work for two reasons:

(1) (and the VFP code suffers from this) after we re-enable preemption,
    we really should check for a pending preemption event in every case.

(2) v7_flush_dcache_all() is called from __v7_setup() using a very small
    private stack.  This doesn't have a thread info structure at the
    bottom.

So, if we need to disable preemption here, we need to find a different
solution to it.

> Should we move get_thread_info into assembler.h? It seems odd
> to include entry-header.S but I saw that vfp was doing the same.

Probably yes, and probably also have preempt_disable and preempt_enable
assembler macros.  That's going to get rather icky if we have to
explicitly call the scheduler though (to solve (1)).