On Mon, Jan 24, 2011 at 11:19 AM, Russell King - ARM Linux
[off-list ref] wrote:

On Mon, Jan 24, 2011 at 10:47:36AM +0200, saeed bishara wrote:

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I've port 2.6.35 to SMP system that runs in V6 mode, this system
doesn't support TLB operations broadcasting by hw, so it uses IPI
messages for that. ?when enabling DEBUG_LOCKDEP, I got the following
error message while booting the system from NFS:

You've bypassed this check:

? ? ? ? ? ? ? ?if (is_smp() && tlb_ops_need_broadcast()) {
? ? ? ? ? ? ? ? ? ? ? ?/*
? ? ? ? ? ? ? ? ? ? ? ? * kmap_high needs to occasionally flush TLB entries,
? ? ? ? ? ? ? ? ? ? ? ? * however, if the TLB entries need to be broadcast
? ? ? ? ? ? ? ? ? ? ? ? * we may deadlock:
? ? ? ? ? ? ? ? ? ? ? ? * ?kmap_high(irqs off)->flush_all_zero_pkmaps->
? ? ? ? ? ? ? ? ? ? ? ? * ?flush_tlb_kernel_range->smp_call_function_many
? ? ? ? ? ? ? ? ? ? ? ? * ? (must not be called with irqs off)
? ? ? ? ? ? ? ? ? ? ? ? */
? ? ? ? ? ? ? ? ? ? ? ?reason = "without hardware TLB ops broadcasting";
? ? ? ? ? ? ? ?}

so you lose. ?There's reasons why such checks are put in. ?We can not
support SMP and highmem on systems which do not have TLB broadcasting.
That's not because the code doesn't support it, it's because there are
deadlocks which will occur.

thanks, I missed that

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The fact is that it is unsafe to send IPIs with IRQs disabled, which
means you can't IPI a TLB operation and wait for it to complete with IRQs
disabled.

as I understand it, the lock_kmap() started to disable IRQs in order
to support the vivt and vipt caches, but in SMP (at least in my case),
the caches are PIPT, so I think I can do the following:
1. undef ?the ?ARCH_NEEDS_KMAP_HIGH_GET
2. use page_address instead of kmap_high_get()
do you think it will work?

Definitely not. ?We use kmap_high_get() so that we can ensure that we've
flushed data out of the PIPT cache for highmem pages. ?highmem pages
which are unmapped do not have a valid page_address() but may have PIPT
cache lines associated with them.

So no, I don't think it'll be safe.

ok, what about the following patch, the idea is to use only the
kmap_high_l1_vipt when doing cache maintenance.

You're really not listening.

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diff --git a/arch/arm/include/asm/highmem.h b/arch/arm/include/asm/highmem.h
index feb988a..457998c 100644
--- a/arch/arm/include/asm/highmem.h
+++ b/arch/arm/include/asm/highmem.h

@@ -19,7 +19,9 @@

?extern pte_t *pkmap_page_table;

+#ifndef CONFIG_SMP
?#define ARCH_NEEDS_KMAP_HIGH_GET
+#endif

?extern void *kmap_high(struct page *page);
?extern void *kmap_high_get(struct page *page);

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index 9e7742f..d22366b 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c

@@ -459,12 +459,15 @@ static void dma_cache_maint_page(struct page

*page, unsigned long offset,
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? }
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? len = PAGE_SIZE - offset;
? ? ? ? ? ? ? ? ? ? ? }
+#ifdef ARCH_NEEDS_KMAP_HIGH_GET
? ? ? ? ? ? ? ? ? ? ? vaddr = kmap_high_get(page);
? ? ? ? ? ? ? ? ? ? ? if (vaddr) {
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? vaddr += offset;
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? op(vaddr, len, dir);
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? kunmap_high(page);
- ? ? ? ? ? ? ? ? ? ? } else if (cache_is_vipt()) {
+ ? ? ? ? ? ? ? ? ? ? } else if (cache_is_vipt())
+#endif

So you're disabling DMA cache maintainence, making DMA support *unsafe*
on your platform. ?You'll get filesystem corruption and other crap like
that. ?Maybe you don't care for users data?

no I'm not disabling DMA cache maintenance, this is how the code looks like:
#ifdef ARCH_NEEDS_KMAP_HIGH_GET
                        vaddr = kmap_high_get(page);
                        if (vaddr) {
                                vaddr += offset;
                                op(vaddr, len, dir);
                                kunmap_high(page);
                        } else if (cache_is_vipt())
#endif
                        {
                                pte_t saved_pte;
                                vaddr = kmap_high_l1_vipt(page, &saved_pte);
                                op(vaddr + offset, len, dir);
                                kunmap_high_l1_vipt(page, saved_pte);
                        }

so I'm doing that cache maintenance using new mapping by kmap_high_l1_vipt.

saeed