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SubjectRe: linux-next: manual merge of the akpm-current tree with the tip tree
Hi Peter,

On Fri, Aug 11, 2017 at 04:04:50PM +0200, Peter Zijlstra wrote:
>
> Ok, so I have the below to still go on-top.
>
> Ideally someone would clarify the situation around
> mm_tlb_flush_nested(), because ideally we'd remove the
> smp_mb__after_atomic() and go back to relying on PTL alone.
>
> This also removes the pointless smp_mb__before_atomic()

I'm not an expert of barrier stuff but IIUC, mm_tlb_flush_nested's
side full memory barrier can go with removing smp_mb__after_atomic
in inc_tlb_flush_pending side?


diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 490af494c2da..5ad0e66df363 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -544,7 +544,12 @@ static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
*/
static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
{
- return atomic_read(&mm->tlb_flush_pending) > 1;
+ /*
+ * atomic_dec_and_test's full memory barrier guarantees
+ * to see uptodate tlb_flush_pending count in other CPU
+ * without relying on page table lock.
+ */
+ return !atomic_dec_and_test(&mm->tlb_flush_pending);
}

static inline void init_tlb_flush_pending(struct mm_struct *mm)
diff --git a/mm/memory.c b/mm/memory.c
index f571b0eb9816..e90b57bc65fb 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -407,6 +407,10 @@ void tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
unsigned long start, unsigned long end)
{
arch_tlb_gather_mmu(tlb, mm, start, end);
+ /*
+ * couterpart is mm_tlb_flush_nested in tlb_finish_mmu
+ * which decreases pending count.
+ */
inc_tlb_flush_pending(tlb->mm);
}

@@ -446,9 +450,7 @@ void tlb_finish_mmu(struct mmu_gather *tlb,
*
*/
bool force = mm_tlb_flush_nested(tlb->mm);
-
arch_tlb_finish_mmu(tlb, start, end, force);
- dec_tlb_flush_pending(tlb->mm);
}

/*
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