Re: [PATCH] fix pgd_lock deadlock

[Jeremy Fitzhardinge <jeremy@goop.org>]

On 02/17/2011 02:19 AM, Johannes Weiner wrote:
> So Xen needs all page tables protected when pinning/unpinning and
> extended page_table_lock to cover kernel range, which it does nowhere
> else AFAICS.  But the places it extended are also taking the pgd_lock,
> so I wonder if Xen could just take the pgd_lock itself in these paths
> and we could revert page_table_lock back to cover user va only?
> Jeremy, could this work?  Untested.

Yes, this looks pretty plausible, but I need to go back and check what
the original bug was to make sure.  Oh, and test it I guess.

But xen_pgd_pin/unpin only operate on the usermode parts of the address
space (since the kernel part is shared and always pinned), so there
shouldn't be any contention there.

Hm, and I don't see why pin/unpin really care about pgd_lock either. 
They're called at well-defined places (fork/exec/exit) on a single pgd. 
pin/unpin_all are a different matter - since they walk the pgd list -
but they were taking the lock anyway.

Will need to think about this a bit.

    J

> 	Hannes
>
> ---
>  arch/x86/include/asm/pgtable.h |    2 --
>  arch/x86/mm/fault.c            |   14 ++------------
>  arch/x86/mm/init_64.c          |    6 ------
>  arch/x86/mm/pgtable.c          |   20 +++-----------------
>  arch/x86/xen/mmu.c             |    8 ++++++++
>  5 files changed, 13 insertions(+), 37 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 18601c8..8c0335a 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -28,8 +28,6 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
>  extern spinlock_t pgd_lock;
>  extern struct list_head pgd_list;
>  
> -extern struct mm_struct *pgd_page_get_mm(struct page *page);
> -
>  #ifdef CONFIG_PARAVIRT
>  #include <asm/paravirt.h>
>  #else  /* !CONFIG_PARAVIRT */
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 7d90ceb..5da4155 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -234,19 +234,9 @@ void vmalloc_sync_all(void)
>  		struct page *page;
>  
>  		spin_lock_irqsave(&pgd_lock, flags);
> -		list_for_each_entry(page, &pgd_list, lru) {
> -			spinlock_t *pgt_lock;
> -			pmd_t *ret;
> -
> -			pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
> -
> -			spin_lock(pgt_lock);
> -			ret = vmalloc_sync_one(page_address(page), address);
> -			spin_unlock(pgt_lock);
> -
> -			if (!ret)
> +		list_for_each_entry(page, &pgd_list, lru)
> +			if (!vmalloc_sync_one(page_address(page), address))
>  				break;
> -		}
>  		spin_unlock_irqrestore(&pgd_lock, flags);
>  	}
>  }
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 71a5929..9332f21 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -114,19 +114,13 @@ void sync_global_pgds(unsigned long start, unsigned long end)
>  		spin_lock_irqsave(&pgd_lock, flags);
>  		list_for_each_entry(page, &pgd_list, lru) {
>  			pgd_t *pgd;
> -			spinlock_t *pgt_lock;
>  
>  			pgd = (pgd_t *)page_address(page) + pgd_index(address);
> -			pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
> -			spin_lock(pgt_lock);
> -
>  			if (pgd_none(*pgd))
>  				set_pgd(pgd, *pgd_ref);
>  			else
>  				BUG_ON(pgd_page_vaddr(*pgd)
>  				       != pgd_page_vaddr(*pgd_ref));
> -
> -			spin_unlock(pgt_lock);
>  		}
>  		spin_unlock_irqrestore(&pgd_lock, flags);
>  	}
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index 500242d..72107ab 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -87,19 +87,7 @@ static inline void pgd_list_del(pgd_t *pgd)
>  #define UNSHARED_PTRS_PER_PGD				\
>  	(SHARED_KERNEL_PMD ? KERNEL_PGD_BOUNDARY : PTRS_PER_PGD)
>  
> -
> -static void pgd_set_mm(pgd_t *pgd, struct mm_struct *mm)
> -{
> -	BUILD_BUG_ON(sizeof(virt_to_page(pgd)->index) < sizeof(mm));
> -	virt_to_page(pgd)->index = (pgoff_t)mm;
> -}
> -
> -struct mm_struct *pgd_page_get_mm(struct page *page)
> -{
> -	return (struct mm_struct *)page->index;
> -}
> -
> -static void pgd_ctor(struct mm_struct *mm, pgd_t *pgd)
> +static void pgd_ctor(pgd_t *pgd)
>  {
>  	/* If the pgd points to a shared pagetable level (either the
>  	   ptes in non-PAE, or shared PMD in PAE), then just copy the
> @@ -113,10 +101,8 @@ static void pgd_ctor(struct mm_struct *mm, pgd_t *pgd)
>  	}
>  
>  	/* list required to sync kernel mapping updates */
> -	if (!SHARED_KERNEL_PMD) {
> -		pgd_set_mm(pgd, mm);
> +	if (!SHARED_KERNEL_PMD)
>  		pgd_list_add(pgd);
> -	}
>  }
>  
>  static void pgd_dtor(pgd_t *pgd)
> @@ -282,7 +268,7 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
>  	 */
>  	spin_lock_irqsave(&pgd_lock, flags);
>  
> -	pgd_ctor(mm, pgd);
> +	pgd_ctor(pgd);
>  	pgd_prepopulate_pmd(mm, pgd, pmds);
>  
>  	spin_unlock_irqrestore(&pgd_lock, flags);
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 5e22810..97fbfce 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -1021,7 +1021,11 @@ static void __xen_pgd_pin(struct mm_struct *mm, pgd_t *pgd)
>  
>  static void xen_pgd_pin(struct mm_struct *mm)
>  {
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&pgd_lock, flags);
>  	__xen_pgd_pin(mm, mm->pgd);
> +	spin_unlock_irqrestore(&pgd_lock, flags);
>  }
>  
>  /*
> @@ -1140,7 +1144,11 @@ static void __xen_pgd_unpin(struct mm_struct *mm, pgd_t *pgd)
>  
>  static void xen_pgd_unpin(struct mm_struct *mm)
>  {
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&pgd_lock, flags);
>  	__xen_pgd_unpin(mm, mm->pgd);
> +	spin_unlock_irqrestore(&pgd_lock, flags);
>  }
>  
>  /*
>