Re: [PATCH] fix pgd_lock deadlock

Jeremy Fitzhardinge
2011-02-21T17:40:32Z

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);
> }
>
> /*
>