[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [Xen-devel] Re: [PATCH] x86: hold mm->page_table_lock while doing vmalloc_sync
Hello, Larry (CC'ed) found a problem with the patch in subject. When USE_SPLIT_PTLOCKS is not defined (NR_CPUS == 2) it will deadlock in ptep_clear_flush_notify in rmap.c because it's sending IPIs with the page_table_lock already held, and the other CPUs now spins on the page_table_lock with irq disabled, so the IPI never runs. With CONFIG_TRANSPARENT_HUGEPAGE=y this deadlocks happens even with USE_SPLIT_PTLOCKS defined so it become visible but it needs to be fixed regardless (for NR_CPUS == 2). I'd like to understand why the pgd_lock needs irq disabled, it sounds too easy that I can just remove the _irqsave, doesn't it? A pgd_free comment says it can run from irq. page_table_lock having to be taken there is for Xen only, but other archs also uses spin_lock_irqsave(pgd_lock) so I guess it's either common code, or it's superfluous and not another Xen special requirement. If we could remove that _irqsave like below it'd solve it... But clearly something must be taking the pgd_lock from irq. (using a rwlock would also be possible as long as nobody takes it in write mode during irq, but if it's pgd_free that really runs in irq, that would need the write_lock so it wouldn't be a solution). I'm trying this fix and the VM_BUG_ON never triggered yet. In short: who takes the pgd_lock from an irq? (__mmdrop shouldn't but maybe I'm overlooking some aio bit?) ====== Subject: fix pgd_lock deadlock From: Andrea Arcangeli <aarcange@xxxxxxxxxx> It's forbidden to take the page_table_lock with the irq disabled or if there's contention the IPIs (for tlb flushes) sent with the page_table_lock held will never run leading to a deadlock. Apparently nobody takes the pgd_lock from irq so the _irqsave can be removed. Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> --- arch/x86/mm/fault.c | 7 ++++--- arch/x86/mm/init_64.c | 7 ++++--- arch/x86/mm/pageattr.c | 21 +++++++++++---------- arch/x86/mm/pgtable.c | 10 ++++++---- arch/x86/xen/mmu.c | 10 ++++------ 5 files changed, 29 insertions(+), 26 deletions(-) --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -230,14 +230,15 @@ void vmalloc_sync_all(void) address >= TASK_SIZE && address < FIXADDR_TOP; address += PMD_SIZE) { - unsigned long flags; struct page *page; - spin_lock_irqsave(&pgd_lock, flags); + VM_BUG_ON(in_interrupt()); + spin_lock(&pgd_lock); list_for_each_entry(page, &pgd_list, lru) { spinlock_t *pgt_lock; pmd_t *ret; + /* the pgt_lock only for Xen */ pgt_lock = &pgd_page_get_mm(page)->page_table_lock; spin_lock(pgt_lock); @@ -247,7 +248,7 @@ void vmalloc_sync_all(void) if (!ret) break; } - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock, flags); } } --- a/arch/x86/mm/init_64.c +++ b/arch/x86/mm/init_64.c @@ -105,18 +105,19 @@ void sync_global_pgds(unsigned long star for (address = start; address <= end; address += PGDIR_SIZE) { const pgd_t *pgd_ref = pgd_offset_k(address); - unsigned long flags; struct page *page; if (pgd_none(*pgd_ref)) continue; - spin_lock_irqsave(&pgd_lock, flags); + VM_BUG_ON(in_interrupt()); + spin_lock(&pgd_lock); list_for_each_entry(page, &pgd_list, lru) { pgd_t *pgd; spinlock_t *pgt_lock; pgd = (pgd_t *)page_address(page) + pgd_index(address); + /* the pgt_lock only for Xen */ pgt_lock = &pgd_page_get_mm(page)->page_table_lock; spin_lock(pgt_lock); @@ -128,7 +129,7 @@ void sync_global_pgds(unsigned long star spin_unlock(pgt_lock); } - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); } } --- a/arch/x86/mm/pageattr.c +++ b/arch/x86/mm/pageattr.c @@ -57,12 +57,11 @@ static unsigned long direct_pages_count[ void update_page_count(int level, unsigned long pages) { - unsigned long flags; - /* Protect against CPA */ - spin_lock_irqsave(&pgd_lock, flags); + VM_BUG_ON(in_interrupt()); + spin_lock(&pgd_lock); direct_pages_count[level] += pages; - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); } static void split_page_count(int level) @@ -402,7 +401,7 @@ static int try_preserve_large_page(pte_t *kpte, unsigned long address, struct cpa_data *cpa) { - unsigned long nextpage_addr, numpages, pmask, psize, flags, addr, pfn; + unsigned long nextpage_addr, numpages, pmask, psize, addr, pfn; pte_t new_pte, old_pte, *tmp; pgprot_t old_prot, new_prot, req_prot; int i, do_split = 1; @@ -411,7 +410,8 @@ try_preserve_large_page(pte_t *kpte, uns if (cpa->force_split) return 1; - spin_lock_irqsave(&pgd_lock, flags); + VM_BUG_ON(in_interrupt()); + spin_lock(&pgd_lock); /* * Check for races, another CPU might have split this page * up already: @@ -506,14 +506,14 @@ try_preserve_large_page(pte_t *kpte, uns } out_unlock: - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); return do_split; } static int split_large_page(pte_t *kpte, unsigned long address) { - unsigned long flags, pfn, pfninc = 1; + unsigned long pfn, pfninc = 1; unsigned int i, level; pte_t *pbase, *tmp; pgprot_t ref_prot; @@ -527,7 +527,8 @@ static int split_large_page(pte_t *kpte, if (!base) return -ENOMEM; - spin_lock_irqsave(&pgd_lock, flags); + VM_BUG_ON(in_interrupt()); + spin_lock(&pgd_lock); /* * Check for races, another CPU might have split this page * up for us already: @@ -599,7 +600,7 @@ out_unlock: */ if (base) __free_page(base); - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); return 0; } --- a/arch/x86/mm/pgtable.c +++ b/arch/x86/mm/pgtable.c @@ -126,9 +126,10 @@ static void pgd_dtor(pgd_t *pgd) if (SHARED_KERNEL_PMD) return; - spin_lock_irqsave(&pgd_lock, flags); + VM_BUG_ON(in_interrupt()); + spin_lock(&pgd_lock); pgd_list_del(pgd); - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); } /* @@ -280,12 +281,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm) * respect to anything walking the pgd_list, so that they * never see a partially populated pgd. */ - spin_lock_irqsave(&pgd_lock, flags); + VM_BUG_ON(in_interrupt()); + spin_lock(&pgd_lock); pgd_ctor(mm, pgd); pgd_prepopulate_pmd(mm, pgd, pmds); - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); return pgd; --- a/arch/x86/xen/mmu.c +++ b/arch/x86/xen/mmu.c @@ -986,10 +986,9 @@ static void xen_pgd_pin(struct mm_struct */ void xen_mm_pin_all(void) { - unsigned long flags; struct page *page; - spin_lock_irqsave(&pgd_lock, flags); + spin_lock(&pgd_lock); list_for_each_entry(page, &pgd_list, lru) { if (!PagePinned(page)) { @@ -998,7 +997,7 @@ void xen_mm_pin_all(void) } } - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); } /* @@ -1099,10 +1098,9 @@ static void xen_pgd_unpin(struct mm_stru */ void xen_mm_unpin_all(void) { - unsigned long flags; struct page *page; - spin_lock_irqsave(&pgd_lock, flags); + spin_lock(&pgd_lock); list_for_each_entry(page, &pgd_list, lru) { if (PageSavePinned(page)) { @@ -1112,7 +1110,7 @@ void xen_mm_unpin_all(void) } } - spin_unlock_irqrestore(&pgd_lock, flags); + spin_unlock(&pgd_lock); } void xen_activate_mm(struct mm_struct *prev, struct mm_struct *next) _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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