[Xen-devel] Re: [PATCH] fix pgd_lock deadlock

[Larry Woodman <lwoodman@xxxxxxxxxx>]

On 02/15/2011 02:26 PM, Thomas Gleixner wrote:

On Tue, 15 Feb 2011, Andrea Arcangeli wrote:

Hello,

Without this patch we can deadlock in the page_table_lock with NR_CPUS
< 4 or THP on, with this patch we hopefully won't deadlock in the
pgd_lock (if taken from irq). I can't see anything taking it from irq
(maybe aio? to check I also tried the libaio testuite with no apparent
VM_BUG_ON triggering), so unless somebody sees it, I think we should
apply it. I've been running for a while with this patch applied
without apparent problems. Other archs may follow suit if it's proven
that there's nothing taking the pgd_lock from irq.

===
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.

I really read this thing 5 times and still cannot make any sense of it.

You talk about page_table_lock and then fiddle with pgd_lock.

-ENOSENSE

	tglx

 

I put this expanation in the redhat BZ, says it all:

Larry Woodman 2011-01-21 15:54:58 EST

The problem is with THP.  The page reclaim code calls page_referenced_one()
which takes the mm->page_table_lock on one CPU before sending an IPI to other
CPU(s):

On CPU1 we take the mm->page_table_lock, send IPIs and wait for a response:
page_referenced_one(...)
        if (unlikely(PageTransHuge(page))) {
                pmd_t *pmd;

                spin_lock(&mm->page_table_lock);
                pmd = page_check_address_pmd(page, mm, address,
                                             PAGE_CHECK_ADDRESS_PMD_FLAG);
                if (pmd && !pmd_trans_splitting(*pmd) &&
                    pmdp_clear_flush_young_notify(vma, address, pmd))
                        referenced++;
                spin_unlock(&mm->page_table_lock);
        } else {


CPU2 can race in vmalloc_sync_all() because it disables interrupt(preventing a
response to the IPI from CPU1) and takes the pgd_lock then spins in the
mm->page_table_lock which is already held on CPU1.

                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);


At this point the system is deadlocked.  The pmdp_clear_flush_young_notify
needs to do its PDG business with the page_table_lock held then release that
lock before sending the IPIs to the other CPUs.


Larry

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