[2.6.32.y][PATCH] fix pgd_lock deadlock

Philipp Hahn
2012-04-23T09:18:10Z

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Hello,

On Wednesday 16 February 2011 15:49:47 Andrea Arcangeli wrote:
> Subject: fix pgd_lock deadlock
>
> From: Andrea Arcangeli <aarcange@redhat.com>
>
> 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@redhat.com>

This patch (original commit Id for 2.6.38
a79e53d85683c6dd9f99c90511028adc2043031f) needs to be back-ported to 2.6.32.x
as well.
I observed a dead-lock problem when running a PAE enabled Debian 2.6.32.46+
kernel with 6 VCPUs as a KVM on (2.6.32, 3.2, 3.3) kernel, which showed the
following behaviour:

1 VCPU is stuck in
pgd_alloc() → pgd_prepopulate_pmb() →... → flush_tlb_others_ipi()
while (!cpumask_empty(to_cpumask(f->flush_cpumask)))
cpu_relax();
(gdb) print f->flush_cpumask
$5 = {1}

while all other VCPUs are stuck in
pgd_alloc() → spin_lock_irqsave(pgd_lock)

I tracked it down to the commit
2.6.39-rc1: 4981d01eada5354d81c8929d5b2836829ba3df7b
2.6.32.34: ba456fd7ec1bdc31a4ad4a6bd02802dcaa730a33
x86: Flush TLB if PGD entry is changed in i386 PAE mode
which when reverted made the bug disappear.

Comparing 3.2 to 2.6.32.34 showed that the 'pgd-deadlock'-patch went into
2.6.38, that is before the 'PAE correctness'-patch, so the problem was
probably never observed in the main development branch.
But for 2.6.32 the 'pgd-deadlock' patch is still missing, so the 'PAE
corretness'-patch made the problem worse with 2.6.32.

The Patch was also back-ported to the OpenSUSE Kernel
<http://kernel.opensuse.org/cgit/kernel-source/commit/?id=ac27c01aa880c65d17043ab87249c613ac4c3635>,
Since the patch didn't apply cleanly on the current Debian kernel, I had to
backport it for us and Debian. The patch is also available from our (German)
Bugzilla <https://forge.univention.org/bugzilla/show_bug.cgi?id=26661> or
from the Debian BTS at
<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=669335>.

I have no easy test case, but running multiple parallel builds inside the VM
normally triggers the bug within seconds to minutes. With the patch applied
the VM survived a night building packages without any problem.

Signed-off-by: Philipp Hahn <hahn@univention.de>

Sincerely
Philipp
--
Philipp Hahn Open Source Software Engineer hahn@univention.de
Univention GmbH be open. fon: +49 421 22 232- 0
Mary-Somerville-Str.1 D-28359 Bremen fax: +49 421 22 232-99
http://www.univention.de/

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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.

Nobody takes the pgd_lock from irq context so the _irqsave can be
removed.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@kernel.org>
LKML-Reference: <201102162345.p1GNjMjm021738@imap1.linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Git-commit: a79e53d85683c6dd9f99c90511028adc2043031f
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -223,15 +223,14 @@ 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);
+ spin_lock(&pgd_lock);
list_for_each_entry(page, &pgd_list, lru) {
if (!vmalloc_sync_one(page_address(page), address))
break;
}
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}
}

@@ -331,13 +330,12 @@ void vmalloc_sync_all(void)
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);
+ spin_lock(&pgd_lock);
list_for_each_entry(page, &pgd_list, lru) {
pgd_t *pgd;
pgd = (pgd_t *)page_address(page) + pgd_index(address);
@@ -346,7 +344,7 @@ void vmalloc_sync_all(void)
else
BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref));
}
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}
}

--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -56,12 +56,10 @@ 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);
+ 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)
@@ -354,7 +352,7 @@ 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;
int i, do_split = 1;
@@ -363,7 +361,7 @@ try_preserve_large_page(pte_t *kpte, uns
if (cpa->force_split)
return 1;

- spin_lock_irqsave(&pgd_lock, flags);
+ spin_lock(&pgd_lock);
/*
* Check for races, another CPU might have split this page
* up already:
@@ -458,14 +456,14 @@ 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;
@@ -479,7 +477,7 @@ static int split_large_page(pte_t *kpte,
if (!base)
return -ENOMEM;

- spin_lock_irqsave(&pgd_lock, flags);
+ spin_lock(&pgd_lock);
/*
* Check for races, another CPU might have split this page
* up for us already:
@@ -551,7 +549,7 @@ 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
@@ -110,14 +110,12 @@ static void pgd_ctor(pgd_t *pgd)

static void pgd_dtor(pgd_t *pgd)
{
- unsigned long flags; /* can be called from interrupt context */
-
if (SHARED_KERNEL_PMD)
return;

- spin_lock_irqsave(&pgd_lock, flags);
+ spin_lock(&pgd_lock);
pgd_list_del(pgd);
- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}

/*
@@ -248,7 +246,6 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
{
pgd_t *pgd;
pmd_t *pmds[PREALLOCATED_PMDS];
- unsigned long flags;

pgd = (pgd_t *)__get_free_page(PGALLOC_GFP);

@@ -268,12 +265,12 @@ 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);
+ spin_lock(&pgd_lock);

pgd_ctor(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
@@ -988,10 +988,9 @@ 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)) {
@@ -1000,7 +999,7 @@ void xen_mm_pin_all(void)
}
}

- spin_unlock_irqrestore(&pgd_lock, flags);
+ spin_unlock(&pgd_lock);
}

/*
@@ -1101,10 +1100,9 @@ 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)) {
@@ -1114,7 +1112,7 @@ 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)

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