Re: [patch] mm: fix lockless pagecache reordering bug (was Re: BUG: soft lockup - is this XFS problem?)

Peter Klotz
2009-01-06T08:38:21Z

Nick Piggin wrote:
> --
> Subject: mm lockless pagecache barrier fix
>
> An XFS workload showed up a bug in the lockless pagecache patch. Basically it
> would go into an "infinite" loop, although it would sometimes be able to break
> out of the loop! The reason is a missing compiler barrier in the "increment
> reference count unless it was zero" case of the lockless pagecache protocol in
> the gang lookup functions.
>
> This would cause the compiler to use a cached value of struct page pointer to
> retry the operation with, rather than reload it. So the page might have been
> removed from pagecache and freed (refcount==0) but the lookup would not correctly
> notice the page is no longer in pagecache, and keep attempting to increment the
> refcount and failing, until the page gets reallocated for something else. This
> isn't a data corruption because the condition will be detected if the page has
> been reallocated. However it can result in a lockup.
>
> Linus points out that ACCESS_ONCE is also required in that pointer load, even
> if it's absence is not causing a bug on our particular build. The most general
> way to solve this is just to put an rcu_dereference in radix_tree_deref_slot.
>
> Assembly of find_get_pages,
> before:
> .L220:
> movq (%rbx), %rax #* ivtmp.1162, tmp82
> movq (%rax), %rdi #, prephitmp.1149
> .L218:
> testb $1, %dil #, prephitmp.1149
> jne .L217 #,
> testq %rdi, %rdi # prephitmp.1149
> je .L203 #,
> cmpq $-1, %rdi #, prephitmp.1149
> je .L217 #,
> movl 8(%rdi), %esi # <variable>._count.counter, c
> testl %esi, %esi # c
> je .L218 #,
>
> after:
> .L212:
> movq (%rbx), %rax #* ivtmp.1109, tmp81
> movq (%rax), %rdi #, ret
> testb $1, %dil #, ret
> jne .L211 #,
> testq %rdi, %rdi # ret
> je .L197 #,
> cmpq $-1, %rdi #, ret
> je .L211 #,
> movl 8(%rdi), %esi # <variable>._count.counter, c
> testl %esi, %esi # c
> je .L212 #,
>
> (notice the obvious infinite loop in the first example, if page->count remains 0)
>
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> ---
> include/linux/radix-tree.h | 2 +-
> mm/filemap.c | 23 ++++++++++++++++++++---
> 2 files changed, 21 insertions(+), 4 deletions(-)
>
> Index: linux-2.6/include/linux/radix-tree.h
> ===================================================================
> --- linux-2.6.orig/include/linux/radix-tree.h
> +++ linux-2.6/include/linux/radix-tree.h
> @@ -136,7 +136,7 @@ do { \
> */
> static inline void *radix_tree_deref_slot(void **pslot)
> {
> - void *ret = *pslot;
> + void *ret = rcu_dereference(*pslot);
> if (unlikely(radix_tree_is_indirect_ptr(ret)))
> ret = RADIX_TREE_RETRY;
> return ret;
>
>

The patch above fixes my problem. I did two complete test runs that
normally fail rather quickly.

Regards, Peter.