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

[Peter Klotz <peter.klotz@aon.at>]

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.

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