On Tue, Jan 03, 2017 at 02:36:05PM -0700, Ross Zwisler wrote:
> Currently in DAX if we have three read faults on the same hole address we
> can end up with the following:
>
> Thread 0 Thread 1 Thread 2
> -------- -------- --------
> dax_iomap_fault
> grab_mapping_entry
> lock_slot
> <locks empty DAX entry>
>
> dax_iomap_fault
> grab_mapping_entry
> get_unlocked_mapping_entry
> <sleeps on empty DAX entry>
>
> dax_iomap_fault
> grab_mapping_entry
> get_unlocked_mapping_entry
> <sleeps on empty DAX entry>
> dax_load_hole
> find_or_create_page
> ...
> page_cache_tree_insert
> dax_wake_mapping_entry_waiter
> <wakes one sleeper>
> __radix_tree_replace
> <swaps empty DAX entry with 4k zero page>
>
> <wakes>
> get_page
> lock_page
> ...
> put_locked_mapping_entry
> unlock_page
> put_page
>
> <sleeps forever on the DAX
> wait queue>
>
> The crux of the problem is that once we insert a 4k zero page, all locking
> from then on is done in terms of that 4k zero page and any additional
> threads sleeping on the empty DAX entry will never be woken. Fix this by
> waking all sleepers when we replace the DAX radix tree entry with a 4k zero
> page. This will allow all sleeping threads to successfully transition from
> locking based on the DAX empty entry to locking on the 4k zero page.
>
> With the test case reported by Xiong this happens very regularly in my test
> setup, with some runs resulting in 9+ threads in this deadlocked state.
> With this fix I've been able to run that same test dozens of times in a
> loop without issue.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Reported-by: Xiong Zhou <xzhou@xxxxxxxxxx>
> Fixes: commit ac401cc78242 ("dax: New fault locking")
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 4.7+
> ---
Positive test result of this patch for this issue and the regression
tests.
Great job!
>
> This issue exists as far back as v4.7, and I was easly able to reproduce it
> with v4.7 using the same test.
>
> Unfortunately this patch won't apply cleanly to the stable trees, but the
> change is very simple and should be easy to replicate by hand. Please ping
> me if you'd like patches that apply cleanly to the v4.9 and v4.8.15 trees.
>
> ---
> mm/filemap.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index d0e4d10..b772a33 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -138,7 +138,7 @@ static int page_cache_tree_insert(struct address_space *mapping,
> dax_radix_locked_entry(0, RADIX_DAX_EMPTY));
> /* Wakeup waiters for exceptional entry lock */
> dax_wake_mapping_entry_waiter(mapping, page->index, p,
> - false);
> + true);
> }
> }
> __radix_tree_replace(&mapping->page_tree, node, slot, page,
> --
> 2.7.4
>
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