On Mon 26-11-18 08:12:40, Matthew Wilcox wrote:
>
> I noticed this path while I was doing the 4.19 backport of
> dax: Avoid losing wakeup in dax_lock_mapping_entry
>
> xa_unlock_irq(&mapping->i_pages);
> revalidate = wait_fn();
> finish_wait(wq, &ewait.wait);
> xa_lock_irq(&mapping->i_pages);
I guess this is a snippet from get_unlocked_entry(), isn't it?
> It's not safe to call xa_lock_irq() if mapping can have been freed while
> we slept. We'll probably get away with it; most filesystems use a unique
> slab for their inodes, so you'll likely get either a freed inode or an
> inode which is now the wrong inode. But if that page has been freed back
> to the page allocator, that pointer could now be pointing at anything.
Correct. Thanks for catching this bug!
> Fixing this in the current codebase is no easier than fixing it in the
> 4.19 codebase. This is the best I've come up with. Could we do better
> by not using the _exclusive form of prepare_to_wait()? I'm not familiar
> with all the things that need to be considered when using this family
> of interfaces.
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 9bcce89ea18e..154b592b18eb 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -232,6 +232,24 @@ static void *get_unlocked_entry(struct xa_state *xas)
> }
> }
>
> +static void wait_unlocked_entry(struct xa_state *xas, void *entry)
> +{
> + struct wait_exceptional_entry_queue ewait;
> + wait_queue_head_t *wq;
> +
> + init_wait(&ewait.wait);
> + ewait.wait.func = wake_exceptional_entry_func;
> +
> + wq = dax_entry_waitqueue(xas, entry, &ewait.key);
> + prepare_to_wait_exclusive(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
> + xas_unlock_irq(xas);
> + /* We can no longer look at xas */
> + schedule();
> + finish_wait(wq, &ewait.wait);
> + if (waitqueue_active(wq))
> + __wake_up(wq, TASK_NORMAL, 1, &ewait.key);
> +}
> +
The code looks good. Maybe can we call this wait_entry_unlocked() to stress
that entry is not really usable after this function returns? And comment
before the function that this is safe to call even if we don't have a
reference keeping mapping alive?
> static void put_unlocked_entry(struct xa_state *xas, void *entry)
> {
> /* If we were the only waiter woken, wake the next one */
> @@ -389,9 +407,7 @@ bool dax_lock_mapping_entry(struct page *page)
> entry = xas_load(&xas);
> if (dax_is_locked(entry)) {
> rcu_read_unlock();
> - entry = get_unlocked_entry(&xas);
> - xas_unlock_irq(&xas);
> - put_unlocked_entry(&xas, entry);
> + wait_unlocked_entry(&xas, entry);
> rcu_read_lock();
> continue;
The continue here actually is not safe either because if the mapping got
freed, page->mapping will be NULL and we oops at the beginning of the loop.
So that !dax_mapping() check should also check for mapping != NULL.
Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR
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