Re: VM: Fix nasty and subtle race in shared mmap'ed page writeback

[Andrew Morton <akpm@osdl.org>]

On Wed, 3 Jan 2007 20:44:36 -0800 (PST)
Linus Torvalds <torvalds@osdl.org> wrote:

> Actually, I think 2.6.18 may have a subtle variation on it. 
> 
> In particular, I look back at the try_to_free_buffers() thing that I hated 
> so much, and it makes me wonder.. It used to do:
> 
> 	spin_lock(&mapping->private_lock);
> 	ret = drop_buffers(page, &buffers_to_free);
> 	spin_unlock(&mapping->private_lock);
> 	if (ret) {
> 		.. crappy comment ..
> 		if (test_clear_page_dirty(page))
> 			task_io_account_cancelled_write(PAGE_CACHE_SIZE);
> 	}
> 
> and I think that at least on SMP, we had a race with another CPU doing the 
> "mark page dirty if it was dirty in the PTE" at the same time. Because the 
> marking dirty would come in, find no buffers (they just got dropped), and 
> then mark the page dirty (ignoring the lack of any buffers), but then the 
> above would do the "test_clear_page_dirty()" thing on it.
> 

That bug was introduced in 2.6.19, with the dirty page tracking patches.

2.6.18 and earlier used ->private_lock coverage in try_to_free_buffers() to
prevent it.

> Ie the race, I think, existed where that crappy comment was.

The comment was complete, accurate and needed.