Re: [PATCH v2 2/3] btrfs: fix race on ENOMEM in alloc_extent_buffer

[David Sterba <dsterba@suse.cz>]

On Tue, Feb 24, 2015 at 02:47:05AM -0800, Omar Sandoval wrote:
> Consider the following interleaving of overlapping calls to
> alloc_extent_buffer:
> 
> Call 1:
> 
> - Successfully allocates a few pages with find_or_create_page
> - find_or_create_page fails, goto free_eb
> - Unlocks the allocated pages
> 
> Call 2:
> - Calls find_or_create_page and gets a page in call 1's extent_buffer
> - Finds that the page is already associated with an extent_buffer
> - Grabs a reference to the half-written extent_buffer and calls
>   mark_extent_buffer_accessed on it
> 
> mark_extent_buffer_accessed will then try to call mark_page_accessed on
> a null page and panic.
> 
> The fix is to decrement the reference count on the half-written
> extent_buffer before unlocking the pages so call 2 won't use it. We
> should also set exists = NULL in the case that we don't use exists to
> avoid accidentally returning a freed extent_buffer in an error case.
> 
> Signed-off-by: Omar Sandoval <osandov@osandov.com>

Reviewed-by: David Sterba <dsterba@suse.cz>