Re: [PATCH] lockdep: introduce lockdep_assert_not_held()

[David Sterba <dsterba@suse.cz>]

On Thu, Jun 13, 2019 at 03:36:04PM +0200, David Sterba wrote:
> Add an assertion that a lock is not held, suitable for the following
> (simplified) usecase in filesystems:
> 
> - filesystem write
>   - lock(&big_filesystem_lock)
>   - kmalloc(GFP_KERNEL)
>     - trigger dirty data write to get more memory
>       - find dirty pages
>       - call filesystem write
>         - lock(&big_filesystem_lock)
> 	  deadlock
> 
> The cause here is the use of GFP_KERNEL that does not exclude poking
> filesystems to allow freeing some memory. Such scenario is a bug, so the
> use of GFP_NOFS is the right flag.
> 
> The annotation can help catch such bugs during development because
> the actual deadlock could be hard to hit in practice.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>

Any comments on that? I just found another case with convoluted
callstacks where the lockdep assertion would catch the potential lock up
earlier than under the testing load.