Re: [PATCH] locking/rwsem: Fix kernel crash when spinning on RWSEM_OWNER_UNKNOWN

[Christoph Hellwig]

On Tue, Jan 14, 2020 at 02:03:03PM -0500, Waiman Long wrote:
> The commit 91d2a812dfb9 ("locking/rwsem: Make handoff writer
> optimistically spin on owner") will allow a recently woken up waiting
> writer to spin on the owner. Unfortunately, if the owner happens to be
> RWSEM_OWNER_UNKNOWN, the code will incorrectly spin on it leading to a
> kernel crash. This is fixed by passing the proper non-spinnable bits
> to rwsem_spin_on_owner() so that RWSEM_OWNER_UNKNOWN will be treated
> as a non-spinnable target.
> 
> Fixes: 91d2a812dfb9 ("locking/rwsem: Make handoff writer optimistically spin on owner")
> 
> Reported-by: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

This survives all the tests that showed the problems with the original
code:

Tested-by: Christoph Hellwig <hch@xxxxxx>

>  		if ((wstate == WRITER_HANDOFF) &&
> -		    (rwsem_spin_on_owner(sem, 0) == OWNER_NULL))
> +		    rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)

Nit: the inner braces in the first half of the conditional aren't required
either.