[PATCH v2] proc/vmcore: fix possible deadlock on concurrent mmap and read

[Andrew Morton]

On Wed, 19 Jan 2022 20:34:17 +0100 David Hildenbrand <david at redhat.com> wrote:

> Lockdep noticed that there is chance for a deadlock if we have
> concurrent mmap, concurrent read, and the addition/removal of a
> callback.
> 
> As nicely explained by Boqun:
> 
> "
> Lockdep warned about the above sequences because rw_semaphore is a fair
> read-write lock, and the following can cause a deadlock:
> 
> 	TASK 1			TASK 2		TASK 3
> 	======			======		======
> 	down_write(mmap_lock);
> 				down_read(vmcore_cb_rwsem)
> 						down_write(vmcore_cb_rwsem); // blocked
> 	down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness
> 				down_read(mmap_lock); // blocked

I'm wondering about cc:stable.  It's hard to believe that this is
likely to be observed in real life.  But the ongoing reports of lockdep
splats will be irritating.