On Sun, May 20, 2018 at 08:33:39AM +0100, Al Viro wrote:

quoted

... get buggered on attempt to dereference a pointer fetched from freed and
reused object.

FWIW, how painful would it be to pull the following trick:
	* insert into wait queue under ->ctx_lock
	* have wakeup do schedule_work() with aio_complete() done from that
	* have ->ki_cancel() grab queue lock, remove from queue and use
the same schedule_work()

That way you'd get ->ki_cancel() with the same semantics as originally for
everything - "ask politely to finish ASAP", and called in the same locking
environment for everyone - under ->ctx_lock, that is.  queue lock nests
inside ->ctx_lock; no magical flags, etc.

The cost is schedule_work() for each async poll-related completion as you
have for fsync.  I don't know whether that's too costly or not; it certainly
simplifies the things, but whether it's OK performance-wise...

I think it is doable:

	http://git.infradead.org/users/hch/vfs.git/commitdiff/c441130e405465268ea10c9ddd5639c155f779e8

downside is that sizeof(struct aio_kiocb) grows a bit.

For the completion performance we can use a spin_trylock to still avoid
the context switch for the common case:

	http://git.infradead.org/users/hch/vfs.git/commitdiff/6cc1827afbea87c52fe425cf533bfcf5f3308163