netdev - Re: [PATCH 07/30] aio: add delayed cancel support

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Date:   Thu, 29 Mar 2018 23:35:00 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Christoph Hellwig <hch@....de>
Cc:     Avi Kivity <avi@...lladb.com>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/30] aio: add delayed cancel support

On Thu, Mar 29, 2018 at 10:33:05PM +0200, Christoph Hellwig wrote:
> The upcoming aio poll support would like to be able to complete the
> iocb inline from the cancellation context, but that would cause a
> double lock of ctx_lock with the current locking scheme.  Move the
> cancelation outside the context lock to avoid this reversal, which
> suits the existing usb gadgets users just fine as well (in fact
> both unconditionally disable irqs and thus seem broken without
> this change).
> 
> To make this safe aio_complete needs to check if this call should
> complete the iocb.  If it didn't the callers must not release any
> other resources.

Uh-oh...  What happens to existing users of kiocb_set_cancel_fn() now?
AFAICS, those guys will *not* get aio_kiocb freed at all in case of
io_cancel(2).  Look: we mark them with AIO_IOCB_CANCELLED and
call whatever ->ki_cancel() the driver has set.  Later the damn
thing calls ->ki_complete() (i.e. aio_complete_rw()), which calls
aio_complete(iocb, res, res2, 0) and gets false.  Nothing's freed,
struct file is leaked.

Frankly, the more I look at that, the less I like what you've done
with ->ki_cancel() overloading.  In regular case it's just accelerating
the call of ->ki_complete(), which will do freeing.  Here you have
->ki_cancel() free the damn thing, with the resulting need to play
silly buggers with locking, freeing logics in aio_complete(), etc.