Re: [PATCH 13/16] block: switch polling to be bio based

From: Ming Lei <ming.lei@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>,
	Jeffle Xu <jefflexu@linux.alibaba.com>,
	Damien Le Moal <Damien.LeMoal@wdc.com>,
	Keith Busch <kbusch@kernel.org>, Sagi Grimberg <sagi@grimberg.me>,
	"Wunderlich, Mark" <mark.wunderlich@intel.com>,
	"Vasudevan, Anil" <anil.vasudevan@intel.com>,
	linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH 13/16] block: switch polling to be bio based
Date: Mon, 21 Jun 2021 16:35:29 +0800	[thread overview]
Message-ID: <YNBPUUVtxSb6/gv1@T590> (raw)
In-Reply-To: <20210621072005.GA6651@lst.de>

On Mon, Jun 21, 2021 at 09:20:05AM +0200, Christoph Hellwig wrote:
> On Fri, Jun 18, 2021 at 10:27:20PM +0800, Ming Lei wrote:
> > > How?  On a block device the caller needs to hold the block device open
> > > to read/write from it.  On a file systems the file systems needs to
> > > be mounted, which also holds a bdev reference.
> > 
> > +       rcu_read_lock();
> > +       bio = READ_ONCE(kiocb->private);
> > +       if (bio && bio->bi_bdev)
> > 
> > The bio may be ended now from another polling job, then the disk is
> > closed & deleted, and released. Then request queue & hctxs are released.
> > 
> > +               ret = bio_poll(bio, flags);
> > 
> > But disk & request queue & hctx can still be referred in above bio_poll().
> 
> I don't see how this can happen.  A bio stashed into kiocb->private needs
> to belong to the correct device initially.  For it to point to the "wrong"
> device it needs to have been completed on the correct one, and then be
> reused for a different device.  At the point it is reused that device
> must obviously have been alive, and for it to be freed a RCU grace
> period must have been passed.  And that grace period can't have started
> earlier than when iocb_bio_iopoll was called.

gendisk isn't freed after a RCU grace period, so even though bio->bi_bdev
may not be freed really, but the gendisk may have been freed already.

+       rcu_read_lock();
+       bio = READ_ONCE(kiocb->private);
+       if (bio && bio->bi_bdev)

The bio may be ended now from another polling job, and it is freed
and re-allocated & freed, then bio->bi_bdev->bd_disk is freed too, which
will be observed in the following bio_poll().

+               ret = bio_poll(bio, flags);

Thanks,
Ming