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Re: [Qemu-devel] [PATCH] quorum: don't share qiov
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH] quorum: don't share qiov |
Date: |
Tue, 3 Feb 2015 10:22:10 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 02.02.2015 um 02:19 hat Wen Congyang geschrieben:
> On 01/30/2015 09:39 PM, Kevin Wolf wrote:
> > Am 30.01.2015 um 09:07 hat Wen Congyang geschrieben:
> >> If the child touches qiov->iov, it will cause unexpected results.
> >>
> >> Signed-off-by: Wen Congyang <address@hidden>
> >
> > Any specific child you're thinking of?
> >
> > I think children are not supposed to modify their qiov (which would also
> > fail for init_external qiovs). Perhaps we should have made it const.
>
> NBD client. The qiov will be modified in iov_send_recv():
> nbd_co_writev()
> nbd_client_session_co_writev()
> nbd_co_writev_1()
> nbd_co_send_request()
> qemu_co_sendv()
> qemu_co_sendv_recvv()
> iov_send_recv()
Paolo, I think it's rather surprising that iov_send_recv() modifies its
iov. The modification is undone at the end, so you seem to have
considered that a caller might be reusing it after and you can't "use it
up", but we still get problems with concurrent accesses.
Was it an intentional design decision that iov_send_recv() is the sole
owner of the iov and the caller must duplicate it if it's used elsewhere
concurrently?
Otherwise I would suggest to fix iov_send_recv(), and possibly try and
make all the qiov/iov arguments in the block layer const.
Kevin