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Re: [Qemu-devel] [Qemu-block] [PATCH v2 06/16] backup: Request BLK_PERM_
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH v2 06/16] backup: Request BLK_PERM_AIO_CONTEXT_CHANGE on target |
Date: |
Thu, 1 Jun 2017 14:26:16 +0100 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
On Wed, May 31, 2017 at 05:57:46PM +0800, Fam Zheng wrote:
> On Wed, 05/31 10:39, Stefan Hajnoczi wrote:
> > On Wed, May 24, 2017 at 10:18:44AM +0800, Fam Zheng wrote:
> > > On Thu, 05/11 15:41, Stefan Hajnoczi wrote:
> > > > On Wed, Apr 19, 2017 at 05:43:46PM +0800, Fam Zheng wrote:
> > > > > What's done in the source's context change notifier is moving the
> > > > > target's context to follow the new one, so we request this permission
> > > > > here.
> > > >
> > > > It's true that the backup block job must be able to set target's
> > > > AioContext, but does this change also allow other users to set target's
> > > > AioContext while the backup job is running? If yes, then we need to
> > > > handle that.
> > >
> > > If through job->target, yes, but I don't think there is any user of
> > > job->target.
> > > Otherwise, it's not allowed, because the second parameter of blk_new
> > > doesn't
> > > have BLK_PERM_AIO_CONTEXT_CHANGE.
> > >
> > > So it's okay.
> >
> > What about blockdev-backup? It allows the user to specify 'target'.
> > Therefore the user can also run other monitor commands on target. Some
> > of them could change the AioContext and the backup job wouldn't know!
>
> That will be rejected.
>
> The contract is that any code that wants to change the AioContext of a BDS, in
> this case the "target BDS", must do this:
>
> 1) create its own BB with perm.BLK_PERM_AIO_CONTEXT_CHANGE
>
> 2) attach BDS to this BB
>
> 3) call blk_set_aio_context and change the AioContext
>
> This is basically how all users of a BDS coordinate through Kevin's new op
> blocker API, and in your concerned case, when a user runs a second monitor
> command that changes AioContext, step 2 will fail, because as in this patch,
> the
> first job->target BB didn't set shared_perm.BLK_PERM_AIO_CONTEXT_CHANGE.
I was wondering how that works since do_blockdev_backup() does not use
BB to access target, but it does check whether a BB is already attached:
target_bs = bdrv_lookup_bs(backup->target, backup->target, errp);
if (!target_bs) {
goto out;
}
if (bdrv_get_aio_context(target_bs) != aio_context) {
if (!bdrv_has_blk(target_bs)) { <----- fails when job is running
/* The target BDS is not attached, we can safely move it to another
* AioContext. */
bdrv_set_aio_context(target_bs, aio_context);
} else {
error_setg(errp, "Target is attached to a different thread from "
"source.");
goto out;
}
}
Thanks!
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- Re: [Qemu-devel] [Qemu-block] [PATCH v2 06/16] backup: Request BLK_PERM_AIO_CONTEXT_CHANGE on target,
Stefan Hajnoczi <=