Re: [Qemu-devel] [PATCH 1/2] Fix Block Hotplug race with drive_del()

[Ryan Harper]

* Markus Armbruster <address@hidden> [2010-11-10 11:40]:
> Ryan Harper <address@hidden> writes:
> 
> > * Markus Armbruster <address@hidden> [2010-11-10 06:48]:
> >> One real question, and a couple of nits.
> >> 
> >> Ryan Harper <address@hidden> writes:
> >> 
> >> > Block hot unplug is racy since the guest is required to acknowlege the 
> >> > ACPI
> >> > unplug event; this may not happen synchronously with the device removal 
> >> > command
> >> 
> >> Well, I wouldn't call unplug "racy".  It just takes an unpredictable
> >> length of time, possibly forever.  To make a race, you need to throw in
> >> a client assuming (incorrectly) that unplug is instantaneous, as
> >> described in your next paragraph.
> >> 
> >> Moreover, all PCI unplug is that way, not just block.
> >> 
> >> > This series aims to close a gap where by mgmt applications that assume 
> >> > the
> >> > block resource has been removed without confirming that the guest has
> >> > acknowledged the removal may re-assign the underlying device to a second 
> >> > guest
> >> > leading to data leakage.
> >> 
> >> Yes, the incorrect assumption is a problem.  But with that fixed (in the
> >> management application), we run right into the next problem: there is no
> >> way for the management application to reliably disconnect the guest from
> >> a block device.  And that's the problem you're fixing.
> >
> > Yeah, that's the right way to word it; providing a method to forcibly
> > disconnect the guest from the host device.
> >> 
> >> > This series introduces a new montor command to decouple asynchornous 
> >> > device
> >> 
> >> Typos "montor" and "asynchornous".  You might want to use a spell
> >> checker :)
> >> 
> >> Lines are a bit long.  Recommend wrap at column 70.
> >> 
> >> > removal from restricting guest access to a block device.  We do this by 
> >> > creating
> >> > a new monitor command drive_del which maps to a bdrv_unplug() command 
> >> > which
> >> > does a qemu_aio_flush; bdrv_flush() and bdrv_close().  Once complete, 
> >> > subsequent
> >> > IO is rejected from the device and the guest will get IO errors but 
> >> > continue to
> >> > function.  In addition to preventing further IO, we clean up state 
> >> > pointers
> >> > between host (BlockDriverState) and guest (DeviceInfo).
> >> >
> >> > A subsequent device removal command can be issued to remove the device, 
> >> > to which
> >> > the guest may or maynot respond, but as long as the unplugged bit is 
> >> > set, no IO
> >> 
> >> "maynot" is not a word.
> >> 
> >> > will be sumbitted.
> >> 
> >> This suggests to drive_del before device_del, which makes the device
> >> goes through a "broken device" state on its way to unplug.  If the guest
> >> accesses the device in that state, it gets I/O errors.  Not nice.
> >> 
> >> Instead, I'd recommend device_del, wait for the device to go away,
> >> drive_del on time out.  If the guest reacts to the ACPI unplug promptly,
> >> it's never exposed to the "broken device" state.  Note: if the drive_del
> >> fails because the device doesn't exist, we lost the race with the
> >> automatic destruction, which is harmless.  Ignore that error.
> >
> > Honestly, other than describing what happens if you sever the connection
> > when the guest isn't aware of it; I don't want to try to capture how the
> > mgmt layer implements the removal.  
> >
> > One may want to force the disconnect before attempting to remove the
> > device; or the other way around; that's really the mgmt layer's call.
> 
> Fair enough.
> 
> >> > Signed-off-by: Ryan Harper <address@hidden>
> >> > ---
> >> >  block.c         |    7 +++++++
> >> >  block.h         |    1 +
> >> >  blockdev.c      |   36 ++++++++++++++++++++++++++++++++++++
> >> >  blockdev.h      |    1 +
> >> >  hmp-commands.hx |   18 ++++++++++++++++++
> >> >  5 files changed, 63 insertions(+), 0 deletions(-)
> >> >
> >> > diff --git a/block.c b/block.c
> >> > index 6b505fb..c76a796 100644
> >> > --- a/block.c
> >> > +++ b/block.c
> >> > @@ -1328,6 +1328,13 @@ void bdrv_set_removable(BlockDriverState *bs, int 
> >> > removable)
> >> >      }
> >> >  }
> >> >  
> >> > +void bdrv_unplug(BlockDriverState *bs)
> >> > +{
> >> > +    qemu_aio_flush();
> >> > +    bdrv_flush(bs);
> >> > +    bdrv_close(bs);
> >> > +}
> >> > +
> >> 
> >> Unless we expect more users, I'd inline this into its only caller.
> >> Matter of taste.
> >
> > Works for me.
> >
> >> 
> >> >  int bdrv_is_removable(BlockDriverState *bs)
> >> >  {
> >> >      return bs->removable;
> >> > diff --git a/block.h b/block.h
> >> > index 78ecfac..581414c 100644
> >> > --- a/block.h
> >> > +++ b/block.h
> >> > @@ -171,6 +171,7 @@ void bdrv_set_on_error(BlockDriverState *bs, 
> >> > BlockErrorAction on_read_error,
> >> >                         BlockErrorAction on_write_error);
> >> >  BlockErrorAction bdrv_get_on_error(BlockDriverState *bs, int is_read);
> >> >  void bdrv_set_removable(BlockDriverState *bs, int removable);
> >> > +void bdrv_unplug(BlockDriverState *bs);
> >> >  int bdrv_is_removable(BlockDriverState *bs);
> >> >  int bdrv_is_read_only(BlockDriverState *bs);
> >> >  int bdrv_is_sg(BlockDriverState *bs);
> >> > diff --git a/blockdev.c b/blockdev.c
> >> > index 6cb179a..ee8c2ec 100644
> >> > --- a/blockdev.c
> >> > +++ b/blockdev.c
> >> > @@ -14,6 +14,8 @@
> >> >  #include "qemu-option.h"
> >> >  #include "qemu-config.h"
> >> >  #include "sysemu.h"
> >> > +#include "hw/qdev.h"
> >> > +#include "block_int.h"
> >> >  
> >> >  static QTAILQ_HEAD(drivelist, DriveInfo) drives = 
> >> > QTAILQ_HEAD_INITIALIZER(drives);
> >> >  
> >> > @@ -597,3 +599,37 @@ int do_change_block(Monitor *mon, const char 
> >> > *device,
> >> >      }
> >> >      return monitor_read_bdrv_key_start(mon, bs, NULL, NULL);
> >> >  }
> >> > +
> >> > +int do_drive_del(Monitor *mon, const QDict *qdict, QObject **ret_data)
> >> > +{
> >> > +    const char *id = qdict_get_str(qdict, "id");
> >> > +    BlockDriverState *bs;
> >> > +    Property *prop;
> >> > +
> >> > +    bs = bdrv_find(id);
> >> > +    if (!bs) {
> >> > +        qerror_report(QERR_DEVICE_NOT_FOUND, id);
> >> > +        return -1;
> >> > +    }
> >> > +
> >> > +    /* quiesce block driver; prevent further io */
> >> > +    bdrv_unplug(bs);
> >> > +
> >> > +    /* clean up guest state from pointing to host resource by
> >> > +     * finding and removing DeviceState "drive" property */
> >> > +    for (prop = bs->peer->info->props; prop && prop->name; prop++) {
> >> > +        if ((prop->info->type == PROP_TYPE_DRIVE) && 
> >> > +            (*(BlockDriverState **)qdev_get_prop_ptr(bs->peer, prop) == 
> >> > bs)) {
> >> > +            if (prop->info->free) {
> >> > +                prop->info->free(bs->peer, prop);
> >> > +            }
> 
> Your use of prop->info->free() in this context is wrong.  More below.
> 
> >> 
> >> Does this null the drive property?  I doubt it.  Quick check in the
> >> debugger?
> >> 
> >> The free callbacks generally don't zap the properties, because they run
> >> from qdev_free().
> >
> > To be honest; I didn't see anything that looked like "remove this
> > property" in the qdev api.  Any pointers?
> 
> The closest we have is indeed the Property method free(), but that's not
> quite right.  It's really only for use by qdev_free().
> 
> > should I be calling qdev_free() on the dev?
> 
> No, because then the whole device is gone, not just the property :)
> 
> >                                              I don't quite understand
> > the distinction between the info list of properties and the device
> > itself, nor specifically what we need to remove in the drive_del()
> > operation versus the device_del() portion.
> 
> device_del / qdev_free() destroy a qdev, such as a "virtio-blk-pci"
> device (C type VirtIOPCIProxy).
> 
> drive_del destroys something else, namely the block device host part
> (BlockDriverState + DeviceInfo).  Obviously, it needs to zap all
> pointers to the host part along with it.  Specifically, it needs to zap
> the device's pointer to it.
> 
> Example: if a "virtio-blk-pci" device is using drive "foo", then
> "drive_del foo" needs to zap its member block.bs.
> 
> Complication: we don't (want to) know what kind of device exactly is
> using the drive.  But we do know that a drive property must be
> describing it.
> 
> So we search the properties (for (prop...)) for a drive property
> (prop->info->type == PROP_TYPE_DRIVE) that points to this drive (... ==
> bs).
> 
> Result:
> 
>     BlockDriverState *bs;
>     Property *prop;
>     BlockDriverState **ptr;
> [...]
>     for (prop = bs->peer->info->props; prop && prop->name; prop++) {
>         if ((prop->info->type == PROP_TYPE_DRIVE)) {
>             ptr = qdev_get_prop_ptr(dev, prop);
>             if (*ptr == bs) {
>                 bdrv_detach(bs, bs->peer);
>                 *ptr = NULL;
>                 break;
>             }
>         }
>     }
> 
> Aside: arguably, bdrv_detach() should zap *both* pointers, i.e. also do
> the *ptr = NULL.  Not your problem to fix.
> 
> Only then are we ready to destroy the host part:
> 
>     drive_uninit(drive_get_by_blockdev(bs));
> 
> Does this help?

Yep; lemme get a v7 out.

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
address@hidden