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

[Markus Armbruster]

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?