Re: [Qemu-devel] [PATCH] block: fix QEMU crash with scsi-hd and drive_del

[Greg Kurz <groug@kaod.org>]

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On Fri, 18 May 2018 16:32:46 +0100
Stefan Hajnoczi <stefanha@redhat.com> wrote:

> On Wed, May 16, 2018 at 01:21:54PM +0200, Greg Kurz wrote:
> > Removing a drive with drive_del while it is being used to run an I/O
> > intensive workload can cause QEMU to crash.
> > 
> > An AIO flush can yield at some point:
> > 
> > blk_aio_flush_entry()
> >  blk_co_flush(blk)
> >   bdrv_co_flush(blk->root->bs)
> >    ...
> >     qemu_coroutine_yield()  
> 
> I'm surprised you didn't hit another crash later on with this patch
> applied.  What happens to this completion after you've set blk->root =
> NULL?
> 

bdrv_co_flush() takes a BDS argument, so I don't see how it would be
affected by blk->root being set to NULL. Then blk_co_flush() returns
to blk_aio_flush_entry(), and the next user of blk->root is....

> > and let the HMP command to run, free blk->root and give control
> > back to the AIO flush:
> > 
> >     hmp_drive_del()
> >      blk_remove_bs()
> >       bdrv_root_unref_child(blk->root)
> >        child_bs = blk->root->bs
> >        bdrv_detach_child(blk->root)
> >         bdrv_replace_child(blk->root, NULL)
> >          blk->root->bs = NULL
> >         g_free(blk->root) <============== blk->root becomes stale
> >        bdrv_unref(child_bs)
> >         bdrv_delete(child_bs)
> >          bdrv_close()
> >           bdrv_drained_begin()
> >            bdrv_do_drained_begin()
> >             bdrv_drain_recurse()
> >              aio_poll()
> >               ...
> >               qemu_coroutine_switch()
> > 
> > and the AIO flush completion ends up dereferencing blk->root:
> > 
> >   blk_aio_complete()
> >    scsi_aio_complete()
> >     blk_get_aio_context(blk)
> >      bs = blk_bs(blk)
> >  ie, bs = blk->root ? blk->root->bs : NULL

... here and the completion ends in the main loop context.

> >             ^^^^^
> >             stale
> > 
> > The solution to this user-after-free situation is is to clear
> > blk->root before calling bdrv_unref() in bdrv_detach_child(),
> > and let blk_get_aio_context() fall back to the main loop context
> > since the BDS has been removed.  
> 
> QEMU should drain I/O requests before making block driver graph changes.
> I think the drained region in blk_remove_bs() needs to begin earlier so
> that requests are completed before we begin to change things.
> 

This looks better indeed. The drained section currently begins in
bdrv_close(), which happens much later than bdrv_detach_child(),
which actually change things.

Maybe change bdrv_root_unref_child() to ensure we don't call
bdrv_close() with pending I/O requests ?

 void bdrv_root_unref_child(BdrvChild *child)
 {
     BlockDriverState *child_bs;
 
     child_bs = child->bs;
+    bdrv_drained_begin(child_bs);
     bdrv_detach_child(child);
+    bdrv_drained_end(child_bs);
     bdrv_unref(child_bs);
 }


> Stefan


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