Re: [Qemu-devel] question： I found a bug which will lead to qemu crash

[Alberto Garcia]

On Tue 12 Sep 2017 01:17:38 PM CEST, WangJie (Captain) wrote:
> Hi, Kevin.
>
> I found a bug about qemu-kvm(version 2.7.0-rc0 adn 2.8.1). but qemu 2.6.0 and 
> current master is OK.
> So I git-bisect the master branch，and I found the patch you commited (block: 
> Decouple throttling from BlockDriverState) lead the bug into qemu.
>
> The patch which lead the bug into qemu: 
> (https://github.com/qemu/qemu/commit/7ca7f0f6db1fedd28d490795d778cf23979a2aa7#diff-ea36ba0f79150cc299732696a069caba)
>
> Because the current master is OK. So I think you had fixed it , can you tell 
> me which patch fixed the bug?        Thank you :>
>
>
> the bug is that: qemu will crash when loop to attach and detach a disk which 
> configured qos to a VM for a while.
>
>
> *Segmentation fault info(qemu 2.7.0-rc0):*
> Using host libthread_db library "/lib64/libthread_db.so.1".
> Core was generated by `/usr/bin/qemu-kvm -name 
> guest=wangjie-i-clone203_rhel_7.3_64_guestosdev,debug-t'.
> Program terminated with signal 11, Segmentation fault.
> #0  0x00007fe960413e3c in throttle_group_next_blk (blk=0x11) at 
> block/throttle-groups.c:160

That's clearly an invalid pointer, so the code is iterating over a
BlockBackend that has either been freed or is not on the throttle_groups
list anymore.

Commit 6bf77e1c2dc24da1bade16e8a9a637f3b127314d fixed a problem in which
the code was not iterating the list correctly, although I don't think
that would have caused any crash.

Did you try using git-bisect to find the commit that fixed the bug?

Berto