Am 08.12.2017 um 14:39 hat Max Reitz geschrieben: > On 2017-12-06 10:12, Kevin Wolf wrote: > > Am 06.12.2017 um 08:28 hat Kangjie Xi geschrieben: > >> Hi, > >> > >> I encountered a qemu-nbd segfault, finally I found it was caused by > >> NULL bs-drv, which is located in block/io.c function bdrv_co_flush > >> line 2377: > >> > >> https://git.qemu.org/?p=qemu.git;a=blob;f=block/io.c;h=4fdf93a0144fa4761a14b8cc6b2a9a6b6e5d5bec;hb=d470ad42acfc73c45d3e8ed5311a491160b4c100#l2377 > >> > >> It is before the patch at line 2402, so the patch needs to be updated > >> to fix NULL bs-drv at line 2337. > >> > >> https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg03425.html > > > > Can you please post a full backtrace? Do you see any error message > > on stderr before the process crashes? > > > > I don't see at the moment how this can happen, except the case that Max > > mentioned where bs->drv = NULL is set when an image corruption is > > detected - this involves an error message, though. > > > > We check bdrv_is_inserted() as the first thing, which includes a NULL > > check for bs->drv. So it must have been non-NULL at the start of the > > function and then become NULL. I suppose this can theoretically happen > > in qemu_co_queue_wait() if another flush request detects image > > corruption. > > > > Max: I think bs->drv = NULL in the middle of a request was a stupid > > idea. In fact, it's already a stupid idea to have any BDS with > > bs->drv = NULL. Maybe it would be better to schedule a BH that replaces > > the qcow2 node with a dummy node (null-co?) and properly closes the > > qcow2 one. > > Yes, that is an idea John had, too. It sounded good to me (we'd just > need to add a new flag to null-co so it would respond with -ENOMEDIUM to > all requests or something)... The only issue I had is how that would > work together with the GRAPH_MOD op blocker.
In order to answer this question, I'd first have to understand what GRAPH_MOD is even supposed to mean and which operations it needs to protect. There aren't currently any users of GRAPH_MOD. Kevin
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