On Thu, Nov 30, 2017 at 03:43:35PM +0100, Kevin Wolf wrote: > Am 30.11.2017 um 15:35 hat Alberto Garcia geschrieben: > > On Thu 30 Nov 2017 01:27:32 PM CET, Kevin Wolf wrote: > > > > >> Destroying a paused block job during bdrv_reopen_multiple() has two > > >> consequences: > > >> > > >> 1) The references to the nodes involved in the job are released, > > >> possibly destroying some of them. If those nodes were in the > > >> reopen queue this would trigger the problem originally described > > >> in commit 40840e419be, crashing QEMU. > > > > > > This specific problem could be avoided by making the BDS reference in > > > the reopen queue strong, i.e. bdrv_ref() in bdrv_reopen_queue_child() > > > and bdrv_unref() only at the end of bdrv_reopen_multiple(). > > > > That is correct. > > > > >> 2) At the end of bdrv_reopen_multiple(), bdrv_drain_all_end() would > > >> not be doing all necessary bdrv_parent_drained_end() calls. > > > > > > If I understand correctly, you don't have a reproducer here. > > > > That's unfortunately not correct. > > > > You can use the very test case that I mentioned in the commit message: > > > > https://lists.gnu.org/archive/html/qemu-block/2017-11/msg00934.html > > > > With that one, QEMU master crashes easily because of problem (1). If I > > hold strong references in the reopen queue as you mentioned, the test > > case hangs because of problem (2). > > Ok, thanks. I'll try to play with this a bit myself later. >
Another data point: I'm able to reproduce that crash, by both increasing STREAM_BUFFER_SIZE as mentioned, and using the new test case, on -rc3. > > > It's certainly not a full solution because keeping a reference to a > > > block job does not prevent it from completing, but only from being > > > freed. Most block jobs do graph modifications, including dropping the > > > references to nodes, already when they complete, not only when they > > > are freed. > > > > Yes but the block job itself holds additional references (thanks to > > block_job_add_bdrv()). > > Mirror and commit call block_job_remove_all_bdrv() during their > completion. So yes, it does help for streaming, but not for all block > jobs. > > Kevin
