>> >> Hi, >> >> >> >> I noticed that bdrv_drain_all is performed in load_vmstate before >> bdrv_snapshot_goto, >> >> and bdrv_drain_all is performed in qmp_transaction before >> internal_snapshot_prepare, >> >> so is it also neccesary to perform bdrv_drain_all in savevm and delvm? >> > >> >Definitely yes for savevm. do_savevm() calls it indirectly via >> >vm_stop(), so that part looks okay. >> > >> Yes, you are right. >> >> >delvm doesn't affect the currently running VM, and therefore doesn't >> >interfere with guest requests that are in flight. So I think that a >> >bdrv_drain_all() isn't needed there. >> > >> I'm worried about that there are still pending IOs while deleting snapshot, >> then is it possible that there is concurrency problem between the >> process of deleting snapshot >> and the coroutine of io read/write(bdrv_co_do_rw) invoked by the >> pending IOs? >> This coroutine is also in main thread. >> Am I missing something? > >What kind of concurrency problem are you thinking of? > I have encountered two problem, 1) double-free of Qcow2DiscardRegion in qcow2_process_discards please see the discussing mail: [PATCH] qcow2: fix double-free of Qcow2DiscardRegion in qcow2_process_discards 2) qcow2 image is truncated to very huge size, but the size shown by qemu-img info is normal please see the discussing mail: [question] is it possible that big-endian l1 table offset referenced by other I/O while updating l1 table offset in qcow2_update_snapshot_refcount? I suspect that both of the two problems are concurrency problem mentioned above, please see the discussing mail.
>I do see that there might be a chance of concurrency, but that doesn't >automatically mean the requests are conflicting. > >Would you feel better with taking s->lock in qcow2_snapshot_delete()? Both deleting snapshot and the coroutine of pending io read/write(bdrv_co_do_rw) are performed in main thread, could BDRVQcowState.lock work? Thanks, Zhang Haoyu >This might actually be a valid concern. > >Kevin
