On 2018-08-23 00:05, John Snow wrote: > > > On 08/22/2018 08:15 AM, Max Reitz wrote: >> On 2018-08-17 21:04, John Snow wrote: >>> Change the manual deferment to mirror_exit into the implicit >>> callback to job_exit and the mirror_exit callback. >>> >>> This does change the order of some bdrv_unref calls and job_completed, >>> but thanks to the new context in which we call .job_exit, this is safe >>> to defer the possible flushing of any nodes to the job_finalize_single >>> cleanup stage. >> >> Ah, right, I forgot this. Hm, what exactly do you mean? This function >> is executed in the main loop, so it can make 'src' go away. I don't see >> any difference to before. >> > > This changes the order in which we unreference these objects; if you > look at this patch the job_completed call I delete is in the middle of > what becomes the .exit() callback, which means there is a subtle change > in the ordering of how references are put down. > > Take a look at the weird ordering of mirror_exit as it exists right now; > we call job_completed first and *then* put down the last references. If > you re-order this upstream right now, you'll deadlock QEMU because this > means job_completed is responsible for putting down the last reference > to some of these block/bds objects. > > However, job_completed takes an additional AIO context lock and calls > job_finalize_single under *two* locks, which will hang QEMU if we > attempt to flush any of these nodes when we put down the last reference.
If you say so... I have to admit I don't really understand. The comment doesn't explain why it's so important to keep src around until job_completed(), so I don't know. I thought AioContexts are recursive so it doesn't matter whether you take them recursively or not. Anyway. So the difference now is that job_defer_to_main_loop() took the lock around the whole exit function, whereas the new exit shim only takes it around the .exit() method, but calls job_complete() without a lock -- and then job_finalize_single() gets its lock again, so the job methods are again called with locks. That sounds OK to me. > Performing the reordering here is *safe* because by removing the call to > job_completed and utilizing the exit shim, the .exit() callback executes > only under one lock, and when the finalize code runs later it is also > executed under only one lock, making this re-ordering safe. > > Clear as mud? Well, I trust you that the drain issue was the reason that src had to stay around until after job_completed(). It seems a bit counter-intuitive, because the comment explaining that src needs to stay around until job_completed() doesn't say much -- but it does imply that without that bdrv_ref(), the BDS might be destroyed before job_completed(). Which is different from simply having only one reference left and then being deleted in job_completed(). Looking at 3f09bfbc7be, I'm inclined to believe the original reason may be that src->job points to the job and that we shouldn't delete it as long as it does (bdrv_delete() asserts that bs->job is NULL). Oh no, a tangent appears. ...I would assume that when bdrv_replace_node() is called, BlockJob.blk is updated to point to the new BDS. But nobody seems to update the BDS.job field. Investigation is in order. Max
signature.asc
Description: OpenPGP digital signature
