Am 18.04.2017 um 16:30 hat Fam Zheng geschrieben: > During block job completion, nothing is preventing > block_job_defer_to_main_loop_bh from being called in a nested > aio_poll(), which is a trouble, such as in this code path: > > qmp_block_commit > commit_active_start > bdrv_reopen > bdrv_reopen_multiple > bdrv_reopen_prepare > bdrv_flush > aio_poll > aio_bh_poll > aio_bh_call > block_job_defer_to_main_loop_bh > stream_complete > bdrv_reopen > > block_job_defer_to_main_loop_bh is the last step of the stream job, > which should have been "paused" by the bdrv_drained_begin/end in > bdrv_reopen_multiple, but it is not done because it's in the form of a > main loop BH. > > Similar to why block jobs should be paused between drained_begin and > drained_end, BHs they schedule must be excluded as well. To achieve > this, this patch forces draining the BH in BDRV_POLL_WHILE. > > As a side effect this fixes a hang in block_job_detach_aio_context > during system_reset when a block job is ready: > > #0 0x0000555555aa79f3 in bdrv_drain_recurse > #1 0x0000555555aa825d in bdrv_drained_begin > #2 0x0000555555aa8449 in bdrv_drain > #3 0x0000555555a9c356 in blk_drain > #4 0x0000555555aa3cfd in mirror_drain > #5 0x0000555555a66e11 in block_job_detach_aio_context > #6 0x0000555555a62f4d in bdrv_detach_aio_context > #7 0x0000555555a63116 in bdrv_set_aio_context > #8 0x0000555555a9d326 in blk_set_aio_context > #9 0x00005555557e38da in virtio_blk_data_plane_stop > #10 0x00005555559f9d5f in virtio_bus_stop_ioeventfd > #11 0x00005555559fa49b in virtio_bus_stop_ioeventfd > #12 0x00005555559f6a18 in virtio_pci_stop_ioeventfd > #13 0x00005555559f6a18 in virtio_pci_reset > #14 0x00005555559139a9 in qdev_reset_one > #15 0x0000555555916738 in qbus_walk_children > #16 0x0000555555913318 in qdev_walk_children > #17 0x0000555555916738 in qbus_walk_children > #18 0x00005555559168ca in qemu_devices_reset > #19 0x000055555581fcbb in pc_machine_reset > #20 0x00005555558a4d96 in qemu_system_reset > #21 0x000055555577157a in main_loop_should_exit > #22 0x000055555577157a in main_loop > #23 0x000055555577157a in main > > The rationale is that the loop in block_job_detach_aio_context cannot > make any progress in pausing/completing the job, because bs->in_flight > is 0, so bdrv_drain doesn't process the block_job_defer_to_main_loop > BH. With this patch, it does. > > Reported-by: Jeff Cody <jc...@redhat.com> > Signed-off-by: Fam Zheng <f...@redhat.com>
Fam, do you remember whether this was really only about drain? Because in that case... > diff --git a/include/block/block.h b/include/block/block.h > index 97d4330..5ddc0cf 100644 > --- a/include/block/block.h > +++ b/include/block/block.h > @@ -381,12 +381,13 @@ void bdrv_drain_all(void); > > #define BDRV_POLL_WHILE(bs, cond) ({ \ > bool waited_ = false; \ > + bool busy_ = true; \ > BlockDriverState *bs_ = (bs); \ > AioContext *ctx_ = bdrv_get_aio_context(bs_); \ > if (aio_context_in_iothread(ctx_)) { \ > - while ((cond)) { \ > - aio_poll(ctx_, true); \ > - waited_ = true; \ > + while ((cond) || busy_) { \ > + busy_ = aio_poll(ctx_, (cond)); \ > + waited_ |= !!(cond) | busy_; \ > } \ > } else { \ > assert(qemu_get_current_aio_context() == \ > @@ -398,11 +399,16 @@ void bdrv_drain_all(void); > */ \ > assert(!bs_->wakeup); \ > bs_->wakeup = true; \ > - while ((cond)) { \ > - aio_context_release(ctx_); \ > - aio_poll(qemu_get_aio_context(), true); \ > - aio_context_acquire(ctx_); \ > - waited_ = true; \ > + while (busy_) { \ > + if ((cond)) { \ > + waited_ = busy_ = true; \ > + aio_context_release(ctx_); \ > + aio_poll(qemu_get_aio_context(), true); \ > + aio_context_acquire(ctx_); \ > + } else { \ > + busy_ = aio_poll(ctx_, false); \ > + waited_ |= busy_; \ > + } \ > } \ > bs_->wakeup = false; \ > } \ ...fixing it in BDRV_POLL_WHILE() rather than the drain functions may have been a bad idea. I got a bug assigned where we have a large (200 GB) fragmented qcow2 image, and qemu-img convert takes two hours before it even starts to copy data. What it does in this time is iterating through the whole image with bdrv_block_status_above() in order to estimate the work to be done (and of course it will repeat the same calls while actually copying data). One of the things I saw is that between each pair of lseek() calls, we also have unnecessary poll() calls, and these were introduced by this patch. If I modify bdrv_common_block_status_above() so that it doesn't poll if data.done == true already, the time needed to iterate through my test image is cut in half. Now, of course, I'm still only seeing a few seconds for a 100 GB image, so there must be more that is wrong for the reporter, but it suggests to me that BDRV_POLL_WHILE() shouldn't be polling unconditionally when only one of its users actually needs this. Kevin