block: Drain BH in bdrv_drained_begin

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 <jcody@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com> Message-Id: <20170418143044.12187-3-famz@redhat.com> Reviewed-by: Jeff Cody <jcody@redhat.com> Tested-by: Jeff Cody <jcody@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com>
OSLL · Apr 18, 2017 · 91af091 · 91af091
1 parent 178bd43
commit 91af091
Showing 1 changed file with 14 additions and 8 deletions.
diff --git 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;                               \
     }                                                      \