[Devel,RH7] ms/workqueue: fix ghost PENDING flag while doing MQ IO

Submitted by Pavel Tikhomirov on Aug. 24, 2017, 1 p.m.

Details

Message ID 20170824130003.9481-1-ptikhomirov@virtuozzo.com
State New
Series "ms/workqueue: fix ghost PENDING flag while doing MQ IO"
Headers show

Commit Message

Pavel Tikhomirov Aug. 24, 2017, 1 p.m.
From: Roman Pen <roman.penyaev@profitbricks.com>

We have the hole node hang, many processes hang on similar stack as here:

crash> ps -m ffff8802b7f00000
[0 00:20:36.663] [UN]  PID: 22713  TASK: ffff8802b7f00000  CPU: 1   COMMAND: "worker"

crash> bt ffff8802b7f00000
PID: 22713  TASK: ffff8802b7f00000  CPU: 1   COMMAND: "worker"
 #0 [ffff88031b04f980] __schedule at ffffffff8256cdd1
 #1 [ffff88031b04f9f8] schedule at ffffffff8256e239
 #2 [ffff88031b04fa18] schedule_timeout at ffffffff82561cea
 #3 [ffff88031b04fb88] io_schedule_timeout at ffffffff8256c0d9
 #4 [ffff88031b04fbb8] wait_for_completion_io at ffffffff8256f3e0
 #5 [ffff88031b04fc90] blkdev_issue_flush at ffffffff8193a207
 #6 [ffff88031b04fe08] ext4_sync_file at ffffffffa0af6d34 [ext4]
 #7 [ffff88031b04fe68] vfs_fsync_range at ffffffff8173212c
 #8 [ffff88031b04fec8] do_fsync at ffffffff817330dc
 #9 [ffff88031b04ff68] sys_fdatasync at ffffffff8173437e
    RIP: 00007f474a581ddd  RSP: 00007f46ba3fe8a0  RFLAGS: 00000282
    RAX: 000000000000004b  RBX: ffffffff8258f609  RCX: ffffffffffffffff
    RDX: 00007f4754ffd458  RSI: 0000000000000000  RDI: 0000000000000011
    RBP: 0000000000000000   R8: 0000000000000000   R9: 00000000000058b9
    R10: 00007f46ba3fe8b0  R11: 0000000000000293  R12: 00007f475be25d80
    R13: ffffffff8173437e  R14: ffff88031b04ff78  R15: 00007f4755141452
    ORIG_RAX: 000000000000004b  CS: 0033  SS: 002b

crash> ps -m ffff8802b7f00000
[0 00:20:36.663] [UN]  PID: 22713  TASK: ffff8802b7f00000  CPU: 1   COMMAND: "worker"

Sleeps for 20 minutes on bio completion:

blkdev_issue_flush:
        submit_bio(WRITE_FLUSH, bio);
here>	wait_for_completion_io(&wait);

As bio->bi_rw = (WRITE | REQ_SYNC | REQ_NOIDLE | REQ_FLUSH), we had:

submit_bio->generic_make_request->dm_make_request->queue_io->queue_work

So in wait_for_completion_io we wait for dm_wq_work to complete these
bio. But work is not in the workqueue already, as work->entry is empty
list, so the work seem completed. That could happen only if md->flags
had DMF_BLOCK_IO_FOR_SUSPEND bit set. But it is already unset, when we
clear the bit we queue another dm_wq_work on these wq in dm_queue_flush.

So what could've happened here is that operation reordering loads
DMF_BLOCK_IO_FOR_SUSPEND bit in dm_wq_work before it was cleared in
dm_queue_flush. Adding smp_mb in set_work_pool_and_clear_pending should
order operations properly.

https://jira.sw.ru/browse/PSBM-69788

original commit message:

The bug in a workqueue leads to a stalled IO request in MQ ctx->rq_list
with the following backtrace:

[  601.347452] INFO: task kworker/u129:5:1636 blocked for more than 120 seconds.
[  601.347574]       Tainted: G           O    4.4.5-1-storage+ #6
[  601.347651] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  601.348142] kworker/u129:5  D ffff880803077988     0  1636      2 0x00000000
[  601.348519] Workqueue: ibnbd_server_fileio_wq ibnbd_dev_file_submit_io_worker [ibnbd_server]
[  601.348999]  ffff880803077988 ffff88080466b900 ffff8808033f9c80 ffff880803078000
[  601.349662]  ffff880807c95000 7fffffffffffffff ffffffff815b0920 ffff880803077ad0
[  601.350333]  ffff8808030779a0 ffffffff815b01d5 0000000000000000 ffff880803077a38
[  601.350965] Call Trace:
[  601.351203]  [<ffffffff815b0920>] ? bit_wait+0x60/0x60
[  601.351444]  [<ffffffff815b01d5>] schedule+0x35/0x80
[  601.351709]  [<ffffffff815b2dd2>] schedule_timeout+0x192/0x230
[  601.351958]  [<ffffffff812d43f7>] ? blk_flush_plug_list+0xc7/0x220
[  601.352208]  [<ffffffff810bd737>] ? ktime_get+0x37/0xa0
[  601.352446]  [<ffffffff815b0920>] ? bit_wait+0x60/0x60
[  601.352688]  [<ffffffff815af784>] io_schedule_timeout+0xa4/0x110
[  601.352951]  [<ffffffff815b3a4e>] ? _raw_spin_unlock_irqrestore+0xe/0x10
[  601.353196]  [<ffffffff815b093b>] bit_wait_io+0x1b/0x70
[  601.353440]  [<ffffffff815b056d>] __wait_on_bit+0x5d/0x90
[  601.353689]  [<ffffffff81127bd0>] wait_on_page_bit+0xc0/0xd0
[  601.353958]  [<ffffffff81096db0>] ? autoremove_wake_function+0x40/0x40
[  601.354200]  [<ffffffff81127cc4>] __filemap_fdatawait_range+0xe4/0x140
[  601.354441]  [<ffffffff81127d34>] filemap_fdatawait_range+0x14/0x30
[  601.354688]  [<ffffffff81129a9f>] filemap_write_and_wait_range+0x3f/0x70
[  601.354932]  [<ffffffff811ced3b>] blkdev_fsync+0x1b/0x50
[  601.355193]  [<ffffffff811c82d9>] vfs_fsync_range+0x49/0xa0
[  601.355432]  [<ffffffff811cf45a>] blkdev_write_iter+0xca/0x100
[  601.355679]  [<ffffffff81197b1a>] __vfs_write+0xaa/0xe0
[  601.355925]  [<ffffffff81198379>] vfs_write+0xa9/0x1a0
[  601.356164]  [<ffffffff811c59d8>] kernel_write+0x38/0x50

The underlying device is a null_blk, with default parameters:

  queue_mode    = MQ
  submit_queues = 1

Verification that nullb0 has something inflight:

root@pserver8:~# cat /sys/block/nullb0/inflight
       0        1
root@pserver8:~# find /sys/block/nullb0/mq/0/cpu* -name rq_list -print -exec cat {} \;
...
/sys/block/nullb0/mq/0/cpu2/rq_list
CTX pending:
        ffff8838038e2400
...

During debug it became clear that stalled request is always inserted in
the rq_list from the following path:

   save_stack_trace_tsk + 34
   blk_mq_insert_requests + 231
   blk_mq_flush_plug_list + 281
   blk_flush_plug_list + 199
   wait_on_page_bit + 192
   __filemap_fdatawait_range + 228
   filemap_fdatawait_range + 20
   filemap_write_and_wait_range + 63
   blkdev_fsync + 27
   vfs_fsync_range + 73
   blkdev_write_iter + 202
   __vfs_write + 170
   vfs_write + 169
   kernel_write + 56

So blk_flush_plug_list() was called with from_schedule == true.

If from_schedule is true, that means that finally blk_mq_insert_requests()
offloads execution of __blk_mq_run_hw_queue() and uses kblockd workqueue,
i.e. it calls kblockd_schedule_delayed_work_on().

That means, that we race with another CPU, which is about to execute
__blk_mq_run_hw_queue() work.

Further debugging shows the following traces from different CPUs:

  CPU#0                                  CPU#1
  ----------------------------------     -------------------------------
  reqeust A inserted
  STORE hctx->ctx_map[0] bit marked
  kblockd_schedule...() returns 1
  <schedule to kblockd workqueue>
                                         request B inserted
                                         STORE hctx->ctx_map[1] bit marked
                                         kblockd_schedule...() returns 0
  *** WORK PENDING bit is cleared ***
  flush_busy_ctxs() is executed, but
  bit 1, set by CPU#1, is not observed

As a result request B pended forever.

This behaviour can be explained by speculative LOAD of hctx->ctx_map on
CPU#0, which is reordered with clear of PENDING bit and executed _before_
actual STORE of bit 1 on CPU#1.

The proper fix is an explicit full barrier <mfence>, which guarantees
that clear of PENDING bit is to be executed before all possible
speculative LOADS or STORES inside actual work function.

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Gioh Kim <gi-oh.kim@profitbricks.com>
Cc: Michael Wang <yun.wang@profitbricks.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
---
 kernel/workqueue.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

Patch hide | download patch | download mbox

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index ec41322ac41d..ffd9f4eb8be8 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -638,6 +638,35 @@  static void set_work_pool_and_clear_pending(struct work_struct *work,
 	 */
 	smp_wmb();
 	set_work_data(work, (unsigned long)pool_id << WORK_OFFQ_POOL_SHIFT, 0);
+	/*
+	 * The following mb guarantees that previous clear of a PENDING bit
+	 * will not be reordered with any speculative LOADS or STORES from
+	 * work->current_func, which is executed afterwards.  This possible
+	 * reordering can lead to a missed execution on attempt to qeueue
+	 * the same @work.  E.g. consider this case:
+	 *
+	 *   CPU#0                         CPU#1
+	 *   ----------------------------  --------------------------------
+	 *
+	 * 1  STORE event_indicated
+	 * 2  queue_work_on() {
+	 * 3    test_and_set_bit(PENDING)
+	 * 4 }                             set_..._and_clear_pending() {
+	 * 5                                 set_work_data() # clear bit
+	 * 6                                 smp_mb()
+	 * 7                               work->current_func() {
+	 * 8				      LOAD event_indicated
+	 *				   }
+	 *
+	 * Without an explicit full barrier speculative LOAD on line 8 can
+	 * be executed before CPU#0 does STORE on line 1.  If that happens,
+	 * CPU#0 observes the PENDING bit is still set and new execution of
+	 * a @work is not queued in a hope, that CPU#1 will eventually
+	 * finish the queued @work.  Meanwhile CPU#1 does not see
+	 * event_indicated is set, because speculative LOAD was executed
+	 * before actual STORE.
+	 */
+	smp_mb();
 }
 
 static void clear_work_data(struct work_struct *work)