[BUG] Deadlock due due to interactions of block, RCU, and cpu offline

From: Jeffrey Hugo
Date: Sun Mar 26 2017 - 19:12:16 EST


Hello,

I observe that running stress-ng with the cpu-online and fstat tests results in a deadlock of hung tasks:

[ 366.810486] INFO: task stress-ng-cpu-o:2590 blocked for more than 120 seconds.
[ 366.817689] Not tainted 4.9.0 #39
[ 366.821504] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 366.829320] stress-ng-cpu-o D 0 2590 2589 0x00000008
[ 366.834803] Call trace:
[ 366.837222] [<ffff000008085dd0>] __switch_to+0x60/0x70
[ 366.842338] [<ffff000008a23c18>] __schedule+0x178/0x648
[ 366.847550] [<ffff000008a24120>] schedule+0x38/0x98
[ 366.852408] [<ffff00000848b774>] blk_mq_freeze_queue_wait+0x64/0x1a8
[ 366.858749] [<ffff00000848e9d4>] blk_mq_queue_reinit_work+0x74/0x110
[ 366.865081] [<ffff00000848ea94>] blk_mq_queue_reinit_dead+0x24/0x30
[ 366.871335] [<ffff0000080c9898>] cpuhp_invoke_callback+0x98/0x4a8
[ 366.877411] [<ffff0000080cb084>] cpuhp_down_callbacks+0x114/0x150
[ 366.883484] [<ffff000008a22578>] _cpu_down+0x100/0x1d8
[ 366.888609] [<ffff0000080cbfdc>] do_cpu_down+0x4c/0x78
[ 366.893727] [<ffff0000080cc02c>] cpu_down+0x24/0x30
[ 366.898593] [<ffff0000086aaf28>] cpu_subsys_offline+0x20/0x30
[ 366.904318] [<ffff0000086a53d8>] device_offline+0xa8/0xd8
[ 366.909704] [<ffff0000086a550c>] online_store+0x4c/0xa8
[ 366.914907] [<ffff0000086a241c>] dev_attr_store+0x44/0x60
[ 366.920294] [<ffff0000082b6a24>] sysfs_kf_write+0x5c/0x78
[ 366.925672] [<ffff0000082b5cec>] kernfs_fop_write+0xbc/0x1e8
[ 366.931318] [<ffff000008238320>] __vfs_write+0x48/0x138
[ 366.936526] [<ffff000008239078>] vfs_write+0xa8/0x1c0
[ 366.941557] [<ffff00000823a08c>] SyS_write+0x54/0xb0
[ 366.946511] [<ffff000008083370>] el0_svc_naked+0x24/0x28
[ 366.951800] INFO: task stress-ng-fstat:2591 blocked for more than 120 seconds.
[ 366.959008] Not tainted 4.9.0 #39
[ 366.962823] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 366.970640] stress-ng-fstat D 0 2591 2589 0x00000000
[ 366.976105] Call trace:
[ 366.978540] [<ffff000008085dd0>] __switch_to+0x60/0x70
[ 366.983658] [<ffff000008a23c18>] __schedule+0x178/0x648
[ 366.988870] [<ffff000008a24120>] schedule+0x38/0x98
[ 366.993727] [<ffff00000848b774>] blk_mq_freeze_queue_wait+0x64/0x1a8
[ 367.000068] [<ffff00000848e2d0>] blk_mq_freeze_queue+0x28/0x38
[ 367.005880] [<ffff0000086d480c>] lo_release+0x64/0x90
[ 367.010919] [<ffff000008278bd4>] __blkdev_put+0x26c/0x2c8
[ 367.016300] [<ffff000008278fec>] blkdev_put+0x54/0x128
[ 367.021418] [<ffff0000082790ec>] blkdev_close+0x2c/0x40
[ 367.026631] [<ffff00000823ab58>] __fput+0xa0/0x1e0
[ 367.031401] [<ffff00000823ad10>] ____fput+0x20/0x30
[ 367.036266] [<ffff0000080e7a40>] task_work_run+0xc8/0xe8
[ 367.041557] [<ffff0000080882b4>] do_notify_resume+0xac/0xb8
[ 367.047116] [<ffff000008083294>] work_pending+0x8/0x10

I have tested and found this issue to be reproducible on both x86 and ARM64 architectures on 4.7, 4.8, 4.9, 4.10, and 4.11-rc3 kernels.

Using the below test methodology [1], the issue reproduces within a few minutes.

Using ftrace, I have analyzed the issue on 4.9 and I believe I've found the root cause [2].

Based on my analysis, I have developed a fix [3], which addresses the issue as I am able to run stress-ng for over an hour where I was unable to do so before, however I do not know the full extend of impacts from this fix, and look for guidance from the community to determine the final fix.


[1] Test methodology
--------------------
Boot a multicore system such as a desktop i5 system with nr_cpus=2

Enable all logging to determine when the deadlock occurs (prints from test stop flowing out of the serial port)
echo 1 > /sys/module/printk/parameters/ignore_loglevel

Run stress-ng
stress-ng --fstat 1 --cpu-online 1 -t 3600

Wait for the test output to stop, and the hung task watchdog to fire.


[2] Analysis
------------
Again, this analysis is based upon the 4.9 kernel, but believe it to still apply to newer kernels.

I conclude that the hung tasks occur due to a race condition which results in a deadlock.

The race condition occurs between "normal" work in the block layer on a core (the stress-ng-fstat task in the above dump) and cpu offline of that core (the stress-ng-cpu-o task in the above dump).

The fput() from userspace in the fstat task results in a call to blk_mq_freeze_queue(), which drops the last reference to the queue via percpu_ref_kill(), and then waits for the ref count of the queue to hit 0 in blk_mq_freeze_queue_wait(). percpu_ref_kill() will result in __percpu_ref_switch_to_atomic() which will use call_rcu_sched() to setup delayed work to finalize the percpu_ref cleanup and drop the ref count to 0.

Note that call_rcu_sched() queues the work to a per-cpu queue, thus the work can only be run on the core it is queued on, by the work thread that is pinned to that core.

It is a race between this work running, and the cpu offline processing.

If the cpu offline processing is able to get to and process the RCU/tree:online state before the queued work from the block layer, then the pinned work thread will be migrated to another core via rcutree_offline_cpu(), and the work will not be able to execute.

This race condition does not result in deadlock until later in the cpu offline processing. Once we hit the block/mq:prepare state the block layer freezes all the queues and waits for the ref counts to hit 0. This normally works because at this point the cpu being offlined is dead from cpu:teardown, and the offline processing is occuring on another active cpu, so call_rcu_sched() will queue work to an active cpu where it can get processed. However the fstat process already did that work for one of the queues to be frozen in the block layer, so the processing of the block/mq:prepare state waits on the same ref count as fstat to hit 0. Thus we see the result of this as the stress-ng-cpu-o task above.

The block/mq:prepare processing stalls the cpu offline processing which causes a deadlock because the processing does not get to the RCU/tree:prepare state which migrates all of the queued work from the offline cpu to another cpu, which would allow the work that the fstat task queued to execute, drop the ref count to 0, and unblock both stalled tasks.

By reordering the cpu offline states such the shutdown processing of RCU/tree:prepare occurs before block/mq:prepare [3], we prevent deadlock by enabling the queued work in the RCU framework to run elsewhere, and eventually unblock the tasks waiting on the ref count.

However, it is not entirely clear what are the full ramifications of this reorder. I assume the ordering of these cpu online/offline states is carefully considered, and without that knowledge, I could not say for certain that my fix [3] is safe.

What is the opinion of the domain experts?

--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.


[3] Proposed fix
---8>---
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index afe641c..9b86db9 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -49,6 +49,7 @@ enum cpuhp_state {
CPUHP_ARM_SHMOBILE_SCU_PREPARE,
CPUHP_SH_SH3X_PREPARE,
CPUHP_BLK_MQ_PREPARE,
+ CPUHP_RCUTREE_PREP2,
CPUHP_TIMERS_DEAD,
CPUHP_NOTF_ERR_INJ_PREPARE,
CPUHP_MIPS_SOC_PREPARE,
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 29de1a9..b46c573 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -1289,6 +1289,11 @@ static int __init cpu_hotplug_pm_sync_init(void)
[CPUHP_RCUTREE_PREP] = {
.name = "RCU/tree:prepare",
.startup.single = rcutree_prepare_cpu,
+ .teardown.single = NULL,
+ },
+ [CPUHP_RCUTREE_PREP2] = {
+ .name = "RCU/tree:dead",
+ .startup.single = NULL,
.teardown.single = rcutree_dead_cpu,
},
/*