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Groups > linux.kernel > #1609413 > unrolled thread
| Started by | Jeffrey Hugo <jhugo@codeaurora.org> |
|---|---|
| First post | 2017-03-27 01:20 +0200 |
| Last post | 2017-03-27 01:30 +0200 |
| Articles | 2 — 2 participants |
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[BUG] Deadlock due due to interactions of block, RCU, and cpu offline Jeffrey Hugo <jhugo@codeaurora.org> - 2017-03-27 01:20 +0200
Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-27 01:30 +0200
| From | Jeffrey Hugo <jhugo@codeaurora.org> |
|---|---|
| Date | 2017-03-27 01:20 +0200 |
| Subject | [BUG] Deadlock due due to interactions of block, RCU, and cpu offline |
| Message-ID | <tppC1-79S-3@gated-at.bofh.it> |
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,
},
/*
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| From | "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> |
|---|---|
| Date | 2017-03-27 01:30 +0200 |
| Subject | Re: [BUG] Deadlock due due to interactions of block, RCU, and cpu offline |
| Message-ID | <tppLH-7da-3@gated-at.bofh.it> |
| In reply to | #1609413 |
On Sun, Mar 26, 2017 at 05:10:40PM -0600, Jeffrey Hugo wrote:
> 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.
One quick way to test this assumption is to build a kernel with Kconfig
options CONFIG_RCU_NOCB_CPU=y and CONFIG_RCU_NOCB_CPU_ALL=y. This will
cause call_rcu_sched() to queue the work to a kthread, which can migrate
to some other CPU. If your analysis is correct, this should avoid
the deadlock. (Note that the deadlock should be fixed in any case,
just a diagnostic assumption-check procedure.)
> 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?
I do hope that we can come up with a better fix. No offense intended,
as coming up with -any- fix in the CPU-hotplug domain is not to be
denigrated, but this looks to be at vest quite fragile.
Thanx, Paul
> --
> 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,
> },
> /*
>
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