mcelog stalls on 2.6.39-rc5

DISCUSSION Jim Schutt 2 participants 5 messages Apr 27, 2011 – Apr 29, 2011

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Jim Schutt

author Tested-by Zdenek Kabelac

2011-04-27 19:59 UTC #0 lore raw

Hi,

Testing 2.6.39-rc5 is giving me the following stall:

    [ 5767.731001] INFO: rcu_sched_state detected stall on CPU 1 (t=60001 jiffies)
    [ 5767.732001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=60002 jiffies)
    [ 5947.763001] INFO: rcu_sched_state detected stall on CPU 1 (t=240032 jiffies)
    [ 5947.764001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=240034 jiffies)
    [ 6024.489362] libceph: mon0 172.17.40.34:6789 socket closed
    [ 6121.281139] INFO: task mcelog:6513 blocked for more than 120 seconds.
    [ 6121.287575] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 6121.295397]  ffff880177aefce8 0000000000000082 ffffffff810339b6 ffff880226d35a40
    [ 6121.302840]  ffff88018226c3b0 ffff88018226c3b0 0000000000011e80 ffff880226d35a40
    [ 6121.310284]  ffff88018226c760 ffff880177aefe80 ffff880177aefd18 ffffffff813af047
    [ 6121.317725] Call Trace:
    [ 6121.320176]  [<ffffffff810339b6>] ? calc_load_account_idle+0xe/0x1d
    [ 6121.326437]  [<ffffffff813af047>] schedule+0x159/0x193
    [ 6121.331569]  [<ffffffff813af449>] schedule_timeout+0x36/0xe2
    [ 6121.337223]  [<ffffffff810ad9eb>] ? trace_hardirqs_on+0x9/0x20
    [ 6121.343047]  [<ffffffff813aee2b>] do_wait_for_common+0x97/0xe3
    [ 6121.348967]  [<ffffffff8103e8fa>] ? try_to_wake_up+0x200/0x200
    [ 6121.354794]  [<ffffffff8107a4bf>] ? __raw_spin_lock_irq+0x17/0x2f
    [ 6121.360878]  [<ffffffff813af2a9>] wait_for_common+0x36/0x4d
    [ 6121.366441]  [<ffffffff813af378>] wait_for_completion+0x1d/0x1f
    [ 6121.372356]  [<ffffffff8109ae6d>] synchronize_sched+0x40/0x49
    [ 6121.378096]  [<ffffffff810635b8>] ? find_get_pid+0x1b/0x1b
    [ 6121.383574]  [<ffffffff81015fe5>] mce_read+0x17f/0x25d
    [ 6121.388707]  [<ffffffff81111af5>] ? rw_verify_area+0xac/0xdb
    [ 6121.394358]  [<ffffffff811121f1>] vfs_read+0xa9/0xe1
    [ 6121.399317]  [<ffffffff8111248d>] sys_read+0x4c/0x70
    [ 6121.404278]  [<ffffffff813b6deb>] system_call_fastpath+0x16/0x1b
    [ 6127.795001] INFO: rcu_sched_state detected stall on CPU 1 (t=420064 jiffies)
    [ 6127.796001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=420066 jiffies)
    [ 6241.410171] INFO: task mcelog:6513 blocked for more than 120 seconds.

Reverting commit a4dd99250dc makes the stalls go away:

rcu: create new rcu_access_index() and use in mce

The MCE subsystem needs to sample an RCU-protected index outside of any protection for that index. If this was a pointer, we would use rcu_access_pointer(), but there is no corresponding rcu_access_index(). This commit therefore creates an rcu_access_index() and applies it to MCE.

Signed-off-by Paul E. McKenney <[email protected]>

Tested-by Zdenek Kabelac <[email protected]>

Here's some bits from my config that might be relevant:

# RCU Subsystem
CONFIG_TREE_RCU=y
# CONFIG_PREEMPT_RCU is not set
# CONFIG_RCU_TRACE is not set
CONFIG_RCU_FANOUT=64
# CONFIG_RCU_FANOUT_EXACT is not set
# CONFIG_RCU_FAST_NO_HZ is not set
# CONFIG_TREE_RCU_TRACE is not set
CONFIG_PREEMPT_NOTIFIERS=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_X86_MCE=y
CONFIG_X86_MCE_INTEL=y
CONFIG_X86_MCE_AMD=y
CONFIG_X86_MCE_THRESHOLD=y
# CONFIG_X86_MCE_INJECT is not set
CONFIG_EDAC_DECODE_MCE=y
# CONFIG_EDAC_MCE_INJ is not set
CONFIG_EDAC_MCE=y
# CONFIG_SPARSE_RCU_POINTER is not set
# CONFIG_RCU_TORTURE_TEST is not set
CONFIG_RCU_CPU_STALL_DETECTOR=y
CONFIG_RCU_CPU_STALL_TIMEOUT=60
CONFIG_RCU_CPU_STALL_DETECTOR_RUNNABLE=y

Please let me know if there is anything I can do to help sort this out.

-- Jim

Paul E. McKenney

2011-04-27 23:03 UTC #1 lore raw

Quoted 39 lines · Jim Schutt

Hi,

Testing 2.6.39-rc5 is giving me the following stall:

    [ 5767.731001] INFO: rcu_sched_state detected stall on CPU 1 (t=60001 jiffies)
    [ 5767.732001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=60002 jiffies)
    [ 5947.763001] INFO: rcu_sched_state detected stall on CPU 1 (t=240032 jiffies)
    [ 5947.764001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=240034 jiffies)
    [ 6024.489362] libceph: mon0 172.17.40.34:6789 socket closed
    [ 6121.281139] INFO: task mcelog:6513 blocked for more than 120 seconds.
    [ 6121.287575] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 6121.295397]  ffff880177aefce8 0000000000000082 ffffffff810339b6 ffff880226d35a40
    [ 6121.302840]  ffff88018226c3b0 ffff88018226c3b0 0000000000011e80 ffff880226d35a40
    [ 6121.310284]  ffff88018226c760 ffff880177aefe80 ffff880177aefd18 ffffffff813af047
    [ 6121.317725] Call Trace:
    [ 6121.320176]  [<ffffffff810339b6>] ? calc_load_account_idle+0xe/0x1d
    [ 6121.326437]  [<ffffffff813af047>] schedule+0x159/0x193
    [ 6121.331569]  [<ffffffff813af449>] schedule_timeout+0x36/0xe2
    [ 6121.337223]  [<ffffffff810ad9eb>] ? trace_hardirqs_on+0x9/0x20
    [ 6121.343047]  [<ffffffff813aee2b>] do_wait_for_common+0x97/0xe3
    [ 6121.348967]  [<ffffffff8103e8fa>] ? try_to_wake_up+0x200/0x200
    [ 6121.354794]  [<ffffffff8107a4bf>] ? __raw_spin_lock_irq+0x17/0x2f
    [ 6121.360878]  [<ffffffff813af2a9>] wait_for_common+0x36/0x4d
    [ 6121.366441]  [<ffffffff813af378>] wait_for_completion+0x1d/0x1f
    [ 6121.372356]  [<ffffffff8109ae6d>] synchronize_sched+0x40/0x49
    [ 6121.378096]  [<ffffffff810635b8>] ? find_get_pid+0x1b/0x1b
    [ 6121.383574]  [<ffffffff81015fe5>] mce_read+0x17f/0x25d
    [ 6121.388707]  [<ffffffff81111af5>] ? rw_verify_area+0xac/0xdb
    [ 6121.394358]  [<ffffffff811121f1>] vfs_read+0xa9/0xe1
    [ 6121.399317]  [<ffffffff8111248d>] sys_read+0x4c/0x70
    [ 6121.404278]  [<ffffffff813b6deb>] system_call_fastpath+0x16/0x1b
    [ 6127.795001] INFO: rcu_sched_state detected stall on CPU 1 (t=420064 jiffies)
    [ 6127.796001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=420066 jiffies)
    [ 6241.410171] INFO: task mcelog:6513 blocked for more than 120 seconds.

Reverting commit a4dd99250dc makes the stalls go away:

rcu: create new rcu_access_index() and use in mce

Quoted 8 lines

The MCE subsystem needs to sample an RCU-protected index outside of any protection for that index. If this was a pointer, we would use rcu_access_pointer(), but there is no corresponding rcu_access_index(). This commit therefore creates an rcu_access_index() and applies it to MCE.
Signed-off-by Paul E. McKenney <[email protected]>
Tested-by Zdenek Kabelac <[email protected]>

Wow! This is just instructions, just wrapping the access in ACCESS_ONCE().

Was the original RCU CPU stall repeatable?

Thanx, Paul

Quoted 32 lines

Here's some bits from my config that might be relevant:
# RCU Subsystem
CONFIG_TREE_RCU=y
# CONFIG_PREEMPT_RCU is not set
# CONFIG_RCU_TRACE is not set
CONFIG_RCU_FANOUT=64
# CONFIG_RCU_FANOUT_EXACT is not set
# CONFIG_RCU_FAST_NO_HZ is not set
# CONFIG_TREE_RCU_TRACE is not set
CONFIG_PREEMPT_NOTIFIERS=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_X86_MCE=y
CONFIG_X86_MCE_INTEL=y
CONFIG_X86_MCE_AMD=y
CONFIG_X86_MCE_THRESHOLD=y
# CONFIG_X86_MCE_INJECT is not set
CONFIG_EDAC_DECODE_MCE=y
# CONFIG_EDAC_MCE_INJ is not set
CONFIG_EDAC_MCE=y
# CONFIG_SPARSE_RCU_POINTER is not set
# CONFIG_RCU_TORTURE_TEST is not set
CONFIG_RCU_CPU_STALL_DETECTOR=y
CONFIG_RCU_CPU_STALL_TIMEOUT=60
CONFIG_RCU_CPU_STALL_DETECTOR_RUNNABLE=y
Please let me know if there is anything I can do to help sort this out.
-- Jim

Jim Schutt

author

2011-04-28 14:06 UTC #2 lore raw

Quoted 46 lines · Paul E. McKenney

Quoted 45 lines · Jim Schutt

Hi,

Testing 2.6.39-rc5 is giving me the following stall:

    [ 5767.731001] INFO: rcu_sched_state detected stall on CPU 1 (t=60001 jiffies)
    [ 5767.732001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=60002 jiffies)
    [ 5947.763001] INFO: rcu_sched_state detected stall on CPU 1 (t=240032 jiffies)
    [ 5947.764001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=240034 jiffies)
    [ 6024.489362] libceph: mon0 172.17.40.34:6789 socket closed
    [ 6121.281139] INFO: task mcelog:6513 blocked for more than 120 seconds.
    [ 6121.287575] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 6121.295397]  ffff880177aefce8 0000000000000082 ffffffff810339b6 ffff880226d35a40
    [ 6121.302840]  ffff88018226c3b0 ffff88018226c3b0 0000000000011e80 ffff880226d35a40
    [ 6121.310284]  ffff88018226c760 ffff880177aefe80 ffff880177aefd18 ffffffff813af047
    [ 6121.317725] Call Trace:
    [ 6121.320176]  [<ffffffff810339b6>] ? calc_load_account_idle+0xe/0x1d
    [ 6121.326437]  [<ffffffff813af047>] schedule+0x159/0x193
    [ 6121.331569]  [<ffffffff813af449>] schedule_timeout+0x36/0xe2
    [ 6121.337223]  [<ffffffff810ad9eb>] ? trace_hardirqs_on+0x9/0x20
    [ 6121.343047]  [<ffffffff813aee2b>] do_wait_for_common+0x97/0xe3
    [ 6121.348967]  [<ffffffff8103e8fa>] ? try_to_wake_up+0x200/0x200
    [ 6121.354794]  [<ffffffff8107a4bf>] ? __raw_spin_lock_irq+0x17/0x2f
    [ 6121.360878]  [<ffffffff813af2a9>] wait_for_common+0x36/0x4d
    [ 6121.366441]  [<ffffffff813af378>] wait_for_completion+0x1d/0x1f
    [ 6121.372356]  [<ffffffff8109ae6d>] synchronize_sched+0x40/0x49
    [ 6121.378096]  [<ffffffff810635b8>] ? find_get_pid+0x1b/0x1b
    [ 6121.383574]  [<ffffffff81015fe5>] mce_read+0x17f/0x25d
    [ 6121.388707]  [<ffffffff81111af5>] ? rw_verify_area+0xac/0xdb
    [ 6121.394358]  [<ffffffff811121f1>] vfs_read+0xa9/0xe1
    [ 6121.399317]  [<ffffffff8111248d>] sys_read+0x4c/0x70
    [ 6121.404278]  [<ffffffff813b6deb>] system_call_fastpath+0x16/0x1b
    [ 6127.795001] INFO: rcu_sched_state detected stall on CPU 1 (t=420064 jiffies)
    [ 6127.796001] INFO: rcu_sched_state detected stalls on CPUs/tasks: { 1} (detected by 0, t=420066 jiffies)
    [ 6241.410171] INFO: task mcelog:6513 blocked for more than 120 seconds.

Reverting commit a4dd99250dc makes the stalls go away:

rcu: create new rcu_access_index() and use in mce

Quoted 7 lines

Quoted 2 lines
Signed-off-by Paul E. McKenney <[email protected]>
Tested-by Zdenek Kabelac <[email protected]>
Wow! This is just instructions, just wrapping the access in ACCESS_ONCE().
Was the original RCU CPU stall repeatable?

Yes. I saw it on two different machines. Both were running as Ceph clients, doing sustained streaming writes, if that helps any.

I can attempt to repeat with any extra debugging you'd like me to try.

-- Jim

Quoted 36 lines

Thanx, Paul
Quoted 32 lines
Here's some bits from my config that might be relevant:
# RCU Subsystem
CONFIG_TREE_RCU=y
# CONFIG_PREEMPT_RCU is not set
# CONFIG_RCU_TRACE is not set
CONFIG_RCU_FANOUT=64
# CONFIG_RCU_FANOUT_EXACT is not set
# CONFIG_RCU_FAST_NO_HZ is not set
# CONFIG_TREE_RCU_TRACE is not set
CONFIG_PREEMPT_NOTIFIERS=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_X86_MCE=y
CONFIG_X86_MCE_INTEL=y
CONFIG_X86_MCE_AMD=y
CONFIG_X86_MCE_THRESHOLD=y
# CONFIG_X86_MCE_INJECT is not set
CONFIG_EDAC_DECODE_MCE=y
# CONFIG_EDAC_MCE_INJ is not set
CONFIG_EDAC_MCE=y
# CONFIG_SPARSE_RCU_POINTER is not set
# CONFIG_RCU_TORTURE_TEST is not set
CONFIG_RCU_CPU_STALL_DETECTOR=y
CONFIG_RCU_CPU_STALL_TIMEOUT=60
CONFIG_RCU_CPU_STALL_DETECTOR_RUNNABLE=y
Please let me know if there is anything I can do to help sort this out.
-- Jim

Jim Schutt

author

2011-04-28 16:07 UTC #3 lore raw

Quoted 67 lines · Jim Schutt

Quoted 66 lines · Paul E. McKenney

Quoted 59 lines · Jim Schutt

Hi,

Testing 2.6.39-rc5 is giving me the following stall:

    [ 5767.731001] INFO: rcu_sched_state detected stall on CPU 1 
(t=60001 jiffies)
    [ 5767.732001] INFO: rcu_sched_state detected stalls on 
CPUs/tasks: { 1} (detected by 0, t=60002 jiffies)
    [ 5947.763001] INFO: rcu_sched_state detected stall on CPU 1 
(t=240032 jiffies)
    [ 5947.764001] INFO: rcu_sched_state detected stalls on 
CPUs/tasks: { 1} (detected by 0, t=240034 jiffies)
    [ 6024.489362] libceph: mon0 172.17.40.34:6789 socket closed
    [ 6121.281139] INFO: task mcelog:6513 blocked for more than 120 
seconds.
    [ 6121.287575] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" 
disables this message.
    [ 6121.295397]  ffff880177aefce8 0000000000000082 
ffffffff810339b6 ffff880226d35a40
    [ 6121.302840]  ffff88018226c3b0 ffff88018226c3b0 
0000000000011e80 ffff880226d35a40
    [ 6121.310284]  ffff88018226c760 ffff880177aefe80 
ffff880177aefd18 ffffffff813af047
    [ 6121.317725] Call Trace:
    [ 6121.320176]  [<ffffffff810339b6>] ? 
calc_load_account_idle+0xe/0x1d
    [ 6121.326437]  [<ffffffff813af047>] schedule+0x159/0x193
    [ 6121.331569]  [<ffffffff813af449>] schedule_timeout+0x36/0xe2
    [ 6121.337223]  [<ffffffff810ad9eb>] ? trace_hardirqs_on+0x9/0x20
    [ 6121.343047]  [<ffffffff813aee2b>] do_wait_for_common+0x97/0xe3
    [ 6121.348967]  [<ffffffff8103e8fa>] ? try_to_wake_up+0x200/0x200
    [ 6121.354794]  [<ffffffff8107a4bf>] ? __raw_spin_lock_irq+0x17/0x2f
    [ 6121.360878]  [<ffffffff813af2a9>] wait_for_common+0x36/0x4d
    [ 6121.366441]  [<ffffffff813af378>] wait_for_completion+0x1d/0x1f
    [ 6121.372356]  [<ffffffff8109ae6d>] synchronize_sched+0x40/0x49
    [ 6121.378096]  [<ffffffff810635b8>] ? find_get_pid+0x1b/0x1b
    [ 6121.383574]  [<ffffffff81015fe5>] mce_read+0x17f/0x25d
    [ 6121.388707]  [<ffffffff81111af5>] ? rw_verify_area+0xac/0xdb
    [ 6121.394358]  [<ffffffff811121f1>] vfs_read+0xa9/0xe1
    [ 6121.399317]  [<ffffffff8111248d>] sys_read+0x4c/0x70
    [ 6121.404278]  [<ffffffff813b6deb>] system_call_fastpath+0x16/0x1b
    [ 6127.795001] INFO: rcu_sched_state detected stall on CPU 1 
(t=420064 jiffies)
    [ 6127.796001] INFO: rcu_sched_state detected stalls on 
CPUs/tasks: { 1} (detected by 0, t=420066 jiffies)
    [ 6241.410171] INFO: task mcelog:6513 blocked for more than 120

seconds.

Reverting commit a4dd99250dc makes the stalls go away:

rcu: create new rcu_access_index() and use in mce

The MCE subsystem needs to sample an RCU-protected index outside of any protection for that index. If this was a pointer, we would use rcu_access_pointer(), but there is no corresponding
rcu_access_index().
This commit therefore creates an rcu_access_index() and applies it to MCE.

Quoted 2 lines

Signed-off-by Paul E. McKenney <[email protected]>
Tested-by Zdenek Kabelac <[email protected]>

Wow! This is just instructions, just wrapping the access in ACCESS_ONCE().

Quoted 8 lines

Quoted 1 line
Was the original RCU CPU stall repeatable?
Yes. I saw it on two different machines. Both were running as Ceph clients, doing sustained streaming writes, if that helps any.
I can attempt to repeat with any extra debugging you'd like me to try.

Evidently I cannot reproduce this after all, so my revert test was incorrect.

If I can learn how to reproduce this I'll post with details; in the meantime ignore this, and sorry for the noise.

-- Jim

Quoted 6 lines

-- Jim
Quoted 3 lines
Thanx, Paul

Paul E. McKenney

2011-04-29 00:31 UTC #4 lore raw

Quoted 75 lines · Jim Schutt

Quoted 74 lines · Jim Schutt

Quoted 61 lines · Paul E. McKenney

Quoted 60 lines · Jim Schutt

Hi,

Testing 2.6.39-rc5 is giving me the following stall:

   [ 5767.731001] INFO: rcu_sched_state detected stall on CPU
1 (t=60001 jiffies)
   [ 5767.732001] INFO: rcu_sched_state detected stalls on
CPUs/tasks: { 1} (detected by 0, t=60002 jiffies)
   [ 5947.763001] INFO: rcu_sched_state detected stall on CPU
1 (t=240032 jiffies)
   [ 5947.764001] INFO: rcu_sched_state detected stalls on
CPUs/tasks: { 1} (detected by 0, t=240034 jiffies)
   [ 6024.489362] libceph: mon0 172.17.40.34:6789 socket closed
   [ 6121.281139] INFO: task mcelog:6513 blocked for more
than 120 seconds.
   [ 6121.287575] "echo 0 >
/proc/sys/kernel/hung_task_timeout_secs" disables this
message.
   [ 6121.295397]  ffff880177aefce8 0000000000000082
ffffffff810339b6 ffff880226d35a40
   [ 6121.302840]  ffff88018226c3b0 ffff88018226c3b0
0000000000011e80 ffff880226d35a40
   [ 6121.310284]  ffff88018226c760 ffff880177aefe80
ffff880177aefd18 ffffffff813af047
   [ 6121.317725] Call Trace:
   [ 6121.320176]  [<ffffffff810339b6>] ?
calc_load_account_idle+0xe/0x1d
   [ 6121.326437]  [<ffffffff813af047>] schedule+0x159/0x193
   [ 6121.331569]  [<ffffffff813af449>] schedule_timeout+0x36/0xe2
   [ 6121.337223]  [<ffffffff810ad9eb>] ? trace_hardirqs_on+0x9/0x20
   [ 6121.343047]  [<ffffffff813aee2b>] do_wait_for_common+0x97/0xe3
   [ 6121.348967]  [<ffffffff8103e8fa>] ? try_to_wake_up+0x200/0x200
   [ 6121.354794]  [<ffffffff8107a4bf>] ? __raw_spin_lock_irq+0x17/0x2f
   [ 6121.360878]  [<ffffffff813af2a9>] wait_for_common+0x36/0x4d
   [ 6121.366441]  [<ffffffff813af378>] wait_for_completion+0x1d/0x1f
   [ 6121.372356]  [<ffffffff8109ae6d>] synchronize_sched+0x40/0x49
   [ 6121.378096]  [<ffffffff810635b8>] ? find_get_pid+0x1b/0x1b
   [ 6121.383574]  [<ffffffff81015fe5>] mce_read+0x17f/0x25d
   [ 6121.388707]  [<ffffffff81111af5>] ? rw_verify_area+0xac/0xdb
   [ 6121.394358]  [<ffffffff811121f1>] vfs_read+0xa9/0xe1
   [ 6121.399317]  [<ffffffff8111248d>] sys_read+0x4c/0x70
   [ 6121.404278]  [<ffffffff813b6deb>] system_call_fastpath+0x16/0x1b
   [ 6127.795001] INFO: rcu_sched_state detected stall on CPU
1 (t=420064 jiffies)
   [ 6127.796001] INFO: rcu_sched_state detected stalls on
CPUs/tasks: { 1} (detected by 0, t=420066 jiffies)
   [ 6241.410171] INFO: task mcelog:6513 blocked for more

than 120 seconds.

Reverting commit a4dd99250dc makes the stalls go away:

rcu: create new rcu_access_index() and use in mce

The MCE subsystem needs to sample an RCU-protected index outside of any protection for that index. If this was a pointer, we would use rcu_access_pointer(), but there is no corresponding
rcu_access_index().
This commit therefore creates an rcu_access_index() and applies it to MCE.

Quoted 7 lines

Quoted 2 lines
Signed-off-by Paul E. McKenney <[email protected]>
Tested-by Zdenek Kabelac <[email protected]>
Wow! This is just instructions, just wrapping the access in ACCESS_ONCE().
Was the original RCU CPU stall repeatable?

Yes. I saw it on two different machines. Both were running as Ceph clients, doing sustained streaming writes, if that helps any.

Quoted 8 lines

Quoted 2 lines
I can attempt to repeat with any extra debugging you'd like me to try.
Evidently I cannot reproduce this after all, so my revert test was incorrect.
If I can learn how to reproduce this I'll post with details; in the meantime ignore this, and sorry for the noise.

OK -- I must confess that I could not understand how this change could cause the problem! ;-)

Thanx, Paul