[PATCH Xenial/Yakkety SRU] percpu-refcount: fix reference leak during percpu-atomic transition

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[PATCH Xenial/Yakkety SRU] percpu-refcount: fix reference leak during percpu-atomic transition

Tim Gardner-2
From: Douglas Miller <[hidden email]>

BugLink: http://bugs.launchpad.net/bugs/1662673

percpu_ref_tryget() and percpu_ref_tryget_live() should return
"true" IFF they acquire a reference. But the return value from
atomic_long_inc_not_zero() is a long and may have high bits set,
e.g. PERCPU_COUNT_BIAS, and the return value of the tryget routines
is bool so the reference may actually be acquired but the routines
return "false" which results in a reference leak since the caller
assumes it does not need to do a corresponding percpu_ref_put().

This was seen when performing CPU hotplug during I/O, as hangs in
blk_mq_freeze_queue_wait where percpu_ref_kill (blk_mq_freeze_queue_start)
raced with percpu_ref_tryget (blk_mq_timeout_work).
Sample stack trace:

__switch_to+0x2c0/0x450
__schedule+0x2f8/0x970
schedule+0x48/0xc0
blk_mq_freeze_queue_wait+0x94/0x120
blk_mq_queue_reinit_work+0xb8/0x180
blk_mq_queue_reinit_prepare+0x84/0xa0
cpuhp_invoke_callback+0x17c/0x600
cpuhp_up_callbacks+0x58/0x150
_cpu_up+0xf0/0x1c0
do_cpu_up+0x120/0x150
cpu_subsys_online+0x64/0xe0
device_online+0xb4/0x120
online_store+0xb4/0xc0
dev_attr_store+0x68/0xa0
sysfs_kf_write+0x80/0xb0
kernfs_fop_write+0x17c/0x250
__vfs_write+0x6c/0x1e0
vfs_write+0xd0/0x270
SyS_write+0x6c/0x110
system_call+0x38/0xe0

Examination of the queue showed a single reference (no PERCPU_COUNT_BIAS,
and __PERCPU_REF_DEAD, __PERCPU_REF_ATOMIC set) and no requests.
However, conditions at the time of the race are count of PERCPU_COUNT_BIAS + 0
and __PERCPU_REF_DEAD and __PERCPU_REF_ATOMIC set.

The fix is to make the tryget routines use an actual boolean internally instead
of the atomic long result truncated to a int.

Fixes: e625305b3907 percpu-refcount: make percpu_ref based on longs instead of ints
Link: https://bugzilla.kernel.org/show_bug.cgi?id=190751
Signed-off-by: Douglas Miller <[hidden email]>
Reviewed-by: Jens Axboe <[hidden email]>
Signed-off-by: Tejun Heo <[hidden email]>
Fixes: e625305b3907 ("percpu-refcount: make percpu_ref based on longs instead of ints")
Cc: [hidden email] # v3.18+
(cherry picked from commit 966d2b04e070bc040319aaebfec09e0144dc3341)
Signed-off-by: Tim Gardner <[hidden email]>
---
 include/linux/percpu-refcount.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 1c7eec0..3a481a4 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -204,7 +204,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 {
  unsigned long __percpu *percpu_count;
- int ret;
+ bool ret;
 
  rcu_read_lock_sched();
 
@@ -238,7 +238,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 {
  unsigned long __percpu *percpu_count;
- int ret = false;
+ bool ret = false;
 
  rcu_read_lock_sched();
 
--
2.7.4


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ACK: [PATCH Xenial/Yakkety SRU] percpu-refcount: fix reference leak during percpu-atomic transition

Colin Ian King-2
On 08/02/17 20:33, Tim Gardner wrote:

> From: Douglas Miller <[hidden email]>
>
> BugLink: http://bugs.launchpad.net/bugs/1662673
>
> percpu_ref_tryget() and percpu_ref_tryget_live() should return
> "true" IFF they acquire a reference. But the return value from
> atomic_long_inc_not_zero() is a long and may have high bits set,
> e.g. PERCPU_COUNT_BIAS, and the return value of the tryget routines
> is bool so the reference may actually be acquired but the routines
> return "false" which results in a reference leak since the caller
> assumes it does not need to do a corresponding percpu_ref_put().
>
> This was seen when performing CPU hotplug during I/O, as hangs in
> blk_mq_freeze_queue_wait where percpu_ref_kill (blk_mq_freeze_queue_start)
> raced with percpu_ref_tryget (blk_mq_timeout_work).
> Sample stack trace:
>
> __switch_to+0x2c0/0x450
> __schedule+0x2f8/0x970
> schedule+0x48/0xc0
> blk_mq_freeze_queue_wait+0x94/0x120
> blk_mq_queue_reinit_work+0xb8/0x180
> blk_mq_queue_reinit_prepare+0x84/0xa0
> cpuhp_invoke_callback+0x17c/0x600
> cpuhp_up_callbacks+0x58/0x150
> _cpu_up+0xf0/0x1c0
> do_cpu_up+0x120/0x150
> cpu_subsys_online+0x64/0xe0
> device_online+0xb4/0x120
> online_store+0xb4/0xc0
> dev_attr_store+0x68/0xa0
> sysfs_kf_write+0x80/0xb0
> kernfs_fop_write+0x17c/0x250
> __vfs_write+0x6c/0x1e0
> vfs_write+0xd0/0x270
> SyS_write+0x6c/0x110
> system_call+0x38/0xe0
>
> Examination of the queue showed a single reference (no PERCPU_COUNT_BIAS,
> and __PERCPU_REF_DEAD, __PERCPU_REF_ATOMIC set) and no requests.
> However, conditions at the time of the race are count of PERCPU_COUNT_BIAS + 0
> and __PERCPU_REF_DEAD and __PERCPU_REF_ATOMIC set.
>
> The fix is to make the tryget routines use an actual boolean internally instead
> of the atomic long result truncated to a int.
>
> Fixes: e625305b3907 percpu-refcount: make percpu_ref based on longs instead of ints
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=190751
> Signed-off-by: Douglas Miller <[hidden email]>
> Reviewed-by: Jens Axboe <[hidden email]>
> Signed-off-by: Tejun Heo <[hidden email]>
> Fixes: e625305b3907 ("percpu-refcount: make percpu_ref based on longs instead of ints")
> Cc: [hidden email] # v3.18+
> (cherry picked from commit 966d2b04e070bc040319aaebfec09e0144dc3341)
> Signed-off-by: Tim Gardner <[hidden email]>
> ---
>  include/linux/percpu-refcount.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
> index 1c7eec0..3a481a4 100644
> --- a/include/linux/percpu-refcount.h
> +++ b/include/linux/percpu-refcount.h
> @@ -204,7 +204,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
>  static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>  {
>   unsigned long __percpu *percpu_count;
> - int ret;
> + bool ret;
>  
>   rcu_read_lock_sched();
>  
> @@ -238,7 +238,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
>  static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
>  {
>   unsigned long __percpu *percpu_count;
> - int ret = false;
> + bool ret = false;
>  
>   rcu_read_lock_sched();
>  
>
Clean upstream cherrypick, looks sane to me.

Acked-by: Colin Ian King <[hidden email]>

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ACK: [PATCH Xenial/Yakkety SRU] percpu-refcount: fix reference leak during percpu-atomic transition

Marcelo Henrique Cerri
In reply to this post by Tim Gardner-2
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APPLIED: [PATCH Xenial/Yakkety SRU] percpu-refcount: fix reference leak during percpu-atomic transition

Thadeu Lima de Souza Cascardo-3
In reply to this post by Tim Gardner-2
Applied to yakkety master-next branch.

This is already included in xenial as part of the update to v4.4.48
stable release.

Thanks.
Cascardo.

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