[PATCH V2 0/1] percpu-refcount: fix reference leak during percpu-atomic transition

[PATCH V2 0/1] percpu-refcount: fix reference leak during percpu-atomic transition

[Douglas Miller]

Removed extraneous parentheses. Did not switch to "bool" as that would necessitate
more testing and raises the question whether other platforms might have a different
definition for "bool" that would not fix this problem.

[PATCH V2 1/1] percpu-refcount: fix reference leak during percpu-atomic transition

[Douglas Miller]

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 return an actual boolean instead
of the atomic long result truncated to a bool.

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 <dougmill@linux.vnet.ibm.com>
Reviewed-by: Jens Axboe <axboe@fb.com>
---
 include/linux/percpu-refcount.h |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 1c7eec0..ac3518a 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -212,7 +212,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 		this_cpu_inc(*percpu_count);
 		ret = true;
 	} else {
-		ret = atomic_long_inc_not_zero(&ref->count);
+		ret = atomic_long_inc_not_zero(&ref->count) != 0;
 	}
 
 	rcu_read_unlock_sched();
@@ -246,7 +246,7 @@ static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 		this_cpu_inc(*percpu_count);
 		ret = true;
 	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
-		ret = atomic_long_inc_not_zero(&ref->count);
+		ret = atomic_long_inc_not_zero(&ref->count) != 0;
 	}
 
 	rcu_read_unlock_sched();
-- 
1.7.1

Re: [PATCH V2 0/1] percpu-refcount: fix reference leak during percpu-atomic transition

[Tejun Heo]

On Fri, Jan 27, 2017 at 05:32:15PM -0600, Douglas Miller wrote:
> Removed extraneous parentheses. Did not switch to "bool" as that would necessitate
> more testing and raises the question whether other platforms might have a different
> definition for "bool" that would not fix this problem.

Please switch to bool.

Thanks.

-- 
tejun

Re: [PATCH V2 0/1] percpu-refcount: fix reference leak during percpu-atomic transition

[Douglas Miller]

I will re-test and post the patch using "bool" when finished.

On 01/27/2017 06:13 PM, Tejun Heo wrote:
> On Fri, Jan 27, 2017 at 05:32:15PM -0600, Douglas Miller wrote:
>> Removed extraneous parentheses. Did not switch to "bool" as that would necessitate
>> more testing and raises the question whether other platforms might have a different
>> definition for "bool" that would not fix this problem.
> Please switch to bool.
>
> Thanks.
>