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

[Jens Axboe <axboe@kernel.dk>]

On 01/27/2017 01:59 PM, Douglas Miller wrote:
> 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>
> ---
>  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..88dc96b 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);
>  	}

Fix looks good to me, but let's drop the extraneous parentheses:

	ret = atomic_long_inc_not_zero(&ref->count) != 0;

in both spots. With that, you can add my

Reviewed-by: Jens Axboe <axboe@fb.com>

and let's fast-track this into 4.10.

-- 
Jens Axboe