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Subject: Re: [PATCH] sample-trace-array: Fix sleeping function called from
 invalid context
To:     Kefeng Wang <wangkefeng.wang@huawei.com>,
        linux-kernel@vger.kernel.org
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
References: <20200608075437.1760242-1-wangkefeng.wang@huawei.com>
From:   Divya Indi <divya.indi@oracle.com>
Message-ID: <0047f80f-4fff-16ff-7ac0-20181f03f06a@oracle.com>
Date:   Mon, 8 Jun 2020 22:14:28 -0700
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Hi Kefeng,

Thanks for catching this issue.

Please find my comments line -

On 6/8/20 12:54 AM, Kefeng Wang wrote:
>  BUG: sleeping function called from invalid context at kernel/locking/mutex.c:935
>  in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 0, name: swapper/5
>  1 lock held by swapper/5/0:
>   #0: ffff80001002bd90 (samples/ftrace/sample-trace-array.c:38){+.-.}-{0:0}, at: call_timer_fn+0x8/0x3e0
>  CPU: 5 PID: 0 Comm: swapper/5 Not tainted 5.7.0+ #8
>  Hardware name: QEMU QEMU Virtual Machine, BIOS 0.0.0 02/06/2015
>  Call trace:
>   dump_backtrace+0x0/0x1a0
>   show_stack+0x20/0x30
>   dump_stack+0xe4/0x150
>   ___might_sleep+0x160/0x200
>   __might_sleep+0x58/0x90
>   __mutex_lock+0x64/0x948
>   mutex_lock_nested+0x3c/0x58
>   __ftrace_set_clr_event+0x44/0x88
>   trace_array_set_clr_event+0x24/0x38
>   mytimer_handler+0x34/0x40 [sample_trace_array]
>
> mutex_lock() will be called in interrupt context, using workqueueu to fix it.

/s/workqueueu/workqueue

Fixes: 89ed42495ef4 ("tracing: Sample module to demonstrate kernel access to Ftrace instances.")

> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  samples/ftrace/sample-trace-array.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/samples/ftrace/sample-trace-array.c b/samples/ftrace/sample-trace-array.c
> index d523450d73eb..41684c7dbd7b 100644
> --- a/samples/ftrace/sample-trace-array.c
> +++ b/samples/ftrace/sample-trace-array.c
> @@ -20,6 +20,16 @@ struct trace_array *tr;
>  static void mytimer_handler(struct timer_list *unused);
>  static struct task_struct *simple_tsk;
>  
> +static void trace_work_fn(struct work_struct *work)
> +{
> +	/*
> +	 * Disable tracing for event "sample_event".
> +	 */
> +	trace_array_set_clr_event(tr, "sample-subsystem", "sample_event",
> +			false);
> +}
> +static DECLARE_WORK(trace_work, trace_work_fn);
> +
>  /*
>   * mytimer: Timer setup to disable tracing for event "sample_event". This
>   * timer is only for the purposes of the sample module to demonstrate access of
> @@ -29,11 +39,7 @@ static DEFINE_TIMER(mytimer, mytimer_handler);
>  
>  static void mytimer_handler(struct timer_list *unused)
>  {
> -	/*
> -	 * Disable tracing for event "sample_event".
> -	 */
> -	trace_array_set_clr_event(tr, "sample-subsystem", "sample_event",
> -			false);
> +	schedule_work(&trace_work);
>  }
>  
>  static void simple_thread_func(int count)

I think we also need to use cancel_work_sync() to handle the case -
1. Module unloaded
2. Timer already ran and scheduled work to disable trace event
3. When the work runs we no longer have the relevant trace array.

static int simple_thread(void *arg)
{
.....

 del_timer(&mytimer);

 <cancel_work_sync> 
.....
 return 0;
}

Thanks,
Divya