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Subject: Re: [BUG] kernel: rcu: a possible sleep-in-atomic-context bug in
 srcu_read_delay()
To:     paulmck@linux.vnet.ibm.com
Cc:     dave@stgolabs.net, josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <4179b4d2-b832-abcd-d407-b865765f28d6@gmail.com>
 <20180813041826.GL24813@linux.vnet.ibm.com>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <5b3fd721-541c-5165-ef54-d6c476318d92@gmail.com>
Date:   Mon, 13 Aug 2018 17:26:49 +0800
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On 2018/8/13 12:18, Paul E. McKenney wrote:
> On Mon, Aug 13, 2018 at 11:04:10AM +0800, Jia-Ju Bai wrote:
>> The kernel may sleep with holding a spinlock.
>>
>> The function call paths (from bottom to top) in Linux-4.16 are:
>>
>> [FUNC] schedule_timeout_interruptible
>> kernel/rcu/rcutorture.c, 523: schedule_timeout_interruptible in
>> srcu_read_delay
>> kernel/rcu/rcutorture.c, 1105: [FUNC_PTR]srcu_read_delay in
>> rcu_torture_timer
>> kernel/rcu/rcutorture.c, 1104: spin_lock in rcu_torture_timer
>>
>> Note that [FUNC_PTR] means a function pointer call is used.
>>
>> I do not find a good way to fix, so I only report.
>> This is found by my static analysis tool (DSAC).
> Interesting.  I would have expected to have gotten a "scheduling while
> atomic" error message, which I do not recall seeing.  And I ran a great
> deal of rcutorture on v4.16.
>
> So let's see...  As you say, the rcu_torture_timer() function does in
> fact acquire rand_lock in 4.16 and 4.17, in which case sleeping would
> indeed be illegal.  But let's take a look at srcu_read_delay():
>
> static void
> srcu_read_delay(struct torture_random_state *rrsp, struct rt_read_seg *rtrsp)
> {
> 	long delay;
> 	const long uspertick = 1000000 / HZ;
> 	const long longdelay = 10;
>
> 	/* We want there to be long-running readers, but not all the time. */
>
> 	delay = torture_random(rrsp) %
> 		(nrealreaders * 2 * longdelay * uspertick);
> 	if (!delay && in_task()) {
> 		schedule_timeout_interruptible(longdelay);
> 		rtrsp->rt_delay_jiffies = longdelay;
> 	} else {
> 		rcu_read_delay(rrsp, rtrsp);
> 	}
> }
>
> The call to schedule_timeout_interruptible() cannot happen unless the
> in_task() macro returns true, which it won't if the SOFTIRQ_OFFSET bit
> is set:
>
> #define in_task()		(!(preempt_count() & \
> 				   (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
>
> And the SOFTIRQ_OFFSET bit will be set if srcu_read_delay()
> is invoked from a timer handler, which is the case for the
> call from rcu_torture_timer().  So if that lock is held,
> schedule_timeout_interruptible() won't ever be invoked.

Thanks for your reply :)
My tool does not track this bit...
Sorry for this false report.


Best wishes,
Jia-Ju Bai