Re: [BUG] kernel: rcu: a possible sleep-in-atomic-context bug in srcu_read_delay()

From: Jia-Ju Bai
Date: Mon Aug 13 2018 - 05:26:59 EST

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
kernel/rcu/rcutorture.c, 1105: [FUNC_PTR]srcu_read_delay in
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()) {
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() & \

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