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

So what am I missing here?

Thanx, Paul

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