Messages in this thread |  | | Date | Sun, 12 Aug 2018 21:18:26 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [BUG] kernel: rcu: a possible sleep-in-atomic-context bug in srcu_read_delay() |
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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.
So what am I missing here?
Thanx, Paul
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