Messages in this thread | | | Date | Mon, 13 Aug 2018 05:42:32 -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 05:26:49PM +0800, Jia-Ju Bai wrote: > > > 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.
Not a problem, a few false positives are to be expected. And it looks like you have some work to do on your tool -- which is good, because I would not want you to be bored. ;-)
Thanx, Paul
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