Re: [PATCH] kfence: Avoid stalling work queue task without allocations

Paul E. McKenney
2020-11-11T18:23:38Z

On Wed, Nov 11, 2020 at 01:05:43PM -0500, Steven Rostedt wrote:
> On Wed, 11 Nov 2020 14:38:13 +0100
> Marco Elver <elver@google.com> wrote:
>
> > [+Cc folks who can maybe help figure out what's going on, since I get
> > warnings even without KFENCE on next-20201110.]
> >
> > On Wed, Nov 11, 2020 at 09:29AM +0100, Marco Elver wrote:
> > > On Wed, 11 Nov 2020 at 00:23, Anders Roxell <anders.roxell@linaro.org> wrote:
> > > [...]
> > > > I gave them a spin on next-20201105 [1] and on next-20201110 [2].
> > > >
> > > > I eventually got to a prompt on next-20201105.
> > > > However, I got to this kernel panic on the next-20201110:
> > > >
> > > > [...]
> > > > [ 1514.089966][ T1] Testing event system initcall: OK
> > > > [ 1514.806232][ T1] Running tests on all trace events:
> > > > [ 1514.857835][ T1] Testing all events:
> > > > [ 1525.503262][ C0] hrtimer: interrupt took 10902600 ns
> > > > [ 1623.861452][ C0] BUG: workqueue lockup - pool cpus=0 node=0
> > > > flags=0x0 nice=0 stuck for 65s!
> > > > [...]
>
> OK, so this blows up when you enable all events?
>
> Note, it could just be adding overhead (which is exasperated with other
> debug options enabled), which could open up a race window.
>
>
> > > > [ 7823.104349][ T28] Tainted: G W
> > > > 5.10.0-rc3-next-20201110-00008-g8dc06700529d #3
> > > > [ 7833.206491][ T28] "echo 0 >
> > > > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > > [ 7840.750700][ T28] task:kworker/0:1 state:D stack:26640 pid:
> > > > 1872 ppid: 2 flags:0x00000428
> > > > [ 7875.642531][ T28] Workqueue: events toggle_allocation_gate
> > > > [ 7889.178334][ T28] Call trace:
> > > > [ 7897.066649][ T28] __switch_to+0x1cc/0x1e0
> > > > [ 7905.326856][ T28] 0xffff00000f7077b0
> > > > [ 7928.354644][ T28] INFO: lockdep is turned off.
> > > > [ 7934.022572][ T28] Kernel panic - not syncing: hung_task: blocked tasks
> > > > [ 7934.032039][ T28] CPU: 0 PID: 28 Comm: khungtaskd Tainted: G
> > > > W 5.10.0-rc3-next-20201110-00008-g8dc06700529d #3
> > > > [ 7934.045586][ T28] Hardware name: linux,dummy-virt (DT)
> > > > [ 7934.053677][ T28] Call trace:
> > > > [ 7934.060276][ T28] dump_backtrace+0x0/0x420
> > > > [ 7934.067635][ T28] show_stack+0x38/0xa0
> > > > [ 7934.091277][ T28] dump_stack+0x1d4/0x278
> > > > [ 7934.098878][ T28] panic+0x304/0x5d8
> > > > [ 7934.114923][ T28] check_hung_uninterruptible_tasks+0x5e4/0x640
> > > > [ 7934.123823][ T28] watchdog+0x138/0x160
> > > > [ 7934.131561][ T28] kthread+0x23c/0x260
> > > > [ 7934.138590][ T28] ret_from_fork+0x10/0x18
> > > > [ 7934.146631][ T28] Kernel Offset: disabled
> > > > [ 7934.153749][ T28] CPU features: 0x0240002,20002004
> > > > [ 7934.161476][ T28] Memory Limit: none
> > > > [ 7934.171272][ T28] ---[ end Kernel panic - not syncing: hung_task:
> > > > blocked tasks ]---
> > > >
> > > > Cheers,
> > > > Anders
> > > > [1] https://people.linaro.org/~anders.roxell/output-next-20201105-test.log
> > > > [2] https://people.linaro.org/~anders.roxell/output-next-20201110-test.log
> > >
> > > Thanks for testing. The fact that it passes on next-20201105 but not
> > > on 20201110 is strange. If you boot with KFENCE disabled (boot param
> > > kfence.sample_interval=0), does it boot?
> > [...]
> >
> > Right, so I think this is no longer KFENCE's fault. This looks like
> > something scheduler/RCU/ftrace related?! I notice that there have been
> > scheduler changes between next-20201105 and next-20201110.
>
> I'm not sure any of that would cause this.
>
> >
> > I get this with KFENCE disabled:
> >
> > | Running tests on all trace events:
> > | Testing all events:
> > | BUG: workqueue lockup - pool cpus=0 node=0 flags=0x0 nice=0 stuck for 32s!
> > | Showing busy workqueues and worker pools:
> > | workqueue events: flags=0x0
> > | pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=1/256 refcnt=2
> > | pending: vmstat_shepherd
> > | workqueue events_power_efficient: flags=0x82
> > | pwq 2: cpus=0 flags=0x5 nice=0 active=2/256 refcnt=4
> > | in-flight: 107:neigh_periodic_work
> > | pending: do_cache_clean
> > | pool 2: cpus=0 flags=0x5 nice=0 hung=3s workers=2 manager: 7
> > | rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> > | (detected by 0, t=6502 jiffies, g=2885, q=4)
> > | rcu: All QSes seen, last rcu_preempt kthread activity 5174 (4295523265-4295518091), jiffies_till_next_fqs=1, root ->qsmask 0x0
> > | rcu: rcu_preempt kthread starved for 5174 jiffies! g2885 f0x2 RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=0
> > | rcu: Unless rcu_preempt kthread gets sufficient CPU time, OOM is now expected behavior.
> > | rcu: RCU grace-period kthread stack dump:
> > | task:rcu_preempt state:R running task stack: 0 pid: 10 ppid: 2 flags:0x00000428
> > | Call trace:
> > | __switch_to+0x100/0x1e0
> > | __schedule+0x2d0/0x890
> > | preempt_schedule_notrace+0x70/0x1c0
> > | ftrace_ops_no_ops+0x174/0x250
> > | ftrace_graph_call+0x0/0xc
>
> Note, just because ftrace is called here, the blocked task was preempted
> when the ftrace code called preempt_enable_notrace().
>
>
> > | preempt_count_add+0x1c/0x180
> > | schedule+0x44/0x108
> > | schedule_timeout+0x394/0x530
> > | rcu_gp_kthread+0x76c/0x19a8
> > | kthread+0x174/0x188
> > | ret_from_fork+0x10/0x18
> > |
> > | ================================
> > | WARNING: inconsistent lock state
> > | 5.10.0-rc3-next-20201110-00001-gc07b306d7fa5-dirty #18 Not tainted
> > | --------------------------------
> > | inconsistent {IN-HARDIRQ-W} -> {HARDIRQ-ON-W} usage.
> > | kcompactd0/26 [HC0[0]:SC0[0]:HE0:SE1] takes:
> > | ffffae32e6bd4358 (rcu_node_0){?.-.}-{2:2}, at: rcu_sched_clock_irq+0x4a0/0xd18
> > | {IN-HARDIRQ-W} state was registered at:
>
> I did some digging here and it looks like the rcu_node lock could be taken
> without interrupts enabled when it does a stall print. That probably should
> be fixed, but it's a symptom of the underlining bug and not the cause.

Does this patch (in -next) help?

Thanx, Paul

------------------------------------------------------------------------

commit c583bcb8f5edd48c1798798e341f78afb9bf4f6f
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Thu Sep 24 15:11:55 2020 -0700

rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled

The try_invoke_on_locked_down_task() function requires that
interrupts be enabled, but it is called with interrupts disabled from
rcu_print_task_stall(), resulting in an "IRQs not enabled as expected"
diagnostic. This commit therefore updates rcu_print_task_stall()
to accumulate a list of the first few tasks while holding the current
leaf rcu_node structure's ->lock, then releases that lock and only then
uses try_invoke_on_locked_down_task() to attempt to obtain per-task
detailed information. Of course, as soon as ->lock is released, the
task might exit, so the get_task_struct() function is used to prevent
the task structure from going away in the meantime.

Link: https://lore.kernel.org/lkml/000000000000903d5805ab908fc4@google.com/
Fixes: 5bef8da66a9c ("rcu: Add per-task state to RCU CPU stall warnings")
Reported-by: syzbot+cb3b69ae80afd6535b0e@syzkaller.appspotmail.com
Reported-by: syzbot+f04854e1c5c9e913cc27@syzkaller.appspotmail.com
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 0fde39b..ca21d28 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -249,13 +249,16 @@ static bool check_slow_task(struct task_struct *t, void *arg)

/*
* Scan the current list of tasks blocked within RCU read-side critical
- * sections, printing out the tid of each.
+ * sections, printing out the tid of each of the first few of them.
*/
-static int rcu_print_task_stall(struct rcu_node *rnp)
+static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
+ __releases(rnp->lock)
{
+ int i = 0;
int ndetected = 0;
struct rcu_stall_chk_rdr rscr;
struct task_struct *t;
+ struct task_struct *ts[8];

if (!rcu_preempt_blocked_readers_cgp(rnp))
return 0;
@@ -264,6 +267,14 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
t = list_entry(rnp->gp_tasks->prev,
struct task_struct, rcu_node_entry);
list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
+ get_task_struct(t);
+ ts[i++] = t;
+ if (i >= ARRAY_SIZE(ts))
+ break;
+ }
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ for (i--; i; i--) {
+ t = ts[i];
if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
pr_cont(" P%d", t->pid);
else
@@ -273,6 +284,7 @@ static int rcu_print_task_stall(struct rcu_node *rnp)
".q"[rscr.rs.b.need_qs],
".e"[rscr.rs.b.exp_hint],
".l"[rscr.on_blkd_list]);
+ put_task_struct(t);
ndetected++;
}
pr_cont("\n");
@@ -293,8 +305,9 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
* Because preemptible RCU does not exist, we never have to check for
* tasks blocked within RCU read-side critical sections.
*/
-static int rcu_print_task_stall(struct rcu_node *rnp)
+static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
{
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
return 0;
}
#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
@@ -472,7 +485,6 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
rcu_for_each_leaf_node(rnp) {
raw_spin_lock_irqsave_rcu_node(rnp, flags);
- ndetected += rcu_print_task_stall(rnp);
if (rnp->qsmask != 0) {
for_each_leaf_node_possible_cpu(rnp, cpu)
if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
@@ -480,7 +492,7 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
ndetected++;
}
}
- raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
}

for_each_possible_cpu(cpu)