[PATCH] sched/deadline: correctly handle active 0-lag timers

[luca abeni <luca.abeni@santannapisa.it>]

syzbot reported the following warning:
[  948.126369] WARNING: CPU: 4 PID: 17089 at kernel/sched/deadline.c:255 task_non_contending+0xae0/0x1950
[  948.130198] Kernel panic - not syncing: panic_on_warn set ...
[  948.130198]
[  948.134221] CPU: 4 PID: 17089 Comm: syz-executor.1 Not tainted 4.19.27 #2
[  948.139072] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1 04/01/2014
[  948.141603] Call Trace:
[  948.142277]  dump_stack+0xca/0x13e
[  948.164636]  panic+0x1f7/0x543
[  948.168704]  ? refcount_error_report+0x29d/0x29d
[  948.172438]  ? __warn+0x1d1/0x210
[  948.183359]  ? task_non_contending+0xae0/0x1950
[  948.191747]  __warn+0x1ec/0x210
[  948.196276]  ? task_non_contending+0xae0/0x1950
[  948.202476]  report_bug+0x1ee/0x2b0
[  948.204622]  fixup_bug.part.7+0x37/0x80
[  948.206879]  do_error_trap+0x22c/0x290
[  948.211340]  ? math_error+0x2f0/0x2f0
[  948.217033]  ? trace_hardirqs_off_caller+0x40/0x190
[  948.222477]  ? trace_hardirqs_off_thunk+0x1a/0x1c
[  948.229877]  invalid_op+0x14/0x20
[  948.238317] RIP: 0010:task_non_contending+0xae0/0x1950
[  948.253825] Code: 6d 29 83 48 89 4c 24 20 48 89 54 24 10 c6 05 d0 89
5a 03 01 e8 11 ea ee ff 0f 0b 48 8b 4c 24 20 48 8b 54 24 10 e9 bb f7 ff
ff <0f> 0b e9 1d f6 ff ff e8 d4 a7 09 00 85 c0 0f 85 74 f8 ff ff 48 c7
[  948.272329] RSP: 0018:ffff8883d443f8c0 EFLAGS: 00010002
[  948.293045] RAX: 0000000000000001 RBX: ffff8883d3572468 RCX: ffffffff813a6571
[  948.300323] RDX: 00000000000008ab RSI: ffffc900030e4000 RDI: ffff8883e2fe6278
[  948.305278] RBP: ffff8883e2f00000 R08: ffffed1078ea3ab2 R09: ffffed1078ea3ab2
[  948.316441] R10: 0000000000000001 R11: ffffed1078ea3ab1 R12: 000000000002c680
[  948.320257] R13: ffff8883d357217c R14: 0000000000000001 R15: ffff8883d3572140
[  948.324500]  ? hrtimer_active+0x171/0x1f0
[  948.327421]  ? dequeue_task_dl+0x38/0x970
[  948.330572]  __schedule+0x94b/0x1a80
[  948.333578]  ? __sched_text_start+0x8/0x8
[  948.336141]  ? lock_downgrade+0x5e0/0x5e0
[  948.338111]  ? plist_add+0x23e/0x480
[  948.339706]  schedule+0x7c/0x1a0
[  948.341395]  futex_wait_queue_me+0x319/0x600
[  948.343329]  ? get_futex_key_refs+0xd0/0xd0
[  948.345037]  ? lock_downgrade+0x5e0/0x5e0
[  948.347206]  ? get_futex_key_refs+0xa4/0xd0
[  948.353007]  futex_wait+0x1e7/0x590
[  948.355328]  ? futex_wait_setup+0x2b0/0x2b0
[  948.360578]  ? __lock_acquire+0x60c/0x3b70
[  948.369186]  ? __save_stack_trace+0x92/0x100
[  948.374344]  ? hash_futex+0x15/0x210
[  948.376832]  ? drop_futex_key_refs+0x3c/0xd0
[  948.378591]  ? futex_wake+0x14e/0x450
[  948.381609]  do_futex+0x5c9/0x15e0
[  948.384567]  ? perf_syscall_enter+0xb1/0xc80
[  948.390307]  ? exit_robust_list+0x240/0x240
[  948.393566]  ? ftrace_syscall_exit+0x5c0/0x5c0
[  948.396369]  ? lock_downgrade+0x5e0/0x5e0
[  948.401748]  ? __might_fault+0x17c/0x1c0
[  948.404171]  __x64_sys_futex+0x296/0x380
[  948.406472]  ? __ia32_sys_futex+0x370/0x370
[  948.440630]  ? trace_hardirqs_on_thunk+0x1a/0x1c
[  948.441774]  ? trace_hardirqs_off_caller+0x40/0x190
[  948.442770]  ? do_syscall_64+0x3b/0x580
[  948.486728]  do_syscall_64+0xc8/0x580
[  948.489138]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  948.492072] RIP: 0033:0x462eb9
[  948.492788] Code: f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00
48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f
05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
[  948.532016] RSP: 002b:00007f7ac8a67cd8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
[  948.536811] RAX: ffffffffffffffda RBX: 000000000073bf08 RCX: 0000000000462eb9
[  948.542138] RDX: 0000000000000000 RSI: 0000000000000080 RDI: 000000000073bf08
[  948.548077] RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
[  948.562535] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000073bf0c
[  948.569184] R13: 0000000000000000 R14: 000000000073bf00 R15: 00007fff106d8c10

line 255 of deadline.c is
	WARN_ON(hrtimer_active(&dl_se->inactive_timer));
in task_non_contending().
Unfortunately, in some cases (for example, a deadline task
continuosly blocking and waking immediately) it can happen that
a task blocks (and task_non_contending() is called) while the
0-lag timer is still active. In this case, the safest thing to
do is to immediately decrease the running bandwidth of the task,
without trying to re-arm the 0-lag timer.

Signed-off-by: luca abeni <luca.abeni@santannapisa.it>
---
 kernel/sched/deadline.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 6a73e41a2016..43901fa3f269 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -252,7 +252,6 @@ static void task_non_contending(struct task_struct *p)
 	if (dl_entity_is_special(dl_se))
 		return;
 
-	WARN_ON(hrtimer_active(&dl_se->inactive_timer));
 	WARN_ON(dl_se->dl_non_contending);
 
 	zerolag_time = dl_se->deadline -
@@ -269,7 +268,7 @@ static void task_non_contending(struct task_struct *p)
 	 * If the "0-lag time" already passed, decrease the active
 	 * utilization now, instead of starting a timer
 	 */
-	if (zerolag_time < 0) {
+	if ((zerolag_time < 0) || hrtimer_active(&dl_se->inactive_timer)) {
 		if (dl_task(p))
 			sub_running_bw(dl_se, dl_rq);
 		if (!dl_task(p) || p->state == TASK_DEAD) {
-- 
2.17.1