3.2-rc7: Hang when calling clone()

Hi all,

During recent fuzzer tests (Trinity over KVM tool), I've managed to cause the following kernel oops:

[10080.793053] INFO: task kworker/u:0:5 blocked for more than 120 seconds.
[10080.794297] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[10080.795751] kworker/u:0     D ffff880015320bd8  4576     5      2 0x00000000
[10080.797127]  ffff880019d5ba00 0000000000000082 ffff8800ffffffff 000016b2b9b199b6
[10080.798571]  ffff88001a7d3ec0 00000000001d33c0 ffff880019d3d800 00000000001d33c0
[10080.800050]  ffff880019d5bfd8 ffff880019d5a000 00000000001d33c0 00000000001d33c0
[10080.802092] Call Trace:
[10080.802546]  [<ffffffff823d87fa>] schedule+0x3a/0x50
[10080.803466]  [<ffffffff823d90e5>] schedule_timeout+0x245/0x2c0
[10080.804522]  [<ffffffff810e7e2e>] ? mark_held_locks+0x6e/0x130
[10080.805580]  [<ffffffff810e54f2>] ? lock_release_holdtime+0xb2/0x160
[10080.806743]  [<ffffffff823db2ab>] ? _raw_spin_unlock_irq+0x2b/0x70
[10080.807885]  [<ffffffff810a4831>] ? get_parent_ip+0x11/0x50
[10080.808933]  [<ffffffff823d8d70>] wait_for_common+0x120/0x170
[10080.810118]  [<ffffffff810a3150>] ? try_to_wake_up+0x350/0x350
[10080.811205]  [<ffffffff810a33e4>] ? wake_up_new_task+0x124/0x1f0
[10080.812318]  [<ffffffff823d8e68>] wait_for_completion+0x18/0x20
[10080.813416]  [<ffffffff810ae9c4>] do_fork+0xf4/0x330
[10080.814339]  [<ffffffff823d8c94>] ? wait_for_common+0x44/0x170
[10080.815429]  [<ffffffff8104bd91>] kernel_thread+0x71/0x80
[10080.816448]  [<ffffffff810c6bd0>] ? proc_cap_handler+0x1c0/0x1c0
[10080.817565]  [<ffffffff823deac0>] ? gs_change+0x13/0x13
[10080.818543]  [<ffffffff810c6de2>] __call_usermodehelper+0x32/0xa0
[10080.819679]  [<ffffffff810c7bd7>] process_one_work+0x1c7/0x460
[10080.820754]  [<ffffffff810c7b76>] ? process_one_work+0x166/0x460
[10080.821843]  [<ffffffff810c6db0>] ? call_usermodehelper_freeinfo+0x30/0x30
[10080.823078]  [<ffffffff810c9152>] worker_thread+0x162/0x340
[10080.824108]  [<ffffffff810c8ff0>] ? manage_workers.clone.20+0x240/0x240
[10080.825286]  [<ffffffff810cf716>] kthread+0xb6/0xc0
[10080.826171]  [<ffffffff823deac4>] kernel_thread_helper+0x4/0x10
[10080.827235]  [<ffffffff823dc038>] ? retint_restore_args+0x13/0x13
[10080.828368]  [<ffffffff810cf660>] ? kthread_flush_work_fn+0x10/0x10
[10080.829538]  [<ffffffff823deac0>] ? gs_change+0x13/0x13
[10080.843036] 2 locks held by kworker/u:0/5:
[10080.843803]  #0:  (khelper){.+.+.+}, at: [<ffffffff810c7b76>] process_one_work+0x166/0x460
[10080.845447]  #1:  ((&sub_info->work)){+.+.+.}, at: [<ffffffff810c7b76>] process_one_work+0x166/0x460
[10080.847223] Kernel panic - not syncing: hung_task: blocked tasks
[10080.848338] Pid: 947, comm: khungtaskd Not tainted 3.2.0-rc7-sasha-00039-g89307ba #93
[10080.849787] Call Trace:
[10080.850361]  [<ffffffff823d7811>] panic+0x96/0x1c5
[10080.851259]  [<ffffffff810e6021>] ? print_lock+0x61/0xb0
[10080.852251]  [<ffffffff81126a46>] watchdog+0x2b6/0x2f0
[10080.853205]  [<ffffffff81126800>] ? watchdog+0x70/0x2f0
[10080.854188]  [<ffffffff823db273>] ? _raw_spin_unlock_irqrestore+0x93/0xa0
[10080.855421]  [<ffffffff81126790>] ? hung_task_panic+0x20/0x20
[10080.856463]  [<ffffffff810cf716>] kthread+0xb6/0xc0
[10080.857363]  [<ffffffff823deac4>] kernel_thread_helper+0x4/0x10
[10080.858458]  [<ffffffff823dc038>] ? retint_restore_args+0x13/0x13
[10080.859570]  [<ffffffff810cf660>] ? kthread_flush_work_fn+0x10/0x10
[10080.860743]  [<ffffffff823deac0>] ? gs_change+0x13/0x13

This is the syscall that caused that:

clone(clone_flags=0xd8220000, newsp=0xf3f270[page_0xff], parent_tid=0xf41290[page_allocs], child_tid=0xf3f270[page_0xff], regs=0x7f1b066a1000)

I've seen two variants of this, one where the hang was in the same process that called clone(), and one (like the above) where it happened in kworker. In both cases, the stack above do_fork() is the same.

Sasha.

vfork parent uninterruptibly and unkillably waits for its child to exec/exit. This wait is of unbounded length. Ignore such waits in the hung_task detector.

LKML-Reference: <1325344394.28904.43.camel@lappy>
Cc: Linus Torvalds <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: John Kacur <[email protected]>

---
 kernel/hung_task.c |   14 ++++++++++----
 1 files changed, 10 insertions(+), 4 deletions(-)

index 8b1748d..2e48ec0 100644
@@ -74,11 +74,17 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 
 	/*
 	 * Ensure the task is not frozen.
-	 * Also, when a freshly created task is scheduled once, changes
-	 * its state to TASK_UNINTERRUPTIBLE without having ever been
-	 * switched out once, it musn't be checked.
+	 * Also, skip vfork and any other user process that freezer should skip.
 	 */
-	if (unlikely(t->flags & PF_FROZEN || !switch_count))
+	if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
+	    return;
+
+	/*
+	 * When a freshly created task is scheduled once, changes its state to
+	 * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
+	 * musn't be checked.
+	 */
+	if (unlikely(!switch_count))
 		return;
 
 	if (switch_count != t->last_switch_count) {

1.7.3.1