Re: [PATCH] futex: fix unbalanced spin_lock/spin_unlock() inexit_pi_state_list()

[Thomas Gleixner]

On Fri, 1 Mar 2013, Yong Zhang wrote:

> From: Yong Zhang <yong.zhang@xxxxxxxxxxxxx>
> 
> Otherwise, below warning is shown somtimes when running some test:
> 
> WARNING: at kernel/sched/core.c:3423 migrate_disable+0xbf/0xd0()
> Hardware name: OptiPlex 755
> Modules linked in: floppy parport parport_pc minix
> Pid: 1800, comm: tst-robustpi8 Tainted: G        W    3.4.28-rt40 #1
> Call Trace:
>  [<ffffffff81031f3f>] warn_slowpath_common+0x7f/0xc0
>  [<ffffffff81031f9a>] warn_slowpath_null+0x1a/0x20
>  [<ffffffff81066eaf>] migrate_disable+0xbf/0xd0
>  [<ffffffff81085d95>] exit_pi_state_list+0xa5/0x170
>  [<ffffffff8102f71f>] mm_release+0x12f/0x170
>  [<ffffffff81036906>] exit_mm+0x26/0x140
>  [<ffffffff81090fc6>] ? acct_collect+0x186/0x1c0
>  [<ffffffff81036b66>] do_exit+0x146/0x930
>  [<ffffffff810658d1>] ? get_parent_ip+0x11/0x50
>  [<ffffffff8103760d>] do_group_exit+0x4d/0xc0
>  [<ffffffff8104828f>] get_signal_to_deliver+0x23f/0x6a0
>  [<ffffffff810019e5>] do_signal+0x65/0x5e0
>  [<ffffffff81047816>] ? group_send_sig_info+0x76/0x80
>  [<ffffffff81002018>] do_notify_resume+0x98/0xd0
>  [<ffffffff8165779b>] int_signal+0x12/0x17
> ---[ end trace 0000000000000004 ]---
> 
> The reason is that spin_lock() is taken in atomic context, but
> spin_unlock() is not.

This doesn't make sense. The spin_lock() happens in non atomic
context.

	spin_lock(&hb->lock);
	raw_spin_lock_irq(&curr->pi_lock);

The unlock is in atomic context and the unlock does not call
migrate_disable().

Though on RT this is caused by the in_atomic check of migrate_enable()
and this results in asymetry. See below.

> Signed-off-by: Yong Zhang <yong.zhang0@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> ---
>  kernel/futex.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 9e26e87..2b676a2 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -562,16 +562,17 @@ void exit_pi_state_list(struct task_struct *curr)
>  
>  		spin_lock(&hb->lock);
>  
> -		raw_spin_lock_irq(&curr->pi_lock);
>  		/*
>  		 * We dropped the pi-lock, so re-check whether this
>  		 * task still owns the PI-state:
>  		 */

Did you read and understand this comment ?

The logic here is

    raw_spin_lock_irq(&curr->pi_lock);
    next = head->next;
    raw_spin_unlock_irq(&curr->pi_lock);
    spin_lock(&hb->lock);
    raw_spin_lock_irq(&curr->pi_lock);
    if (head->next != next)

We must drop pi_lock before locking the hash bucket lock. That opens a
window for another task to modify head list. So we must relock pi_lock
and verify whether head->next is unmodified. If it changed, we need to
reevaluate.

>  		if (head->next != next) {
>  			spin_unlock(&hb->lock);
> +			raw_spin_lock_irq(&curr->pi_lock);
>  			continue;
>  		}
>  
> +		raw_spin_lock_irq(&curr->pi_lock);
>  		WARN_ON(pi_state->owner != curr);
>  		WARN_ON(list_empty(&pi_state->list));
>  		list_del_init(&pi_state->list);

So both your patch description and your patch are patently wrong.
Correct solution below.

Thanks,

	tglx
---
futex: Ensure lock/unlock symetry versus pi_lock and hash bucket lock

In exit_pi_state_list() we have the following locking construct:

   spin_lock(&hb->lock);
   raw_spin_lock_irq(&curr->pi_lock);
   
   ...
   spin_unlock(&hb->lock);

In !RT this works, but on RT the migrate_enable() function which is
called from spin_unlock() sees atomic context due to the held pi_lock
and just decrements the migrate_disable_atomic counter of the
task. Now the next call to migrate_disable() sees the counter being
negative and issues a warning. That check should be in
migrate_enable() already.

Fix this by dropping pi_lock before unlocking hb->lock and reaquire
pi_lock after that again. This is safe as the loop code reevaluates
head again under the pi_lock.

Reported-by: Yong Zhang <yong.zhang@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

diff --git a/kernel/futex.c b/kernel/futex.c
index f15f0e4..c795c9c 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -568,7 +568,9 @@ void exit_pi_state_list(struct task_struct *curr)
 		 * task still owns the PI-state:
 		 */
 		if (head->next != next) {
+			raw_spin_unlock_irq(&curr->pi_lock);
 			spin_unlock(&hb->lock);
+			raw_spin_lock_irq(&curr->pi_lock);
 			continue;
 		}
 
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