Cc Mel, who added seqcount to cpuset.

On 2013/9/14 8:19, John Stultz wrote:
> After adding lockdep support to seqlock/seqcount structures,
> I started seeing the following warning:
> 
> [    1.070907] ======================================================
> [    1.072015] [ INFO: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected ]
> [    1.073181] 3.11.0+ #67 Not tainted
> [    1.073801] ------------------------------------------------------
> [    1.074882] kworker/u4:2/708 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
> [    1.076088]  (&p->mems_allowed_seq){+.+...}, at: [<ffffffff81187d7f>] new_slab+0x5f/0x280
> [    1.077572]
> [    1.077572] and this task is already holding:
> [    1.078593]  (&(&q->__queue_lock)->rlock){..-...}, at: [<ffffffff81339f03>] blk_execute_rq_nowait+0x53/0xf0
> [    1.080042] which would create a new lock dependency:
> [    1.080042]  (&(&q->__queue_lock)->rlock){..-...} -> (&p->mems_allowed_seq){+.+...}
> [    1.080042]
> [    1.080042] but this new dependency connects a SOFTIRQ-irq-safe lock:
> [    1.080042]  (&(&q->__queue_lock)->rlock){..-...}
> [    1.080042] ... which became SOFTIRQ-irq-safe at:
> [    1.080042]   [<ffffffff810ec179>] __lock_acquire+0x5b9/0x1db0
> [    1.080042]   [<ffffffff810edfe5>] lock_acquire+0x95/0x130
> [    1.080042]   [<ffffffff818968a1>] _raw_spin_lock+0x41/0x80
> [    1.080042]   [<ffffffff81560c9e>] scsi_device_unbusy+0x7e/0xd0
> [    1.080042]   [<ffffffff8155a612>] scsi_finish_command+0x32/0xf0
> [    1.080042]   [<ffffffff81560e91>] scsi_softirq_done+0xa1/0x130
> [    1.080042]   [<ffffffff8133b0f3>] blk_done_softirq+0x73/0x90
> [    1.080042]   [<ffffffff81095dc0>] __do_softirq+0x110/0x2f0
> [    1.080042]   [<ffffffff81095fcd>] run_ksoftirqd+0x2d/0x60
> [    1.080042]   [<ffffffff810bc506>] smpboot_thread_fn+0x156/0x1e0
> [    1.080042]   [<ffffffff810b3916>] kthread+0xd6/0xe0
> [    1.080042]   [<ffffffff818980ac>] ret_from_fork+0x7c/0xb0
> [    1.080042]
> [    1.080042] to a SOFTIRQ-irq-unsafe lock:
> [    1.080042]  (&p->mems_allowed_seq){+.+...}
> [    1.080042] ... which became SOFTIRQ-irq-unsafe at:
> [    1.080042] ...  [<ffffffff810ec1d3>] __lock_acquire+0x613/0x1db0
> [    1.080042]   [<ffffffff810edfe5>] lock_acquire+0x95/0x130
> [    1.080042]   [<ffffffff810b3df2>] kthreadd+0x82/0x180
> [    1.080042]   [<ffffffff818980ac>] ret_from_fork+0x7c/0xb0
> [    1.080042]
> [    1.080042] other info that might help us debug this:
> [    1.080042]
> [    1.080042]  Possible interrupt unsafe locking scenario:
> [    1.080042]
> [    1.080042]        CPU0                    CPU1
> [    1.080042]        ----                    ----
> [    1.080042]   lock(&p->mems_allowed_seq);
> [    1.080042]                                local_irq_disable();
> [    1.080042]                                lock(&(&q->__queue_lock)->rlock);
> [    1.080042]                                lock(&p->mems_allowed_seq);
> [    1.080042]   <Interrupt>
> [    1.080042]     lock(&(&q->__queue_lock)->rlock);
> [    1.080042]
> [    1.080042]  *** DEADLOCK ***
> 
> The issue stems from the kthreadd() function calling set_mems_allowed
> with irqs enabled. While its possibly unlikely for the actual deadlock
> to trigger, a fix is fairly simple: disable irqs before taking the
> mems_allowed_seq lock.
> 

Now I get it. I'm fine with this change.

Acked-by: Li Zefan <lizefan@huawei.com>

> Let me know if you have any other suggestions or alternative fixes you'd
> prefer.
> 
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
>  include/linux/cpuset.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index cc1b01c..3fe661f 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -110,10 +110,14 @@ static inline bool put_mems_allowed(unsigned int seq)
> 
>  static inline void set_mems_allowed(nodemask_t nodemask)
>  {
> +	unsigned long flags;
> +
>  	task_lock(current);
> +	local_irq_save(flags);
>  	write_seqcount_begin(&current->mems_allowed_seq);
>  	current->mems_allowed = nodemask;
>  	write_seqcount_end(&current->mems_allowed_seq);
> +	local_irq_restore(flags);
>  	task_unlock(current);
>  }
> 
>