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 <[email protected]>
> Let me know if you have any other suggestions or alternative fixes you'd
> prefer.
>
> Cc: Li Zefan <[email protected]>
> Cc: Mathieu Desnoyers <[email protected]>
> Cc: Steven Rostedt <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Signed-off-by: John Stultz <[email protected]>
> ---
> 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(¤t->mems_allowed_seq);
> current->mems_allowed = nodemask;
> write_seqcount_end(¤t->mems_allowed_seq);
> + local_irq_restore(flags);
> task_unlock(current);
> }
>
>
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