Re: sata_mv port lockup on hotplug (kernel 2.6.38.2)

[Tejun Heo <htejun@gmail.com>]

Hello,

On Wed, Sep 07, 2011 at 01:09:10PM +0100, Bruce Stenning wrote:
> Sorry for sending so many emails yesterday; I blame the dental anaesthetic
> I received in the morning for being so jumpy on the send button ;-)

Oh the fun. :)

> I can certainly try this.  Could you confirm whether my thoughts about a race
> between the scsi_eh thread and the wake-up are plausible?  I backtracked
> yesterday because I thought the scsi_eh thread would get rescheduled naturally,
> not realising that when the task state is TASK_INTERRUPTIBLE schedule() takes
> the task off the run queue (so it needs to be explicitly woken.)
> 
> Here is my thinking again:
> 
> shost->host_eh_scheduled is read here in scsi_error_handler:
> 
>         set_current_state(TASK_INTERRUPTIBLE);
>         while (!kthread_should_stop()) {
>                 if ((shost->host_failed == 0 && shost->host_eh_scheduled == 0) ||
> 
> There's no locking in scsi_error_handler (though functions it calls probably
> claim locks.)
> 
> When scheduling an EH, scsi_schedule_eh takes the shost->host_lock, increments
> shost->host_eh_scheduled, and then wakes the EH thread.  If this happens
> between the scsi_eh thread reading host_eh_scheduled and sending itself back
> to sleep (when the scsi_eh thread's state is TASK_INTERRUPTIBLE) nothing will
> wake up the thread again and host_eh_scheduled will not get inspected.
> host_eh_scheduled is stuck at 1 with the scsi_eh thread asleep, and it won't
> get woken again because the ata port has been frozen and irqs are masked off.

I don't think there's a race condition there.  set_current_state()
implies memory barrier and wake_up_process() implies wmb().  host_eh
either sees the inrecremented eh_scheduled count or TASK_RUNNING set
by wake_up_process(), so it can't miss an event.

Thanks.

-- 
tejun