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
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