Easy, it happens all the time, there are just no tests in place to see it.
We are keeping a histogram of how long every IDE DMA transfer takes
place. In ide_intr we record the time and set the start time in
ide_drive_t to 0. In ide_dma_proc, ide_dma_begin right AFTER activating
the dma, we store the current value of jiffies in start time in ide_drive_t.
In both those places we check to make sure that the value of start_time
is sensible. In ide_dma_begin, we make sure it's 0 and in ide_dma_intr,
we make sure its non-zero. Because of this race condition, we often saw
DMAs finish before they began.
In the normal kernel, the only thing I can see that could go wrong would
be that the printk
printk("%s: ide_intr: huh? expected NULL handler on exit\n", drive->name);
in ide_intr could be triggered. I've never seen it happen, but I
believe with enough effort, it could be made to occur.
Ross
Andre Hedrick wrote:
>Okay, how do you reproduce it to see the effects?
>
>On Mon, 27 Jan 2003, Ross Biro wrote:
>
>
>
>>The net effect of this race condition and the other one I spotted is
>>that you may see some interesting messages in your log file and you can
>>detect the race condition if you look for it hard enough. I don't
>>currently see any bad effects.
>>
>> Ross
>>
>>Ross Biro wrote:
>>
>>
>>
>>>There is at least one more IDE race condition in 2.4.18 and
>>>2.4.21-pre3. Basically the interrupt for the controller being serviced
>>>is left on while setting up the next command. I'm not sure how much
>>>trouble it can cause but it does lead to some interesting stack traces.
>>>
>>>The condition
>>>if (masked_irq && hwif->irq != masked_irq)
>>>in ide_do_request should be replaced with
>>>if (!masked_irq || hwif->irq != masked_irq)
>>>in two places.
>>>
>>>
>>>
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>>
>>
>
>Andre Hedrick
>LAD Storage Consulting Group
>
>
>
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