Messages in this thread | | | Date | Sat, 07 Apr 2007 01:09:31 +0400 | From | Sergei Shtylyov <> | Subject | Re: [PATCH] Correctly prevent IDE timer expiry function to run if request was already handled |
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Hello.
Suleiman Souhlal wrote:
>>>It is possible for the timer expiry function to run even though the >>>request has already been handled: ide_timer_expiry() only checks that the >>>handler is not NULL, but it is possible that we have handled a request >>>(thus clearing the handler) and then started a new request >>>(thus starting the timer again, and setting a handler).
>>>A simple way to exhibit this is to set the DMA timeout to 1 jiffy and >>>run dd: The kernel will panic after a few minutes because >>>ide_timer_expiry() tries to add a timer when it's already active.
>>>To fix this, we simply add a request generation count that gets >>>incremented at every interrupt, and check in ide_timer_expiry() that we >>>have not already handled a new interrupt before running the expiry >>>function.
>>Couldn't this be addressed by simply changing add_timer() to mod_timer()?
> No, we don't want to run the expiry function at all, in this case, since > the request might have correctly been handled already by the time we > would try to run the expiry function/restart the timer.
> Also, if we just change the add_timer() to mod_timer(), we will just be > hiding the problem because you might end up changing the timeout of a > timer whose purpose is different (for a new request, for example). > The timer should not be active when ide_timer_expiry() tries to restart > it, since that function is called when the timer has expired (meaning it > is not active anymore).
Yeah, that was stupid idea. Been looking at network schedulers too much recently. :-)
> The reason the timer could have been active at that point, before applying > this patch, is that we try to dispatch a new request after handling one. > The new request will then have its own expiry timer, along with a handler. > Since before this patch ide_timer_expiry() only looked at whether or not > a handler was present, it would incorrectly think the request had not been > handled already, and incorrectly tried to restart the timer.
Hm, I'm still not sure why this happens at all, probably need to try reproducing it (unless you post a stack trace :-).
> -- Suleiman
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