Re: [PATCH] Correctly prevent IDE timer expiry function to run if request was already handled

[Sergei Shtylyov]

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

MBR, Sergei
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