From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sergei Shtylyov Subject: Re: [PATCH] Correctly prevent IDE timer expiry function to run if request was already handled Date: Sat, 07 Apr 2007 01:09:31 +0400 Message-ID: <4616B70B.3070501@ru.mvista.com> References: <20070406173603.GA82722@freefall.freebsd.org> <46169DDA.6020008@ru.mvista.com> <50660.65.57.245.11.1175892938.squirrel@ssl.mu.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <50660.65.57.245.11.1175892938.squirrel@ssl.mu.org> Sender: linux-kernel-owner@vger.kernel.org To: Suleiman Souhlal Cc: bzolnier@gmail.com, linux-ide@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-ide@vger.kernel.org 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