[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [patch] netback: Xennet half die---netback driver didn't detect the jiffies wrapping correctly.
On Fri, 2012-11-30 at 09:25 +0000, Jan Beulich wrote: > >>> On 30.11.12 at 09:35, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote: > > On Fri, 2012-11-30 at 08:09 +0000, Yi, Shunli wrote: > >> Netback driver use " time_after_eq()" to check the jiffies wrapping, > >> while this function was only called when the credit is running out. > >> So, if the jiffies wrapped and the credit isn't run out in first half > >> jiffies circle, the time_after_eq() cannot check the wrapping any > >> more. > > > > Which tree is this against? It doesn't appear to be mainline Linux, > > which is all I am really interested in these days. > > > > Also your patch is missing a Signed-off-by and is whitespace damaged. > > Please read Documentation/SubmittingPatches and > > Documentation/SubmitChecklist. > > > >> This will cause the credit_timerout.expires is set to dozens of days > >> in future. > >> > >> The netback will stop receiving data from netfront. > >> > >> For example: > >> Jiffies initialized to 0xffffff-(300*HZ), and the > >> credit_timeout.expires was initialized to 0xffffff00, > >> After dozens of days, when the jiffies grow to upper than 0x80000000, > >> and the time_after_eq() will cannot check for the wrapping. > >> > >> > >> > >> --- drivers/xen/netback/netback.c.org 2012-11-30 15:48:13.109039998 > >> -0500 > >> +++ drivers/xen/netback/netback.c 2012-11-30 15:48:55.212072898 -0500 > >> @@ -1272,6 +1272,10 @@ static void net_tx_action(unsigned long > >> rmb(); /* Ensure that we see the request before we copy it. */ > >> memcpy(&txreq, RING_GET_REQUEST(&netif->tx, i), sizeof(txreq)); > >> > >> + /* Check for the jiffies wrapping */ > >> + if (time_after_eq(jiffies, netif->credit_timeout.expires)) > >> + netif->credit_timeout.expires = jiffies; > > > > Do you not need to remove the similar check from the following block? > > I don't think so, but I also can't see how that adjustment would > help in the first place: If it gets executed after a very long period > of no traffic, it would itself not be able to reliably tell whether the > clock wrapped. Hrm, yes, This change would help in the case of a dribble of traffic which never hits the limit, but not in the case of no timer at all. > That said, I agree that the code as is appears to have a problem > (with 32-bit jiffies at least), but I can't see how to easily deal with > it. Would it help to always have the pending timer armed, for either the next tick if credit needs replenishing or for, say MAX_JIFFIES/4 as a backstop to avoid wrapping issues? Ian. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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