Re: [PATCH 2/2] Avoid unnecessary error messages handling udev events

On 4/20/20 3:09 PM, Mark Asselstine wrote: > On Monday, April 20, 2020 6:11:25 A.M. EDT Michal Privoznik wrote: >> On 4/16/20 5:57 PM, Mark Asselstine wrote: >>> The udev monitor thread "udevEventHandleThread()" will lag the >>> actual/real view of devices in sysfs as it serially processes udev >>> monitor events. So for instance if you were to run the following cmd >>> to create a new veth pair and rename one of the veth endpoints >>> >>> you might see the following monitor events and real world that looks >>> like >>> >>> time >>> | >>> | create v0 sysfs entry >>> >>> wake udevEventHandleThread | create v1 sysfs entry >>> udev_monitor_receive_device(v1-add) | move v0 sysfs to v2 >>> udevHandleOneDevice(v1) | >>> udev_monitor_receive_device(v0-add) | >>> udevHandleOneDevice(v0) | <--- error msgs in >>> virNetDevGetLinkInfo() udev_monitor_receive_device(v2-move) | as >>> v0 >>> no longer exists udevHandleOneDevice(v2) | >>> >>> \/ >>> >>> As you can see the changes in sysfs can take place well before we get >>> to act on the events in the udevEventHandleThread(), so by the time we >>> get around to processing the v0 add event, the sysfs entry has been >>> moved to v2. >>> >>> To work around this we check if the sysfs entry is valid before >>> attempting to read it and don't bother trying to read link info if >>> not. This is safe since we will never read sysfs entries earlier than >>> it existing, ie. if the entry is not there it has either been removed >>> in the time since we enumerated the device or something bigger is >>> busted, in either case, no sysfs entry, no link info. In the case >>> described above we will eventually get the link info as we work >>> through the queue of monitor events and get to the 'move' event. >>> >>> https://bugzilla.redhat.com/show_bug.cgi?id=1557902 >>> >>> Signed-off-by: Mark Asselstine <mark.asselstine(a)windriver.com&gt; >>> --- >>> >>> src/util/virnetdev.c | 11 +++++++++++ >>> 1 file changed, 11 insertions(+) >>> >>> diff --git a/src/util/virnetdev.c b/src/util/virnetdev.c >>> index b465bdac2e..bf256cbe35 100644 >>> --- a/src/util/virnetdev.c >>> +++ b/src/util/virnetdev.c >>> @@ -2438,6 +2438,17 @@ virNetDevGetLinkInfo(const char *ifname, >>> >>> if (virNetDevSysfsFile(&path, ifname, "operstate") < 0) >>> >>> goto cleanup; >>> >>> + /* The device may have been removed or moved by the time we got >>> here. >>> + * Obviously attempting to get LinkInfo on a no longer existing >>> device >>> + * is useless, so stop processing. If we got here via the udev >>> monitor >>> + * a remove or move event will follow and we will be able to get >>> valid >>> + * LinkInfo at that time */ >>> + if (!virFileExists(path)) { >>> + VIR_WARN("The interface '%s' was removed before we could query >>> it.", >>> + ifname); >> >> This is not aligned. > > Should I send a V2 or can this be correct when merged? No need, I've fixed it (also changed goto cleanup to return, since I've merged a patch that made 'cleanup' label go away this morning) and pushed. >>> + goto cleanup; >>> + } >> >> But I have another idea to consider. We could return a different error >> value here (say -2) and then have the caller decide what to do with it >> (e.g. print the warning). > > I had thought about propagating the error so the caller could possibly > wrap > the warning message in more context but in the end I decided to call it > out at the issue site avoiding having each caller essentially reproducing > essentially the same code. Yeah, what convinced me was that non-Linux version of this function simply prints a debug message and returns 0. I haven't realized that earlier, sorry. Michal