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Date: Mon, 15 Jul 2019 08:38:07 +0300 From: Vasily Averin <vvs@...tuozzo.com> To: Xiaoming Ni <nixiaoming@...wei.com>, "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org> Cc: "adobriyan@...il.com" <adobriyan@...il.com>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "anna.schumaker@...app.com" <anna.schumaker@...app.com>, "arjan@...ux.intel.com" <arjan@...ux.intel.com>, "bfields@...ldses.org" <bfields@...ldses.org>, "chuck.lever@...cle.com" <chuck.lever@...cle.com>, "davem@...emloft.net" <davem@...emloft.net>, "jlayton@...nel.org" <jlayton@...nel.org>, "luto@...nel.org" <luto@...nel.org>, "mingo@...nel.org" <mingo@...nel.org>, "Nadia.Derbey@...l.net" <Nadia.Derbey@...l.net>, "paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>, "semen.protsenko@...aro.org" <semen.protsenko@...aro.org>, "stable@...nel.org" <stable@...nel.org>, "stern@...land.harvard.edu" <stern@...land.harvard.edu>, "tglx@...utronix.de" <tglx@...utronix.de>, "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>, "trond.myklebust@...merspace.com" <trond.myklebust@...merspace.com>, "viresh.kumar@...aro.org" <viresh.kumar@...aro.org>, "Huangjianhui (Alex)" <alex.huangjianhui@...wei.com>, Dailei <dylix.dailei@...wei.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org> Subject: Re: [PATCH v3 0/3] kernel/notifier.c: avoid duplicate registration On 7/14/19 5:45 AM, Xiaoming Ni wrote: > On 2019/7/12 22:07, gregkh@...uxfoundation.org wrote: >> On Fri, Jul 12, 2019 at 09:11:57PM +0800, Xiaoming Ni wrote: >>> On 2019/7/11 21:57, Vasily Averin wrote: >>>> On 7/11/19 4:55 AM, Nixiaoming wrote: >>>>> On Wed, July 10, 2019 1:49 PM Vasily Averin wrote: >>>>>> On 7/10/19 6:09 AM, Xiaoming Ni wrote: >>>>>>> Registering the same notifier to a hook repeatedly can cause the hook >>>>>>> list to form a ring or lose other members of the list. >>>>>> >>>>>> I think is not enough to _prevent_ 2nd register attempt, >>>>>> it's enough to detect just attempt and generate warning to mark host in bad state. >>>>>> >>>>> >>>>> Duplicate registration is prevented in my patch, not just "mark host in bad state" >>>>> >>>>> Duplicate registration is checked and exited in notifier_chain_cond_register() >>>>> >>>>> Duplicate registration was checked in notifier_chain_register() but only >>>>> the alarm was triggered without exiting. added by commit 831246570d34692e >>>>> ("kernel/notifier.c: double register detection") >>>>> >>>>> My patch is like a combination of 831246570d34692e and notifier_chain_cond_register(), >>>>> which triggers an alarm and exits when a duplicate registration is detected. >>>>> >>>>>> Unexpected 2nd register of the same hook most likely will lead to 2nd unregister, >>>>>> and it can lead to host crash in any time: >>>>>> you can unregister notifier on first attempt it can be too early, it can be still in use. >>>>>> on the other hand you can never call 2nd unregister at all. >>>>> >>>>> Since the member was not added to the linked list at the time of the second registration, >>>>> no linked list ring was formed. >>>>> The member is released on the first unregistration and -ENOENT on the second unregistration. >>>>> After patching, the fault has been alleviated >>>> >>>> You are wrong here. >>>> 2nd notifier's registration is a pure bug, this should never happen. >>>> If you know the way to reproduce this situation -- you need to fix it. >>>> >>>> 2nd registration can happen in 2 cases: >>>> 1) missed rollback, when someone forget to call unregister after successfull registration, >>>> and then tried to call register again. It can lead to crash for example when according module will be unloaded. >>>> 2) some subsystem is registered twice, for example from different namespaces. >>>> in this case unregister called during sybsystem cleanup in first namespace will incorrectly remove notifier used >>>> in second namespace, it also can lead to unexpacted behaviour. >>>> >>> So in these two cases, is it more reasonable to trigger BUG() directly when checking for duplicate registration ? >>> But why does current notifier_chain_register() just trigger WARN() without exiting ? >>> notifier_chain_cond_register() direct exit without triggering WARN() ? >> >> It should recover from this, if it can be detected. The main point is >> that not all apis have to be this "robust" when used within the kernel >> as we do allow for the callers to know what they are doing :) >> > In the notifier_chain_register(), the condition ( (*nl) == n) is the same registration of the same hook. > We can intercept this situation and avoid forming a linked list ring to make the API more rob Once again -- yes, you CAN prevent list corruption, but you CANNOT recover the host and return it back to safe state. If double register event was detected -- it means you have bug in kernel. Yes, you can add BUG here and crash the host immediately, but I prefer to use warning in such situations. >> If this does not cause any additional problems or slow downs, it's >> probably fine to add. >> > Notifier_chain_register() is not a system hotspot function. > At the same time, there is already a WARN_ONCE judgment. There is no new judgment in the new patch. > It only changes the processing under the condition of (*nl) == n, which will not cause performance problems. > At the same time, avoiding the formation of a link ring can make the system more robust. I disagree, yes, node will have correct list, but anyway node will work wrong and can crash the host in any time. >> thanks, >> >> greg k-h >> >> . >> > Thanks > > Xiaoming Ni > > >
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