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Date: Fri, 12 Jul 2019 21:11:57 +0800 From: Xiaoming Ni <nixiaoming@...wei.com> To: Vasily Averin <vvs@...tuozzo.com>, "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>, "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>, "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> CC: "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 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() ? Thanks Xiaoming Ni >> It may be more helpful to return an error code when someone tries to register the same >> notification program a second time. > > You are wrong again here, it is senseless. > If you have detected 2nd register -- your node is already in bad state. > >> But I noticed that notifier_chain_cond_register() returns 0 when duplicate registration >> is detected. At the same time, in all the existing export function comments of notify, >> "Currently always returns zero" >> >> I am a bit confused: which is better? >> >>> >>> Unfortunately I do not see any ways to handle such cases properly, >>> and it seems for me your patches does not resolve this problem. >>> >>> Am I missed something probably? >>> >>>> case1: An infinite loop in notifier_chain_register() can cause soft lockup >>>> atomic_notifier_chain_register(&test_notifier_list, &test1); >>>> atomic_notifier_chain_register(&test_notifier_list, &test1); >>>> atomic_notifier_chain_register(&test_notifier_list, &test2); >> >> Thanks >> >> Xiaoming Ni >> > > . >
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