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mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 76F5F19560B5; Sun, 8 Sep 2024 10:30:41 +0000 (UTC) Received: from localhost (unknown [10.72.112.58]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 91F211956056; Sun, 8 Sep 2024 10:30:39 +0000 (UTC) Date: Sun, 8 Sep 2024 18:30:33 +0800 From: Baoquan He To: Sourabh Jain Cc: Michael Ellerman , Hari Bathini , kexec@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, x86@kernel.org, Sachin P Bappalige Subject: Re: [PATCH] kexec/crash: no crash update when kexec in progress Message-ID: References: <20240731152738.194893-1-sourabhjain@linux.ibm.com> <87v80lnf8d.fsf@mail.lhotse> <10c666ae-d528-4f49-82e9-8e0fee7099e0@linux.ibm.com> <355b58b1-6c51-4c42-b6ea-dcd6b1617a18@linux.ibm.com> <1e4a8e18-cda9-45f5-a842-8ffcd725efc9@linux.ibm.com> <0dd94920-b13f-4da7-9ea6-4f008af1f4b3@linux.ibm.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Post: List-Subscribe: , , List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On 09/05/24 at 02:07pm, Sourabh Jain wrote: > Hello Baoquan, > > On 05/09/24 08:53, Baoquan He wrote: > > On 09/04/24 at 02:55pm, Sourabh Jain wrote: > > > Hello Baoquan, > > > > > > On 30/08/24 16:47, Baoquan He wrote: > > > > On 08/20/24 at 12:10pm, Sourabh Jain wrote: > > > > > Hello Baoquan, > > > > > > > ......snip... > > > > > 2. A patch to return early from the `crash_handle_hotplug_event()` function > > > > > if `kexec_in_progress` is > > > > >    set to True. This is essentially my original patch. > > > > There's a race gap between the kexec_in_progress checking and the > > > > setting it to true which Michael has mentioned. > > > The window where kernel is holding kexec_lock to do kexec boot > > > but kexec_in_progress is yet not set to True. > > > > > > If kernel needs to handle crash hotplug event, the function > > > crash_handle_hotplug_event()  will not get the kexec_lock and > > > error out by printing error message about not able to update > > > kdump image. > > But you wanted to avoid the erroring out if it's being in > > kernel_kexec(). Now you are seeing at least one the noising > > message, aren't you? > > Yes, but it is very rare to encounter. > > My comments on your updated code are inline below. > > > > > > I think it should be fine. Given that lock is already taken for > > > kexec kernel boot. > > > > > > Am I missing something major? > > > > > > > That's why I think > > > > maybe checking kexec_in_progress after failing to retriving > > > > __kexec_lock is a little better, not very sure. > > > Try for kexec lock before kexec_in_progress check will not solve > > > the original problem this patch trying to solve. > > > > > > You proposed the below changes earlier: > > > > > > - if (!kexec_trylock()) { > > > + if (!kexec_trylock() && kexec_in_progress) { > > > pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n"); > > > crash_hotplug_unlock(); > > Ah, I meant as below, but wrote it mistakenly. > > > > diff --git a/kernel/crash_core.c b/kernel/crash_core.c > > index 63cf89393c6e..e7c7aa761f46 100644 > > --- a/kernel/crash_core.c > > +++ b/kernel/crash_core.c > > @@ -504,7 +504,7 @@ int crash_check_hotplug_support(void) > > crash_hotplug_lock(); > > /* Obtain lock while reading crash information */ > > - if (!kexec_trylock()) { > > + if (!kexec_trylock() && !kexec_in_progress) { > > pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n"); > > crash_hotplug_unlock(); > > return 0; > > > > > > > > > > Once the kexec_in_progress is set to True there is no way one can get > > > kexec_lock. So kexec_trylock() before kexec_in_progress is not helpful > > > for the problem I am trying to solve. > > With your patch, you could still get the error message if the race gap > > exist. With above change, you won't get it. Please correct me if I am > > wrong. > > The above code will print an error message during the race gap. Here's why: > > Let’s say the kexec lock is acquired in the kernel_kexec() function, > but kexec_in_progress is not yet set to True. In this scenario, the code > will print > an error message. > > There is another issue I see with the above code: > > Consider that the system is on the kexec kernel boot path, and > kexec_in_progress > is set to True. If crash_hotplug_unlock() is called, the kernel will not > only update > the kdump image without acquiring the kexec lock, but it will also release > the > kexec lock in the out label. I believe this is incorrect. > > Please share your thoughts. How about this? diff --git a/kernel/crash_core.c b/kernel/crash_core.c index 63cf89393c6e..8ba7b1da0ded 100644 --- a/kernel/crash_core.c +++ b/kernel/crash_core.c @@ -505,7 +505,8 @@ int crash_check_hotplug_support(void) crash_hotplug_lock(); /* Obtain lock while reading crash information */ if (!kexec_trylock()) { - pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n"); + if (!kexec_in_progress) + pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n"); crash_hotplug_unlock(); return 0; } @@ -540,7 +541,8 @@ static void crash_handle_hotplug_event(unsigned int hp_action, unsigned int cpu, crash_hotplug_lock(); /* Obtain lock while changing crash information */ if (!kexec_trylock()) { - pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n"); + if (!kexec_in_progress) + pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n"); crash_hotplug_unlock(); return; }