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Fri, 30 Aug 2024 07:17:41 -0400 X-MC-Unique: Y6QDg5daOvCkiKXeP_nKMg-1 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 44DF11955BF2; Fri, 30 Aug 2024 11:17:39 +0000 (UTC) Received: from localhost (unknown [10.72.112.42]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 87A6019560AA; Fri, 30 Aug 2024 11:17:36 +0000 (UTC) Date: Fri, 30 Aug 2024 19:17:33 +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> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1e4a8e18-cda9-45f5-a842-8ffcd725efc9@linux.ibm.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 On 08/20/24 at 12:10pm, Sourabh Jain wrote: > Hello Baoquan, > > On 19/08/24 11:45, Baoquan He wrote: > > On 08/19/24 at 09:45am, Sourabh Jain wrote: > > > Hello Michael and Boaquan > > > > > > On 01/08/24 12:21, Sourabh Jain wrote: > > > > Hello Michael, > > > > > > > > On 01/08/24 08:04, Michael Ellerman wrote: > > > > > Sourabh Jain writes: > > > > > > The following errors are observed when kexec is done with SMT=off on > > > > > > powerpc. > > > > > > > > > > > > [  358.458385] Removing IBM Power 842 compression device > > > > > > [  374.795734] kexec_core: Starting new kernel > > > > > > [  374.795748] kexec: Waking offline cpu 1. > > > > > > [  374.875695] crash hp: kexec_trylock() failed, elfcorehdr may > > > > > > be inaccurate > > > > > > [  374.935833] kexec: Waking offline cpu 2. > > > > > > [  375.015664] crash hp: kexec_trylock() failed, elfcorehdr may > > > > > > be inaccurate > > > > > > snip.. > > > > > > [  375.515823] kexec: Waking offline cpu 6. > > > > > > [  375.635667] crash hp: kexec_trylock() failed, elfcorehdr may > > > > > > be inaccurate > > > > > > [  375.695836] kexec: Waking offline cpu 7. > > > > > Are they actually errors though? Do they block the actual kexec from > > > > > happening? Or are they just warnings in dmesg? > > > > The kexec kernel boots fine. > > > > > > > > This warning appears regardless of whether the kdump kernel is loaded. > > > > > > > > However, when the kdump kernel is loaded, we will not be able to update > > > > the kdump image (FDT). > > > > I think this should be fine given that kexec is in progress. > > > > > > > > Please let me know your opinion. > > > > > > > > > Because the fix looks like it could be racy. > > > > It seems like it is racy, but given that kexec takes the lock first and > > > > then > > > > brings the CPU up, which triggers the kdump image, which always fails to > > > > update the kdump image because it could not take the same lock. > > > > > > > > Note: the kexec lock is not released unless kexec boot fails. > > > Any comments or suggestions on this fix? > > Is this a little better? > > > > diff --git a/kernel/crash_core.c b/kernel/crash_core.c > > index 63cf89393c6e..0355ffb712f4 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; > > @@ -539,7 +539,7 @@ 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()) { > > + if (!kexec_trylock() && kexec_in_progress) { > > pr_info("kexec_trylock() failed, elfcorehdr may be inaccurate\n"); > > crash_hotplug_unlock(); > > return; > > Ideally, when `kexec_in_progress` is True, there should be no way to acquire > the kexec lock. > Therefore, calling `kexec_trylock()` before checking `kexec_in_progress` is > not helpful. > The kernel will print the error message "kexec_trylock() failed, elfcorehdr > may be inaccurate." > So, with the above changes, the original problem remains unsolved. > > However, after closely inspecting the `kernel/kexec_core.c:kernel_kexec()` > function, I discovered > an exceptional case where my patch needs an update. The issue arises when > the system returns > from the `machine_kexec()` function, which indicates that kexec has failed. > > In this scenario, the kexec lock is released, but `kexec_in_progress` > remains True. > > I am unsure why `kexec_in_progress` is NOT set to False when kexec fails. > Was this by design, > or was it an oversight because returning from the `machine_kexec()` function > is highly unlikely? > > Here is my proposal to address the original problem along with the > exceptional case I described > above. > > Let's implement two patches: > > 1. A patch that sets `kexec_in_progress` to False if the system returns from > `machine_kexec()` before I don't think we have chance to return from machine_kexec() after triggering kexec/kdump jumping. The KEXEC_JUMP could return, but I'v never heard people using it. >    unlocking the kexec lock in the `kernel_kexec()` function. > >    ``` >    diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c >    index c0caa14880c3..b41277183455 100644 >    --- a/kernel/kexec_core.c >    +++ b/kernel/kexec_core.c >    @@ -1069,6 +1069,7 @@ int kernel_kexec(void) >    #endif > >     Unlock: >    +      kexec_in_progress = false; >            kexec_unlock(); >            return error; >     ``` > > 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. That's why I think maybe checking kexec_in_progress after failing to retriving __kexec_lock is a little better, not very sure. > > Please share your comments on the new approach. > > Thank you for review. > > - Sourabh Jain >