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Mon, 18 May 2026 06:02:38 +0000 (GMT) Received: from [9.123.5.233] (unknown [9.123.5.233]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 18 May 2026 06:02:38 +0000 (GMT) Message-ID: <45af9f95-da07-45b6-babe-dbae0f7eb318@linux.ibm.com> Date: Mon, 18 May 2026 11:32:38 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/3] powerpc/kexec: fix double get_cpu() imbalance in kexec_prepare_cpus To: Aboorva Devarajan , Madhavan Srinivasan , linuxppc-dev@lists.ozlabs.org Cc: Athira Rajeev , Christophe Leroy , linux-kernel@vger.kernel.org, Sourabh Jain , Ritesh Harjani References: <20260518050855.1147242-1-aboorvad@linux.ibm.com> <20260518050855.1147242-4-aboorvad@linux.ibm.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20260518050855.1147242-4-aboorvad@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-ORIG-GUID: UzyFgxX_jkVt8H868ceh3EF4ElsNY7dT X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTE4MDA1MSBTYWx0ZWRfX5qpda25PIm6S Hgpeu/9HRz26Zs3F6ZBX9nan1VrsxMrII4mU9n+vO4bvyzrt0Dwzo2SkqZnmMCW3fR5XHqgwOWH hmzo8YeIUvkBE1KnpCzjHs4TEbA2rvPaajc6RWh+9cZ3CVT6n39xZTLU+ccvgtFdf6Wf9ix1GMh smLZvQNkYtwAGO8oCKShmurGq9O+x52P3XnTC2qQIiZHIjcERil5HjOuvejzGrf3AHSCerx9cLv FD+e+e3MPFodURov/jeWCHRpYd1Xjz6woBTlHAMSF1+1Bq4kDbmO3lREsYA++4IFMeZrD+b4J+R Lpnvzvp63/HORCQvoMMCS93IYaEH4GMNOxMKvI5wmWJdmcYyec+bcZjElr952eDZ4OiBgsWem0x i3IREDtRgwdYwIYEbGSdZuQAEqZezU+14bBA6Y46Xe7daAmucLqMNAmmCyaij+JTOcbEw/eErYl Jg13pUhcQ/nO4gVhcFQ== X-Proofpoint-GUID: 0QB6blTLBVZg7d-qTshchTKYAS6O_sgp X-Authority-Analysis: v=2.4 cv=apyCzyZV c=1 sm=1 tr=0 ts=6a0aab85 cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=NGcC8JguVDcA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VnNF1IyMAAAA:8 a=REhrpbHZYlxIgP8uR5AA:9 a=QEXdDO2ut3YA:10 a=O8hF6Hzn-FEA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-05-18_01,2026-05-15_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 lowpriorityscore=0 priorityscore=1501 impostorscore=0 bulkscore=0 suspectscore=0 adultscore=0 spamscore=0 phishscore=0 clxscore=1015 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2605130000 definitions=main-2605180051 Hi Aboorva. On 5/18/26 10:38 AM, Aboorva Devarajan wrote: > kexec_prepare_cpus_wait() calls get_cpu() internally to obtain the > current CPU id. kexec_prepare_cpus() calls kexec_prepare_cpus_wait() > twice -- once for KEXEC_STATE_IRQS_OFF and once for > KEXEC_STATE_REAL_MODE -- but only issues a single put_cpu() at the end, > leaving preempt_count elevated by one extra nesting level. > > In practice the imbalance does not trigger a 'scheduling while atomic' > splat because the kexec path is a one-way trip: IRQs are already > disabled, no schedule() occurs after the leak, and > default_machine_kexec() overwrites preempt_count with HARDIRQ_OFFSET > before jumping into kexec_sequence() which never returns. However the > bookkeeping is still wrong. > > Lift the get_cpu()/put_cpu() pair into kexec_prepare_cpus() so it is > called exactly once, and pass the CPU id to kexec_prepare_cpus_wait() > as a parameter. This keeps preempt_count correctly balanced. > > Fixes: 1fc711f7ffb01 ("powerpc/kexec: Fix race in kexec shutdown") > Signed-off-by: Aboorva Devarajan > --- > arch/powerpc/kexec/core_64.c | 15 ++++++++------- > 1 file changed, 8 insertions(+), 7 deletions(-) > > diff --git a/arch/powerpc/kexec/core_64.c b/arch/powerpc/kexec/core_64.c > index 825ab8a88f18e..9d7e5a1e6e5b8 100644 > --- a/arch/powerpc/kexec/core_64.c > +++ b/arch/powerpc/kexec/core_64.c > @@ -164,12 +164,11 @@ static void kexec_smp_down(void *arg) > /* NOTREACHED */ > } > > -static void kexec_prepare_cpus_wait(int wait_state) > +static void kexec_prepare_cpus_wait(int wait_state, int my_cpu) > { > - int my_cpu, i, notified=-1; > + int i, notified = -1; > > hw_breakpoint_disable(); > - my_cpu = get_cpu(); > /* Make sure each CPU has at least made it to the state we need. > * > * FIXME: There is a (slim) chance of a problem if not all of the CPUs > @@ -246,6 +245,8 @@ static void wake_offline_cpus(void) > > static void kexec_prepare_cpus(void) > { > + int my_cpu; > + > wake_offline_cpus(); > smp_call_function(kexec_smp_down, NULL, /* wait */0); > local_irq_disable(); > @@ -254,7 +255,8 @@ static void kexec_prepare_cpus(void) > mb(); /* make sure IRQs are disabled before we say they are */ > get_paca()->kexec_state = KEXEC_STATE_IRQS_OFF; > > - kexec_prepare_cpus_wait(KEXEC_STATE_IRQS_OFF); > + my_cpu = get_cpu(); raw_smp_processor_id() is better here. All it needs is get current cpu? caller does irq_disable above and that renders call for get_cpu un-necessary. > + kexec_prepare_cpus_wait(KEXEC_STATE_IRQS_OFF, my_cpu); > /* we are sure every CPU has IRQs off at this point */ > kexec_all_irq_disabled = 1; > > @@ -262,13 +264,12 @@ static void kexec_prepare_cpus(void) > * Before removing MMU mappings make sure all CPUs have entered real > * mode: > */ > - kexec_prepare_cpus_wait(KEXEC_STATE_REAL_MODE); > + kexec_prepare_cpus_wait(KEXEC_STATE_REAL_MODE, my_cpu); > + put_cpu(); > > /* after we tell the others to go down */ > if (ppc_md.kexec_cpu_down) > ppc_md.kexec_cpu_down(0, 0); > - > - put_cpu(); > } > > #else /* ! SMP */