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Wed, 22 Sep 2021 07:57:21 +0000 (GMT) Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BF5FCAE063; Wed, 22 Sep 2021 07:57:19 +0000 (GMT) Received: from linux.vnet.ibm.com (unknown [9.126.150.29]) by d06av26.portsmouth.uk.ibm.com (Postfix) with SMTP; Wed, 22 Sep 2021 07:57:19 +0000 (GMT) Date: Wed, 22 Sep 2021 13:27:18 +0530 From: Srikar Dronamraju To: Nathan Lynch Subject: Re: [PATCH] powerpc/paravirt: correct preempt debug splat in vcpu_is_preempted() Message-ID: <20210922075718.GA2004@linux.vnet.ibm.com> References: <20210921031213.2029824-1-nathanl@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20210921031213.2029824-1-nathanl@linux.ibm.com> X-TM-AS-GCONF: 00 X-Proofpoint-GUID: lnYNZIPub06sGV5PBOSDZ037jyuFF5ul X-Proofpoint-ORIG-GUID: 30covYLPOMs0shHZlrl4iOnfHtwfS5wo X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.182.1,Aquarius:18.0.790,Hydra:6.0.391,FMLib:17.0.607.475 definitions=2021-09-22_02,2021-09-20_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 phishscore=0 mlxlogscore=999 spamscore=0 lowpriorityscore=0 malwarescore=0 clxscore=1015 impostorscore=0 priorityscore=1501 mlxscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2109200000 definitions=main-2109220051 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Srikar Dronamraju Cc: linuxppc-dev@lists.ozlabs.org, npiggin@gmail.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" * Nathan Lynch [2021-09-20 22:12:13]: > vcpu_is_preempted() can be used outside of preempt-disabled critical > sections, yielding warnings such as: > > BUG: using smp_processor_id() in preemptible [00000000] code: systemd-udevd/185 > caller is rwsem_spin_on_owner+0x1cc/0x2d0 > CPU: 1 PID: 185 Comm: systemd-udevd Not tainted 5.15.0-rc2+ #33 > Call Trace: > [c000000012907ac0] [c000000000aa30a8] dump_stack_lvl+0xac/0x108 (unreliable) > [c000000012907b00] [c000000001371f70] check_preemption_disabled+0x150/0x160 > [c000000012907b90] [c0000000001e0e8c] rwsem_spin_on_owner+0x1cc/0x2d0 > [c000000012907be0] [c0000000001e1408] rwsem_down_write_slowpath+0x478/0x9a0 > [c000000012907ca0] [c000000000576cf4] filename_create+0x94/0x1e0 > [c000000012907d10] [c00000000057ac08] do_symlinkat+0x68/0x1a0 > [c000000012907d70] [c00000000057ae18] sys_symlink+0x58/0x70 > [c000000012907da0] [c00000000002e448] system_call_exception+0x198/0x3c0 > [c000000012907e10] [c00000000000c54c] system_call_common+0xec/0x250 > > The result of vcpu_is_preempted() is always subject to invalidation by > events inside and outside of Linux; it's just a best guess at a point in > time. Use raw_smp_processor_id() to avoid such warnings. Typically smp_processor_id() and raw_smp_processor_id() except for the CONFIG_DEBUG_PREEMPT. In the CONFIG_DEBUG_PREEMPT case, smp_processor_id() is actually debug_smp_processor_id(), which does all the checks. I believe these checks in debug_smp_processor_id() are only valid for x86 case (aka cases were they have __smp_processor_id() defined.) i.e x86 has a different implementation of _smp_processor_id() for stable and unstable > > Signed-off-by: Nathan Lynch > Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()") > --- > arch/powerpc/include/asm/paravirt.h | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h > index bcb7b5f917be..e429aca566de 100644 > --- a/arch/powerpc/include/asm/paravirt.h > +++ b/arch/powerpc/include/asm/paravirt.h > @@ -97,7 +97,14 @@ static inline bool vcpu_is_preempted(int cpu) > > #ifdef CONFIG_PPC_SPLPAR > if (!is_kvm_guest()) { > - int first_cpu = cpu_first_thread_sibling(smp_processor_id()); > + int first_cpu; > + > + /* > + * This is only a guess at best, and this function may be > + * called with preemption enabled. Using raw_smp_processor_id() > + * does not damage accuracy. > + */ > + first_cpu = cpu_first_thread_sibling(raw_smp_processor_id()); > > /* > * Preemption can only happen at core granularity. This CPU > -- > 2.31.1 > How about something like the below? diff --git a/include/linux/smp.h b/include/linux/smp.h index 510519e8a1eb..8c669e8ceb73 100644 --- a/include/linux/smp.h +++ b/include/linux/smp.h @@ -256,12 +256,14 @@ static inline int get_boot_cpu_id(void) */ #ifndef __smp_processor_id #define __smp_processor_id(x) raw_smp_processor_id(x) -#endif - +#else #ifdef CONFIG_DEBUG_PREEMPT extern unsigned int debug_smp_processor_id(void); # define smp_processor_id() debug_smp_processor_id() -#else +#endif +#endif + +#ifndef smp_processor_id # define smp_processor_id() __smp_processor_id() #endif -- Thanks and Regards Srikar Dronamraju