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Tue, 28 Sep 2021 21:41:53 +0000 (GMT) Received: from localhost (unknown [9.211.96.63]) by b03ledav003.gho.boulder.ibm.com (Postfix) with ESMTP; Tue, 28 Sep 2021 21:41:53 +0000 (GMT) From: Nathan Lynch To: linuxppc-dev@lists.ozlabs.org Subject: [PATCH v2 2/2] powerpc/paravirt: correct preempt debug splat in vcpu_is_preempted() Date: Tue, 28 Sep 2021 16:41:47 -0500 Message-Id: <20210928214147.312412-3-nathanl@linux.ibm.com> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210928214147.312412-1-nathanl@linux.ibm.com> References: <20210928214147.312412-1-nathanl@linux.ibm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: 6YaREQVzLdXagMRtAKrqPfxukHbL8J5O X-Proofpoint-ORIG-GUID: _nUFcaysbo0cHzHWgAeOAt1JiK8xy_qF 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=2021-09-28_10,2021-09-28_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 lowpriorityscore=0 spamscore=0 malwarescore=0 priorityscore=1501 mlxlogscore=867 impostorscore=0 mlxscore=0 phishscore=0 bulkscore=0 adultscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2109230001 definitions=main-2109280126 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: , Cc: srikar@linux.vnet.ibm.com, npiggin@gmail.com Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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 used speculatively, and the function does not access per-cpu resources in a (Linux) preempt-unsafe way. Use raw_smp_processor_id() to avoid such warnings, adding explanatory comments. Signed-off-by: Nathan Lynch Fixes: ca3f969dcb11 ("powerpc/paravirt: Use is_kvm_guest() in vcpu_is_preempted()") --- arch/powerpc/include/asm/paravirt.h | 18 +++++++++++++++++- 1 file changed, 17 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/include/asm/paravirt.h b/arch/powerpc/include/asm/paravirt.h index 39f173961f6a..eb7df559ae74 100644 --- a/arch/powerpc/include/asm/paravirt.h +++ b/arch/powerpc/include/asm/paravirt.h @@ -110,7 +110,23 @@ 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; + + /* + * The result of vcpu_is_preempted() is used in a + * speculative way, and is always subject to invalidation + * by events internal and external to Linux. While we can + * be called in preemptable context (in the Linux sense), + * we're not accessing per-cpu resources in a way that can + * race destructively with Linux scheduler preemption and + * migration, and callers can tolerate the potential for + * error introduced by sampling the CPU index without + * pinning the task to it. So it is permissible to use + * raw_smp_processor_id() here to defeat the preempt debug + * warnings that can arise from using smp_processor_id() + * in arbitrary contexts. + */ + first_cpu = cpu_first_thread_sibling(raw_smp_processor_id()); /* * The PowerVM hypervisor dispatches VMs on a whole core -- 2.31.1