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b=ZlXgc5A6VXqi0yE8Clplx3zXOmH0VDx1JgSvWRxA8tP7R0e6cqOS2oBPu0RgpizmObraFiphQWkmjAQ9ZFsBJQrrrztFDys8DmlUKk8U44tAOkOXHDcr+j7JgUgK5X7UiBJ8830zjrFMG/GI5ct3YMOTZM/zJLroVsMAnNfewFo= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=amd.com; Received: from PH7PR12MB6588.namprd12.prod.outlook.com (2603:10b6:510:210::10) by PH7PR12MB6787.namprd12.prod.outlook.com (2603:10b6:510:1ad::15) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8272.13; Thu, 19 Dec 2024 09:33:49 +0000 Received: from PH7PR12MB6588.namprd12.prod.outlook.com ([fe80::5e9c:4117:b5e0:cf39]) by PH7PR12MB6588.namprd12.prod.outlook.com ([fe80::5e9c:4117:b5e0:cf39%5]) with mapi id 15.20.8272.005; Thu, 19 Dec 2024 09:33:49 +0000 Message-ID: <8c31f7bd-871d-4a38-ad15-a16a116e1f39@amd.com> Date: Thu, 19 Dec 2024 15:03:40 +0530 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 19/19] perf: Make perf_pmu_unregister() useable To: Peter Zijlstra Cc: "mingo@kernel.org" , "lucas.demarchi@intel.com" , "linux-kernel@vger.kernel.org" , "willy@infradead.org" , "acme@kernel.org" , "namhyung@kernel.org" , "mark.rutland@arm.com" , "alexander.shishkin@linux.intel.com" , "jolsa@kernel.org" , "irogers@google.com" , "adrian.hunter@intel.com" , "kan.liang@linux.intel.com" , Ravi Bangoria References: <20241104133909.669111662@infradead.org> <20241104135519.715883982@infradead.org> <20241217091216.GK35539@noisy.programming.kicks-ass.net> <20241217115219.GH12500@noisy.programming.kicks-ass.net> Content-Language: en-US From: Ravi Bangoria In-Reply-To: <20241217115219.GH12500@noisy.programming.kicks-ass.net> Content-Type: text/plain; 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Thanks! > > Bah, that's a right mess indeed, however did I miss all that. > > The easiest solution is probably to leave the RB around on detach, but > now I need to remember why I did that in the first place :/ > > Oh.. I think I mostly that to serialize against perf_mmap(), which > should reject creating further maps. But we can do that without actually > detaching the RB -- we only need to acquire and release mmap_mutex. > > Ah, there's that perf_event_stop() inside of ring_buffer_attach(), that > must not happen after detach, obviously. So that must be dealt with. > > Hmm, also if we leave ->rb around, then we need to deal with > perf_event_set_output(), someone could try and redirect their things > into our buffer -- which isn't technically broken, but still weird. > > Something like the below. > > How did you test; perf-fuzzer or something? Prepared a simple test that does pmu register(), unregister() and "perf record" in parallel. It's quite dirty, I'll clean it up and share it here. > --- a/include/linux/perf_event.h > +++ b/include/linux/perf_event.h > @@ -1742,7 +1742,7 @@ static inline bool needs_branch_stack(st > > static inline bool has_aux(struct perf_event *event) > { > - return event->pmu->setup_aux; > + return event->pmu && event->pmu->setup_aux; > } > > static inline bool has_aux_action(struct perf_event *event) > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -5409,7 +5409,6 @@ static void _free_event(struct perf_even > security_perf_event_free(event); > > if (event->rb) { > - WARN_ON_ONCE(!event->pmu); > /* > * Can happen when we close an event with re-directed output. > * > @@ -12023,7 +12022,10 @@ static void __pmu_detach_event(struct pm > */ > scoped_guard (mutex, &event->mmap_mutex) { > WARN_ON_ONCE(pmu->event_unmapped); > - ring_buffer_attach(event, NULL); > + /* > + * Mostly an empy lock sequence, such that perf_mmap(), which > + * relies on mmap_mutex, is sure to observe the state change. > + */ > } > > perf_event_free_bpf_prog(event); > @@ -12823,6 +12825,9 @@ perf_event_set_output(struct perf_event > goto unlock; > > if (output_event) { > + if (output_event->state <= PERF_EVENT_STATE_REVOKED) > + goto unlock; > + > /* get the rb we want to redirect to */ > rb = ring_buffer_get(output_event); > if (!rb) I needed this additional diff on top of your change. With this, it survives my test. perf_mmap_close() change seems correct. Not sure about perf_mmap(). I'll inspect the code further. --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -6540,7 +6540,7 @@ static void perf_mmap_close(struct vm_area_struct *vma) bool detach_rest = false; /* FIXIES vs perf_pmu_unregister() */ - if (event->pmu->event_unmapped) + if (event->pmu && event->pmu->event_unmapped) event->pmu->event_unmapped(event, vma->vm_mm); /* @@ -6873,7 +6873,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma) vm_flags_set(vma, VM_DONTCOPY | VM_DONTEXPAND | VM_DONTDUMP); vma->vm_ops = &perf_mmap_vmops; - if (!ret && event->pmu->event_mapped) + if (!ret && event->pmu && event->pmu->event_mapped) event->pmu->event_mapped(event, vma->vm_mm); return ret; Thanks, Ravi