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Mon, 09 Mar 2026 20:37:54 -0700 (PDT) From: Qing Wang To: irogers@google.com Cc: acme@kernel.org, adrian.hunter@intel.com, alexander.shishkin@linux.intel.com, james.clark@linaro.org, jolsa@kernel.org, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, mark.rutland@arm.com, mingo@redhat.com, namhyung@kernel.org, peterz@infradead.org, syzbot+196a82fd904572696b3c@syzkaller.appspotmail.com, wangqing7171@gmail.com, yuhaocheng035@gmail.com Subject: Re: [PATCH] perf: Fix deadlock in perf_mmap() Date: Tue, 10 Mar 2026 11:37:42 +0800 Message-Id: <20260310033742.3606390-1-wangqing7171@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Tue, 10 Mar 2026 at 02:59, Ian Rogers wrote: > Hi Qing, thank you for looking into this. I proposed a similar fix: > https://lore.kernel.org/lkml/CAP-5=fW-wHEv=TCULwk_HVOhWHdqRd8AZoESZsU_vnhLjghUBQ@mail.gmail.com/ > but Haocheng noted it reintroduced the race condition the original fix > was targeting. Haocheng has a larger fix in: > https://lore.kernel.org/lkml/20260306093616.84299-1-yuhaocheng035@gmail.com/ > that should handle both the original race condition and the deadlock. Oh, this fix is too large. > I wonder that fix may be made smaller by passing a parameter like > "holds_event_mmap_mutex" to perf_mmap_close, something like: > ``` > static void __perf_mmap_close(struct vm_area_struct *vma, struct > perf_event *event, bool holds_event_mmap_lock) > { > ... // code from original perf_mmap_close > if ((!holds_event_mmap_lock && > !refcount_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex)) > || > (holds_event_mmap_lock && !refcount_dec_and_test(&event->mmap_count))) > ... > > static void perf_mmap_close(struct vm_area_struct *vma) > { > struct perf_event *event = vma->vm_file->private_data; > __perf_mmap_close(vma, event, /*holds_event_mmap_lock=*/false); > } > > static void perf_mmap_close_locked(struct vm_area_struct *vma, struct > perf_event *event) > { > __perf_mmap_close(vma, event, /*holds_event_mmap_lock=*/true); > } > ``` > > Thanks, > Ian LGTM, your fix is better. But, can you explain why there is still race issue after applying my patch? Thanks, Qing