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[72.235.129.167]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1ff5917496fsm72278125ad.182.2024.08.05.12.48.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Aug 2024 12:48:01 -0700 (PDT) Sender: Tejun Heo Date: Mon, 5 Aug 2024 09:48:00 -1000 From: Tejun Heo To: Jens Axboe Cc: syzbot , asml.silence@gmail.com, io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, Lai Jiangshan Subject: [PATCH wq/for-6.11-fixes] workqueue: Fix spruious data race in __flush_work() Message-ID: References: <000000000000ae429e061eea2157@google.com> <8ada52ac-48e9-48cd-afa0-c738cf25fe4f@kernel.dk> Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <8ada52ac-48e9-48cd-afa0-c738cf25fe4f@kernel.dk> From c5eac4d10384f1ddd728e143ede35eaa6081c61e Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Mon, 5 Aug 2024 09:37:25 -1000 When flushing a work item for cancellation, __flush_work() knows that it exclusively owns the work item through its PENDING bit. 134874e2eee9 ("workqueue: Allow cancel_work_sync() and disable_work() from atomic contexts on BH work items") added a read of @work->data to determine whether to use busy wait for BH work items that are being canceled. While the read is safe when @from_cancel, @work->data was read before testing @from_cancel to simplify code structure: data = *work_data_bits(work); if (from_cancel && !WARN_ON_ONCE(data & WORK_STRUCT_PWQ) && (data & WORK_OFFQ_BH)) { While the read data was never used if !@from_cancel, this could trigger KCSAN data race detection spuriously: ================================================================== BUG: KCSAN: data-race in __flush_work / __flush_work write to 0xffff8881223aa3e8 of 8 bytes by task 3998 on cpu 0: instrument_write include/linux/instrumented.h:41 [inline] ___set_bit include/asm-generic/bitops/instrumented-non-atomic.h:28 [inline] insert_wq_barrier kernel/workqueue.c:3790 [inline] start_flush_work kernel/workqueue.c:4142 [inline] __flush_work+0x30b/0x570 kernel/workqueue.c:4178 flush_work kernel/workqueue.c:4229 [inline] ... read to 0xffff8881223aa3e8 of 8 bytes by task 50 on cpu 1: __flush_work+0x42a/0x570 kernel/workqueue.c:4188 flush_work kernel/workqueue.c:4229 [inline] flush_delayed_work+0x66/0x70 kernel/workqueue.c:4251 ... value changed: 0x0000000000400000 -> 0xffff88810006c00d Reorganize the code so that @from_cancel is tested before @work->data is accessed. The only problem is triggering KCSAN detection spuriously. This shouldn't need READ_ONCE() or other access qualifiers. No functional changes. Signed-off-by: Tejun Heo Reported-by: syzbot+b3e4f2f51ed645fd5df2@syzkaller.appspotmail.com Fixes: 134874e2eee9 ("workqueue: Allow cancel_work_sync() and disable_work() from atomic contexts on BH work items") Link: http://lkml.kernel.org/r/000000000000ae429e061eea2157@google.com Cc: Jens Axboe --- Applied to wq/for-6.11-fixes. Thanks. kernel/workqueue.c | 45 +++++++++++++++++++++++++-------------------- 1 file changed, 25 insertions(+), 20 deletions(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index d56bd2277e58..ef174d8c1f63 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -4166,7 +4166,6 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr, static bool __flush_work(struct work_struct *work, bool from_cancel) { struct wq_barrier barr; - unsigned long data; if (WARN_ON(!wq_online)) return false; @@ -4184,29 +4183,35 @@ static bool __flush_work(struct work_struct *work, bool from_cancel) * was queued on a BH workqueue, we also know that it was running in the * BH context and thus can be busy-waited. */ - data = *work_data_bits(work); - if (from_cancel && - !WARN_ON_ONCE(data & WORK_STRUCT_PWQ) && (data & WORK_OFFQ_BH)) { - /* - * On RT, prevent a live lock when %current preempted soft - * interrupt processing or prevents ksoftirqd from running by - * keeping flipping BH. If the BH work item runs on a different - * CPU then this has no effect other than doing the BH - * disable/enable dance for nothing. This is copied from - * kernel/softirq.c::tasklet_unlock_spin_wait(). - */ - while (!try_wait_for_completion(&barr.done)) { - if (IS_ENABLED(CONFIG_PREEMPT_RT)) { - local_bh_disable(); - local_bh_enable(); - } else { - cpu_relax(); + if (from_cancel) { + unsigned long data = *work_data_bits(work); + + if (!WARN_ON_ONCE(data & WORK_STRUCT_PWQ) && + (data & WORK_OFFQ_BH)) { + /* + * On RT, prevent a live lock when %current preempted + * soft interrupt processing or prevents ksoftirqd from + * running by keeping flipping BH. If the BH work item + * runs on a different CPU then this has no effect other + * than doing the BH disable/enable dance for nothing. + * This is copied from + * kernel/softirq.c::tasklet_unlock_spin_wait(). + */ + while (!try_wait_for_completion(&barr.done)) { + if (IS_ENABLED(CONFIG_PREEMPT_RT)) { + local_bh_disable(); + local_bh_enable(); + } else { + cpu_relax(); + } } + goto out_destroy; } - } else { - wait_for_completion(&barr.done); } + wait_for_completion(&barr.done); + +out_destroy: destroy_work_on_stack(&barr.work); return true; }