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Message-ID: <20250612042005.99602-1-lance.yang@linux.dev> Date: Thu, 12 Jun 2025 12:19:23 +0800 From: Lance Yang <ioworker0@...il.com> To: akpm@...ux-foundation.org Cc: zi.li@...ux.dev, anna.schumaker@...cle.com, boqun.feng@...il.com, joel.granados@...nel.org, jstultz@...gle.com, kent.overstreet@...ux.dev, leonylgao@...cent.com, linux-kernel@...r.kernel.org, longman@...hat.com, mhiramat@...nel.org, mingo@...hat.com, mingzhe.yang@...com, peterz@...radead.org, rostedt@...dmis.org, senozhatsky@...omium.org, tfiga@...omium.org, will@...nel.org Subject: [PATCH RFC 0/3] extend hung task blocker tracking to rwsems Hi all, Inspired by mutex blocker tracking[1], and having already extended it to semaphores, let's now add support for reader-writer semaphores (rwsems). The approach is simple: when a task enters TASK_UNINTERRUPTIBLE while waiting for an rwsem, we just call hung_task_set_blocker(). The hung task detector can then query the rwsem's owner to identify the lock holder. Tracking works reliably for writers, as there can only be a single writer holding the lock, and its task struct is stored in the owner field. The main challenge lies with readers. The owner field points to only one of many concurrent readers, so we might lose track of the blocker if that specific reader unlocks, even while others remain. This is not a significant issue, however. In practice, long-lasting lock contention is almost always caused by a writer. Therefore, reliably tracking the writer is the primary goal of this patch series ;) With this change, the hung task detector can now show blocker task's info like below: [Thu Jun 12 11:01:33 2025] INFO: task rw_sem_thread2:36526 blocked for more than 122 seconds. [Thu Jun 12 11:01:33 2025] Tainted: G S O 6.16.0-rc1 #1 [Thu Jun 12 11:01:33 2025] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [Thu Jun 12 11:01:33 2025] task:rw_sem_thread2 state:D stack:0 pid:36526 tgid:36526 ppid:2 task_flags:0x208040 flags:0x00004000 [Thu Jun 12 11:01:33 2025] Call Trace: [Thu Jun 12 11:01:33 2025] <TASK> [Thu Jun 12 11:01:33 2025] __schedule+0x7c7/0x1930 [Thu Jun 12 11:01:33 2025] ? __pfx___schedule+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? _raw_spin_lock_irq+0x8a/0xe0 [Thu Jun 12 11:01:33 2025] ? __pfx__raw_spin_lock_irq+0x10/0x10 [Thu Jun 12 11:01:33 2025] schedule+0x6a/0x180 [Thu Jun 12 11:01:33 2025] schedule_preempt_disabled+0x15/0x30 [Thu Jun 12 11:01:33 2025] rwsem_down_write_slowpath+0x447/0x1090 [Thu Jun 12 11:01:33 2025] ? __pfx_rwsem_down_write_slowpath+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx___schedule+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx___might_resched+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx_thread2_func+0x10/0x10 [rw_sem_test_2] [Thu Jun 12 11:01:33 2025] down_write+0x125/0x140 [Thu Jun 12 11:01:33 2025] ? __pfx_down_write+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? msleep+0x91/0xf0 [Thu Jun 12 11:01:33 2025] ? __raw_spin_lock_irqsave+0x8c/0xf0 [Thu Jun 12 11:01:33 2025] thread2_func+0x37/0x70 [rw_sem_test_2] [Thu Jun 12 11:01:33 2025] kthread+0x39f/0x750 [Thu Jun 12 11:01:33 2025] ? __pfx_kthread+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx__raw_spin_lock_irq+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx_kthread+0x10/0x10 [Thu Jun 12 11:01:33 2025] ret_from_fork+0x25d/0x320 [Thu Jun 12 11:01:33 2025] ? __pfx_kthread+0x10/0x10 [Thu Jun 12 11:01:33 2025] ret_from_fork_asm+0x1a/0x30 [Thu Jun 12 11:01:33 2025] </TASK> [Thu Jun 12 11:01:33 2025] INFO: task rw_sem_thread2:36526 <writer> blocked on an rw-semaphore likely owned by task rw_sem_thread1:36525 <writer> [Thu Jun 12 11:01:33 2025] task:rw_sem_thread1 state:S stack:0 pid:36525 tgid:36525 ppid:2 task_flags:0x208040 flags:0x00004000 [Thu Jun 12 11:01:33 2025] Call Trace: [Thu Jun 12 11:01:33 2025] <TASK> [Thu Jun 12 11:01:33 2025] __schedule+0x7c7/0x1930 [Thu Jun 12 11:01:33 2025] ? __pfx___schedule+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __mod_timer+0x304/0xa80 [Thu Jun 12 11:01:33 2025] ? irq_work_queue+0x6a/0xa0 [Thu Jun 12 11:01:33 2025] ? __pfx_vprintk_emit+0x10/0x10 [Thu Jun 12 11:01:33 2025] schedule+0x6a/0x180 [Thu Jun 12 11:01:33 2025] schedule_timeout+0xfb/0x230 [Thu Jun 12 11:01:33 2025] ? __pfx_schedule_timeout+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx_process_timeout+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? down_write+0xc4/0x140 [Thu Jun 12 11:01:33 2025] msleep_interruptible+0xbe/0x150 [Thu Jun 12 11:01:33 2025] ? __pfx_thread1_func+0x10/0x10 [rw_sem_test_2] [Thu Jun 12 11:01:33 2025] thread1_func+0x37/0x60 [rw_sem_test_2] [Thu Jun 12 11:01:33 2025] kthread+0x39f/0x750 [Thu Jun 12 11:01:33 2025] ? __pfx_kthread+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx__raw_spin_lock_irq+0x10/0x10 [Thu Jun 12 11:01:33 2025] ? __pfx_kthread+0x10/0x10 [Thu Jun 12 11:01:33 2025] ret_from_fork+0x25d/0x320 [Thu Jun 12 11:01:33 2025] ? __pfx_kthread+0x10/0x10 [Thu Jun 12 11:01:33 2025] ret_from_fork_asm+0x1a/0x30 [Thu Jun 12 11:01:33 2025] </TASK> [1] https://lore.kernel.org/all/174046694331.2194069.15472952050240807469.stgit@mhiramat.tok.corp.google.com/ Thanks, Lance Lance Yang (3): locking/rwsem: make owner helpers globally available locking/rwsem: clear reader-owner on unlock to reduce false positives hung_task: extend hung task blocker tracking to rwsems include/linux/hung_task.h | 18 +++++++++--------- include/linux/rwsem.h | 12 ++++++++++++ kernel/hung_task.c | 29 +++++++++++++++++++++++++---- kernel/locking/rwsem.c | 31 +++++++++++++++++++++++-------- 4 files changed, 69 insertions(+), 21 deletions(-) -- 2.49.0
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