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Sat, 08 Aug 2026 06:02:37 -0700 (PDT) From: ruipengqi To: akpm@linux-foundation.org, peterz@infradead.org, bigeasy@linutronix.de Cc: lance.yang@linux.dev, mhiramat@kernel.org, pmladek@suse.com, mingo@redhat.com, will@kernel.org, boqun@kernel.org, longman@redhat.com, clrkwllms@kernel.org, rostedt@goodmis.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, ruipengqi3@gmail.com, Zhan Xusheng Subject: [PATCH v2 1/2] hung_task: show the blocker task if the task is hung on rtmutex Date: Sat, 8 Aug 2026 21:02:17 +0800 Message-Id: <51d7411207693824ee515b62a31c2a3d76fdf0d4.1786193045.git.ruipengqi3@gmail.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Ruipeng Qi Currently, debug_show_blocker() only tracks mutex, semaphore and rwsem lock types. Extend it to also cover rtmutex or rt_mutex-based lock implementations on PREEMPT_RT, so that when a task is hung on an rtmutex, the hung task detector can identify and report which task holds the lock. Add rt_mutex_task_owner() to get the task's lock owner when a task is blocked by an rtmutex-based lock implementation. Since the LSBs of task->blocker pointer are already fully utilized and leave no room for new blocker types, this version avoids adding a new blocker type by directly leveraging rt_mutex_task_owner() to retrieve the lock owner information. With this change, the hung task detector can now show blocker task's info like below: [ 3000.899985] INFO: task cat:195 blocked for more than 120 seconds. [ 3000.900561] Not tainted 7.2.0-rc4-00366-gf339a6eb59f3-dirty #22 [ 3000.900930] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. [ 3000.901376] task:cat state:D stack:13784 pid:195 tgid:195 ppid:1 task_flags:0x400000 flags:0x00080000 [ 3000.902081] Call Trace: [ 3000.902233] [ 3000.902383] __schedule+0x514/0xf50 [ 3000.902677] rt_mutex_schedule+0x1b/0x30 [ 3000.902916] rt_mutex_slowlock_block.constprop.0+0x3b/0x1c0 [ 3000.903242] __rt_mutex_slowlock_locked.constprop.0+0xa8/0x200 [ 3000.903716] rt_mutex_slowlock.constprop.0+0x48/0xb0 [ 3000.904023] rt_mutex_lock+0x32/0x40 [ 3000.904249] read_dummy_rtmutex+0x2a/0x60 [hung_task_tests] [ 3000.904647] full_proxy_read+0x5b/0x90 [ 3000.904843] vfs_read+0xb0/0x370 [ 3000.905051] ? vm_mmap_pgoff+0xf1/0x1b0 [ 3000.905293] ? vm_mmap_pgoff+0x122/0x1b0 [ 3000.905684] ksys_read+0x68/0xe0 [ 3000.905980] do_syscall_64+0xf9/0x540 [ 3000.906213] entry_SYSCALL_64_after_hwframe+0x77/0x7f [ 3000.906643] RIP: 0033:0x49a182 [ 3000.906844] RSP: 002b:00007ffc431f6528 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [ 3000.907251] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000049a182 [ 3000.907673] RDX: 0000000000010000 RSI: 00007f69b2166000 RDI: 0000000000000003 [ 3000.908044] RBP: 00007f69b2166000 R08: 00000000ffffffff R09: 0000000000000000 [ 3000.908485] R10: 0000000000000022 R11: 0000000000000246 R12: 0000000000010000 [ 3000.908816] R13: 0000000000000003 R14: 0000000000000001 R15: 0000000001000000 [ 3000.909206] [ 3000.909351] INFO: task cat:195 is blocked on a rtmutex likely owned by task cat:194. [ 3000.909832] task:cat state:S stack:13784 pid:194 tgid:194 ppid:1 task_flags:0x400000 flags:0x00080000 [ 3000.910426] Call Trace: [ 3000.910618] [ 3000.910727] __schedule+0x514/0xf50 [ 3000.910912] schedule+0x22/0xa0 [ 3000.911064] schedule_timeout+0x81/0x100 [ 3000.911254] ? __pfx_process_timeout+0x10/0x10 [ 3000.911522] msleep_interruptible+0x28/0x50 [ 3000.911754] read_dummy_rtmutex+0x34/0x60 [hung_task_tests] [ 3000.912019] full_proxy_read+0x5b/0x90 [ 3000.912212] vfs_read+0xb0/0x370 [ 3000.912378] ? vm_mmap_pgoff+0xf1/0x1b0 [ 3000.912630] ? vm_mmap_pgoff+0x122/0x1b0 [ 3000.912839] ksys_read+0x68/0xe0 [ 3000.913007] do_syscall_64+0xf9/0x540 [ 3000.913187] entry_SYSCALL_64_after_hwframe+0x77/0x7f [ 3000.913461] RIP: 0033:0x49a182 [ 3000.913618] RSP: 002b:00007ffd0b4cf048 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 [ 3000.913972] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000049a182 [ 3000.914294] RDX: 0000000000010000 RSI: 00007fbf49a0d000 RDI: 0000000000000003 [ 3000.914648] RBP: 00007fbf49a0d000 R08: 00000000ffffffff R09: 0000000000000000 [ 3000.914955] R10: 0000000000000022 R11: 0000000000000246 R12: 0000000000010000 [ 3000.915208] R13: 0000000000000003 R14: 0000000000000001 R15: 0000000001000000 [ 3000.915439] Suggested-by: Petr Mladek Suggested-by: Zhan Xusheng Signed-off-by: Ruipeng Qi --- v2 changes: - Reduce #ifdef-ery by using rt_mutex_task_owner() instead of adding a new blocker type. - Remove the unused function debug_trace_blocker(). - Clean up and update relevant comments. --- include/linux/rtmutex.h | 2 ++ kernel/hung_task.c | 26 +++++++++++++++++++++++--- kernel/locking/rtmutex_api.c | 29 +++++++++++++++++++++++++++++ lib/Kconfig.debug | 1 - 4 files changed, 54 insertions(+), 4 deletions(-) diff --git a/include/linux/rtmutex.h b/include/linux/rtmutex.h index 9e1f012f89db89..c179f946d2d040 100644 --- a/include/linux/rtmutex.h +++ b/include/linux/rtmutex.h @@ -53,6 +53,8 @@ static inline struct task_struct *rt_mutex_owner(struct rt_mutex_base *lock) return (struct task_struct *) (owner & ~RT_MUTEX_HAS_WAITERS); } + +extern struct task_struct *rt_mutex_task_owner(struct task_struct *task); #endif extern void rt_mutex_base_init(struct rt_mutex_base *rtb); diff --git a/kernel/hung_task.c b/kernel/hung_task.c index 6fcc94ce4ca9d2..9467011f878c95 100644 --- a/kernel/hung_task.c +++ b/kernel/hung_task.c @@ -27,6 +27,7 @@ #include #include +#include /* * The number of tasks checked: @@ -139,9 +140,19 @@ static void debug_show_blocker(struct task_struct *task, unsigned long timeout) struct task_struct *g, *t; unsigned long owner, blocker, blocker_type; const char *rwsem_blocked_by, *rwsem_blocked_as; + bool is_rtmutex = false; RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "No rcu lock held"); +#if defined(CONFIG_RT_MUTEXES) + owner = (unsigned long)rt_mutex_task_owner(task); + if (owner) { + is_rtmutex = true; + blocker_type = -1; + goto found; + } +#endif + blocker = READ_ONCE(task->blocker); if (!blocker) return; @@ -149,12 +160,13 @@ static void debug_show_blocker(struct task_struct *task, unsigned long timeout) blocker_type = hung_task_get_blocker_type(blocker); switch (blocker_type) { - case BLOCKER_TYPE_MUTEX: - owner = mutex_get_owner(hung_task_blocker_to_lock(blocker)); - break; case BLOCKER_TYPE_SEM: owner = sem_last_holder(hung_task_blocker_to_lock(blocker)); break; +#ifndef CONFIG_PREEMPT_RT + case BLOCKER_TYPE_MUTEX: + owner = mutex_get_owner(hung_task_blocker_to_lock(blocker)); + break; case BLOCKER_TYPE_RWSEM_READER: case BLOCKER_TYPE_RWSEM_WRITER: owner = (unsigned long)rwsem_owner( @@ -165,6 +177,7 @@ static void debug_show_blocker(struct task_struct *task, unsigned long timeout) hung_task_blocker_to_lock(blocker)) ? "reader" : "writer"; break; +#endif default: WARN_ON_ONCE(1); return; @@ -190,6 +203,7 @@ static void debug_show_blocker(struct task_struct *task, unsigned long timeout) return; } +found: /* Ensure the owner information is correct. */ for_each_process_thread(g, t) { if ((unsigned long)t != owner) @@ -210,6 +224,12 @@ static void debug_show_blocker(struct task_struct *task, unsigned long timeout) task->comm, task->pid, rwsem_blocked_as, t->comm, t->pid, rwsem_blocked_by); break; + default: + if (is_rtmutex) { + pr_err("INFO: task %s:%d is blocked on a rtmutex likely owned by task %s:%d.\n", + task->comm, task->pid, t->comm, t->pid); + } + break; } /* Avoid duplicated task dump, skip if the task is also hung. */ if (!task_is_hung(t, timeout)) diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c index 5d48d64725b125..d0826b31a148c0 100644 --- a/kernel/locking/rtmutex_api.c +++ b/kernel/locking/rtmutex_api.c @@ -530,6 +530,35 @@ void __sched rt_mutex_postunlock(struct rt_wake_q_head *wqh) rt_mutex_wake_up_q(wqh); } +/** + * rt_mutex_task_owner - Get the lock owner a task is blocked on + * @task: Task to inspect. + * + * Caller must hold an RCU read lock. The owner value is a snapshot read + * without exclusion, so it may become stale immediately. Use only for + * best-effort diagnostics (e.g., hung task detection). + * + * Return: The task_struct owning the lock, or NULL if not blocked. + */ +struct task_struct *rt_mutex_task_owner(struct task_struct *task) +{ + struct task_struct *owner = NULL; + struct rt_mutex_base *lock; + unsigned long flags; + + lockdep_assert(rcu_read_lock_held()); + + raw_spin_lock_irqsave(&task->pi_lock, flags); + lock = task_blocked_on_lock(task); + + if (lock) + owner = rt_mutex_owner(lock); + raw_spin_unlock_irqrestore(&task->pi_lock, flags); + + return owner; +} +EXPORT_SYMBOL(rt_mutex_task_owner); + #ifdef CONFIG_DEBUG_RT_MUTEXES void rt_mutex_debug_task_free(struct task_struct *task) { diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 1244dcac2294ad..92dff380027e78 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -1317,7 +1317,6 @@ config BOOTPARAM_HUNG_TASK_PANIC config DETECT_HUNG_TASK_BLOCKER bool "Dump Hung Tasks Blocker" depends on DETECT_HUNG_TASK - depends on !PREEMPT_RT default y help Say Y here to show the blocker task's stacktrace who acquires -- 2.25.1