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Message-ID: <20250612042005.99602-4-lance.yang@linux.dev>
Date: Thu, 12 Jun 2025 12:19:26 +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,
Lance Yang <lance.yang@...ux.dev>
Subject: [PATCH RFC 3/3] hung_task: extend hung task blocker tracking to rwsems
From: Lance Yang <lance.yang@...ux.dev>
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/
Suggested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Signed-off-by: Lance Yang <lance.yang@...ux.dev>
---
include/linux/hung_task.h | 18 +++++++++---------
kernel/hung_task.c | 29 +++++++++++++++++++++++++----
kernel/locking/rwsem.c | 17 ++++++++++++++++-
3 files changed, 50 insertions(+), 14 deletions(-)
diff --git a/include/linux/hung_task.h b/include/linux/hung_task.h
index 1bc2b3244613..34e615c76ca5 100644
--- a/include/linux/hung_task.h
+++ b/include/linux/hung_task.h
@@ -21,17 +21,17 @@
* type.
*
* Type encoding:
- * 00 - Blocked on mutex (BLOCKER_TYPE_MUTEX)
- * 01 - Blocked on semaphore (BLOCKER_TYPE_SEM)
- * 10 - Blocked on rt-mutex (BLOCKER_TYPE_RTMUTEX)
- * 11 - Blocked on rw-semaphore (BLOCKER_TYPE_RWSEM)
+ * 00 - Blocked on mutex (BLOCKER_TYPE_MUTEX)
+ * 01 - Blocked on semaphore (BLOCKER_TYPE_SEM)
+ * 10 - Blocked on rw-semaphore as READER (BLOCKER_TYPE_RWSEM_READER)
+ * 11 - Blocked on rw-semaphore as WRITER (BLOCKER_TYPE_RWSEM_WRITER)
*/
-#define BLOCKER_TYPE_MUTEX 0x00UL
-#define BLOCKER_TYPE_SEM 0x01UL
-#define BLOCKER_TYPE_RTMUTEX 0x02UL
-#define BLOCKER_TYPE_RWSEM 0x03UL
+#define BLOCKER_TYPE_MUTEX 0x00UL
+#define BLOCKER_TYPE_SEM 0x01UL
+#define BLOCKER_TYPE_RWSEM_READER 0x02UL
+#define BLOCKER_TYPE_RWSEM_WRITER 0x03UL
-#define BLOCKER_TYPE_MASK 0x03UL
+#define BLOCKER_TYPE_MASK 0x03UL
#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
static inline void hung_task_set_blocker(void *lock, unsigned long type)
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index d2432df2b905..8708a1205f82 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -23,6 +23,7 @@
#include <linux/sched/debug.h>
#include <linux/sched/sysctl.h>
#include <linux/hung_task.h>
+#include <linux/rwsem.h>
#include <trace/events/sched.h>
@@ -100,6 +101,7 @@ static void debug_show_blocker(struct task_struct *task)
{
struct task_struct *g, *t;
unsigned long owner, blocker, blocker_type;
+ const char *rwsem_blocked_by, *rwsem_blocked_as;
RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "No rcu lock held");
@@ -111,12 +113,20 @@ static void debug_show_blocker(struct task_struct *task)
switch (blocker_type) {
case BLOCKER_TYPE_MUTEX:
- owner = mutex_get_owner(
- (struct mutex *)hung_task_blocker_to_lock(blocker));
+ owner = mutex_get_owner(hung_task_blocker_to_lock(blocker));
break;
case BLOCKER_TYPE_SEM:
- owner = sem_last_holder(
- (struct semaphore *)hung_task_blocker_to_lock(blocker));
+ owner = sem_last_holder(hung_task_blocker_to_lock(blocker));
+ break;
+ case BLOCKER_TYPE_RWSEM_READER:
+ case BLOCKER_TYPE_RWSEM_WRITER:
+ owner = (unsigned long)rwsem_owner(
+ hung_task_blocker_to_lock(blocker));
+ rwsem_blocked_as = (blocker_type == BLOCKER_TYPE_RWSEM_READER) ?
+ "reader" : "writer";
+ rwsem_blocked_by = is_rwsem_reader_owned(
+ hung_task_blocker_to_lock(blocker)) ?
+ "reader" : "writer";
break;
default:
WARN_ON_ONCE(1);
@@ -134,6 +144,11 @@ static void debug_show_blocker(struct task_struct *task)
pr_err("INFO: task %s:%d is blocked on a semaphore, but the last holder is not found.\n",
task->comm, task->pid);
break;
+ case BLOCKER_TYPE_RWSEM_READER:
+ case BLOCKER_TYPE_RWSEM_WRITER:
+ pr_err("INFO: task %s:%d is blocked on an rw-semaphore, but the owner is not found.\n",
+ task->comm, task->pid);
+ break;
}
return;
}
@@ -152,6 +167,12 @@ static void debug_show_blocker(struct task_struct *task)
pr_err("INFO: task %s:%d blocked on a semaphore likely last held by task %s:%d\n",
task->comm, task->pid, t->comm, t->pid);
break;
+ case BLOCKER_TYPE_RWSEM_READER:
+ case BLOCKER_TYPE_RWSEM_WRITER:
+ pr_err("INFO: task %s:%d <%s> blocked on an rw-semaphore likely owned by task %s:%d <%s>\n",
+ task->comm, task->pid, rwsem_blocked_as, t->comm,
+ t->pid, rwsem_blocked_by);
+ break;
}
sched_show_task(t);
return;
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index a310eb9896de..92c6332da401 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -27,6 +27,7 @@
#include <linux/export.h>
#include <linux/rwsem.h>
#include <linux/atomic.h>
+#include <linux/hung_task.h>
#include <trace/events/lock.h>
#ifndef CONFIG_PREEMPT_RT
@@ -1065,10 +1066,13 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
wake_up_q(&wake_q);
trace_contention_begin(sem, LCB_F_READ);
+ set_current_state(state);
+
+ if (state == TASK_UNINTERRUPTIBLE)
+ hung_task_set_blocker(sem, BLOCKER_TYPE_RWSEM_READER);
/* wait to be given the lock */
for (;;) {
- set_current_state(state);
if (!smp_load_acquire(&waiter.task)) {
/* Matches rwsem_mark_wake()'s smp_store_release(). */
break;
@@ -1083,8 +1087,12 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
}
schedule_preempt_disabled();
lockevent_inc(rwsem_sleep_reader);
+ set_current_state(state);
}
+ if (state == TASK_UNINTERRUPTIBLE)
+ hung_task_clear_blocker();
+
__set_current_state(TASK_RUNNING);
lockevent_inc(rwsem_rlock);
trace_contention_end(sem, 0);
@@ -1146,6 +1154,9 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
set_current_state(state);
trace_contention_begin(sem, LCB_F_WRITE);
+ if (state == TASK_UNINTERRUPTIBLE)
+ hung_task_set_blocker(sem, BLOCKER_TYPE_RWSEM_WRITER);
+
for (;;) {
if (rwsem_try_write_lock(sem, &waiter)) {
/* rwsem_try_write_lock() implies ACQUIRE on success */
@@ -1179,6 +1190,10 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
trylock_again:
raw_spin_lock_irq(&sem->wait_lock);
}
+
+ if (state == TASK_UNINTERRUPTIBLE)
+ hung_task_clear_blocker();
+
__set_current_state(TASK_RUNNING);
raw_spin_unlock_irq(&sem->wait_lock);
lockevent_inc(rwsem_wlock);
--
2.49.0
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