Re: INFO: task hung in __sb_start_write

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2018/06/16 4:40, Tetsuo Handa wrote:
> Hmm, there might be other locations calling percpu_rwsem_release() ?

There are other locations calling percpu_rwsem_release(), but quite few.

include/linux/fs.h:1494:#define __sb_writers_release(sb, lev)   \
include/linux/fs.h-1495-        percpu_rwsem_release(&(sb)->s_writers.rw_sem[(lev)-1], 1, _THIS_IP_)

fs/btrfs/transaction.c:1821:            __sb_writers_release(fs_info->sb, SB_FREEZE_FS);
fs/aio.c:1566:                  __sb_writers_release(file_inode(file)->i_sb, SB_FREEZE_WRITE);
fs/xfs/xfs_aops.c:211:  __sb_writers_release(ioend->io_inode->i_sb, SB_FREEZE_FS);



I'd like to check what atomic_long_read(&sem->rw_sem.count) says
when hung task is reported.

Dmitry, if you succeed to reproduce khungtaskd messages, can you try this patch?

---
 include/linux/sched.h         |  1 +
 kernel/hung_task.c            | 25 +++++++++++++++++++++++++
 kernel/locking/percpu-rwsem.c |  2 ++
 3 files changed, 28 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 87bf02d..af95251 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1179,6 +1179,7 @@ struct task_struct {
 	/* Used by LSM modules for access restriction: */
 	void				*security;
 #endif
+	struct percpu_rw_semaphore	*pcpu_rwsem_read_waiting;
 
 	/*
 	 * New fields for task_struct should be added above here, so that
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 32b4794..409cc82 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -18,6 +18,7 @@
 #include <linux/utsname.h>
 #include <linux/sched/signal.h>
 #include <linux/sched/debug.h>
+#include <linux/percpu-rwsem.h>
 
 #include <trace/events/sched.h>
 
@@ -77,6 +78,29 @@ static int __init hung_task_panic_setup(char *str)
 	.notifier_call = hung_task_panic,
 };
 
+static void dump_percpu_rwsem_state(struct percpu_rw_semaphore *sem)
+{
+	unsigned int sum = 0;
+	int cpu;
+
+	if (!sem)
+		return;
+	pr_info("percpu_rw_semaphore(%p)\n", sem);
+	pr_info("  ->rw_sem.count=0x%lx\n",
+		atomic_long_read(&sem->rw_sem.count));
+	pr_info("  ->rss.gp_state=%d\n", sem->rss.gp_state);
+	pr_info("  ->rss.gp_count=%d\n", sem->rss.gp_count);
+	pr_info("  ->rss.cb_state=%d\n", sem->rss.cb_state);
+	pr_info("  ->rss.gp_type=%d\n", sem->rss.gp_type);
+	pr_info("  ->readers_block=%d\n", sem->readers_block);
+	for_each_possible_cpu(cpu)
+		sum += per_cpu(*sem->read_count, cpu);
+	pr_info("  ->read_count=%d\n", sum);
+	pr_info("  ->list_empty(rw_sem.wait_list)=%d\n",
+	       list_empty(&sem->rw_sem.wait_list));
+	pr_info("  ->writer.task=%p\n", sem->writer.task);
+}
+
 static void check_hung_task(struct task_struct *t, unsigned long timeout)
 {
 	unsigned long switch_count = t->nvcsw + t->nivcsw;
@@ -122,6 +146,7 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 		pr_err("\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
 			" disables this message.\n");
 		sched_show_task(t);
+		dump_percpu_rwsem_state(t->pcpu_rwsem_read_waiting);
 		hung_task_show_lock = true;
 	}
 
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 883cf1b..b3654ab 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -82,7 +82,9 @@ int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
 	/*
 	 * Avoid lockdep for the down/up_read() we already have them.
 	 */
+	current->pcpu_rwsem_read_waiting = sem;
 	__down_read(&sem->rw_sem);
+	current->pcpu_rwsem_read_waiting = NULL;
 	this_cpu_inc(*sem->read_count);
 	__up_read(&sem->rw_sem);
 
-- 
1.8.3.1