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From:   Yu Chen <chen.yu@easystack.cn>
To:     minyard@acm.org
Cc:     openipmi-developer@lists.sourceforge.net,
        linux-kernel@vger.kernel.org, chen.yu@easystack.cn
Subject: [PATCH] watchdog: Avoid 100% CPU usage during reading watchdog when a task get signal
Date:   Mon, 15 May 2023 05:19:41 -0700
Message-Id: <20230515121941.59859-1-chen.yu@easystack.cn>
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A simple reproducer demonstrating the problem: (use ipmi_watchdog.ko)

In one terminal:

$ cat /dev/watchdog
...

In another terminal:

$ ps -aux | grep cat
14755 pts/1    R+    43:00 cat /dev/watchdog
51943 pts/2    S+     0:00 grep --color=auto cat

$ kill -9 14755
$
$ cat /proc/14755/status | grep SigPnd
SigPnd: 0000000000000100
$
$ top

Tasks: 1049 total,   2 running, 1047 sleeping,   0 stopped,   0 zombie
%Cpu(s): 0.0 us, 1.0 sy, 0.0 ni, 98.9 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
MiB Mem : 522594.8 total, 517241.4 free,  2922.1 used,   2431.2 buff/cache
MiB Swap:      0.0 total,      0.0 free,     0.0 used. 516589.2 avail Mem

PID USER    PR  NI    VIRT    RES    SHR S  %CPU  %MEM  TIME+ COMMAND
14755 root  20   0  215552   1024    576 R 100.0  0.0  0:15.12 cat
53417 root  20   0  224960   7040   3648 R   0.7  0.0  0:00.10 top
11 root     20   0       0      0      0 I   0.3  0.0  0:02.85 rcu_sched
1772 root   20   0  512256 387776 380800 S   0.3  0.1  0:32.05 python

We can see that when the cat process gets the signal, the CPU usage
is 100%, Since signal_pending is true, the pick_next_task function
in schedule always returns itself, it retries schedule indefinitely.
ipmi_read() will busyloop.

Signed-off-by: Yu Chen <chen.yu@easystack.cn>
---
 drivers/char/ipmi/ipmi_watchdog.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/char/ipmi/ipmi_watchdog.c b/drivers/char/ipmi/ipmi_watchdog.c
index 0d4a8dcac..173ed4266 100644
--- a/drivers/char/ipmi/ipmi_watchdog.c
+++ b/drivers/char/ipmi/ipmi_watchdog.c
@@ -803,6 +803,11 @@ static ssize_t ipmi_read(struct file *file,
 		init_waitqueue_entry(&wait, current);
 		add_wait_queue(&read_q, &wait);
 		while (!data_to_read) {
+			if (signal_pending(current)) {
+				remove_wait_queue(&read_q, &wait);
+				rv = -ERESTARTSYS;
+				goto out;
+			}
 			set_current_state(TASK_INTERRUPTIBLE);
 			spin_unlock_irq(&ipmi_read_lock);
 			schedule();
@@ -810,10 +815,6 @@ static ssize_t ipmi_read(struct file *file,
 		}
 		remove_wait_queue(&read_q, &wait);
 
-		if (signal_pending(current)) {
-			rv = -ERESTARTSYS;
-			goto out;
-		}
 	}
 	data_to_read = 0;
 
-- 
2.27.0