pstore: Switch pmsg_lock to an rt_mutex to avoid priority inversion

Wei Wang reported seeing priority inversion caused latencies
caused by contention on pmsg_lock, and suggested it be switched
to a rt_mutex.

I was initially hesitant this would help, as the tasks in that
trace all seemed to be SCHED_NORMAL, so the benefit would be
limited to only nice boosting.

However, another similar issue was raised where the priority
inversion was seen did involve a blocked RT task so it is clear
this would be helpful in that case.

Cc: Wei Wang <[email protected]>
Cc: Midas Chien<[email protected]>
Cc: Connor O'Brien <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Anton Vorontsov <[email protected]>
Cc: Colin Cross <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: [email protected]
Fixes: 9d5438f ("pstore: Add pmsg - user-space accessible pstore object")
Reported-by: Wei Wang <[email protected]>
Signed-off-by: John Stultz <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: johnstultz-work

fs/pstore/pmsg.c

@@ -7,9 +7,10 @@
 #include <linux/device.h>
 #include <linux/fs.h>
 #include <linux/uaccess.h>
+#include <linux/rtmutex.h>
 #include "internal.h"
 
-static DEFINE_MUTEX(pmsg_lock);
+static DEFINE_RT_MUTEX(pmsg_lock);
 
 static ssize_t write_pmsg(struct file *file, const char __user *buf,
 			  size_t count, loff_t *ppos)
@@ -28,9 +29,9 @@ static ssize_t write_pmsg(struct file *file, const char __user *buf,
 	if (!access_ok(buf, count))
 		return -EFAULT;
 
-	mutex_lock(&pmsg_lock);
+	rt_mutex_lock(&pmsg_lock);
 	ret = psinfo->write_user(&record, buf);
-	mutex_unlock(&pmsg_lock);
+	rt_mutex_unlock(&pmsg_lock);
 	return ret ? ret : count;
 }