f2fs: fix potential hangtask in f2fs_trace_pid

As Jia-Ju Bai reported:

"According to fs/f2fs/trace.c, the kernel module may sleep under a spinlock.
The function call path is:
f2fs_trace_pid (acquire the spinlock)
f2fs_radix_tree_insert
cond_resched --> may sleep

I do not find a good way to fix it, so I only report.
This possible bug is found by my static analysis tool (DSAC) and my code
review."

Obviously, it's problemetic to schedule in critical region of spinlock,
which will cause uninterruptable sleep if there is no waker.

This patch changes to use mutex lock intead of spinlock to avoid this
condition.

Reported-by: Jia-Ju Bai <[e-mail]>
Signed-off-by: Chao Yu <[e-mail]>
Signed-off-by: Jaegeuk Kim <[e-mail]>

fs/f2fs/trace.c

···
 #include "trace.h"
 
 static RADIX_TREE(pids, GFP_ATOMIC);
-static spinlock_t pids_lock;
+static struct mutex pids_lock;
 static struct last_io_info last_io;
 
 static inline void __print_last_io(void)
···
 	if (radix_tree_preload(GFP_NOFS))
 		return;
 
-	spin_lock(&pids_lock);
+	mutex_lock(&pids_lock);
 	p = radix_tree_lookup(&pids, pid);
 	if (p == current)
 		goto out;
···
 			MAJOR(inode->i_sb->s_dev), MINOR(inode->i_sb->s_dev),
 			pid, current->comm);
 out:
-	spin_unlock(&pids_lock);
+	mutex_unlock(&pids_lock);
 	radix_tree_preload_end();
 }
 
···
 
 void f2fs_build_trace_ios(void)
 {
-	spin_lock_init(&pids_lock);
+	mutex_init(&pids_lock);
 }
 
 #define PIDVEC_SIZE	128
···
 	pid_t next_pid = 0;
 	unsigned int found;
 
-	spin_lock(&pids_lock);
+	mutex_lock(&pids_lock);
 	while ((found = gang_lookup_pids(pid, next_pid, PIDVEC_SIZE))) {
 		unsigned idx;
 
···
 		for (idx = 0; idx < found; idx++)
 			radix_tree_delete(&pids, pid[idx]);
 	}
-	spin_unlock(&pids_lock);
+	mutex_unlock(&pids_lock);
 }