[PATCH 1/2] ocfs2: Fix deadlock in ocfs2_read_virt_blocks

[Mohammed Anees <pvmohammedanees2003@gmail.com>]

syzbot has found a kernel BUG in ocfs2_write_cluster_by_desc,
while the next patch in the series resolves this, another
bug has been detected due to a potential deadlock [1].

The scenario is depicted here,

	CPU0					CPU1
lock(&ocfs2_file_ip_alloc_sem_key);
                               lock(&osb->system_file_mutex);
                               lock(&ocfs2_file_ip_alloc_sem_key);
lock(&osb->system_file_mutex);

The function calls which could lead to this are:

CPU0
ocfs2_write_begin - lock(&ocfs2_file_ip_alloc_sem_key);
.
.
.
ocfs2_get_system_file_inode - lock(&osb->system_file_mutex);

CPU1 -
ocfs2_get_system_file_inode - lock(&osb->system_file_mutex);
.
.
.
ocfs2_read_virt_blocks - lock(&ocfs2_file_ip_alloc_sem_key);

This issue can be resolved by making the down_read -> down_read_try
in the ocfs2_read_virt_blocks.

[1] https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6

Reported-and-tested-by: syzbot+18a87160c7d64ba2e2f6@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=18a87160c7d64ba2e2f6
Signed-off-by: Mohammed Anees <pvmohammedanees2003@gmail.com>
---
 fs/ocfs2/extent_map.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/extent_map.c b/fs/ocfs2/extent_map.c
index 70a768b62..f83d0a3b6 100644
--- a/fs/ocfs2/extent_map.c
+++ b/fs/ocfs2/extent_map.c
@@ -12,6 +12,7 @@
 #include <linux/slab.h>
 #include <linux/types.h>
 #include <linux/fiemap.h>
+#include <linux/delay.h>
 
 #include <cluster/masklog.h>
 
@@ -961,6 +962,8 @@ int ocfs2_read_virt_blocks(struct inode *inode, u64 v_block, int nr,
 	int rc = 0;
 	u64 p_block, p_count;
 	int i, count, done = 0;
+	int retries, max_retries = 5;
+	int retry_delay_ms = 30;
 
 	trace_ocfs2_read_virt_blocks(
 	     inode, (unsigned long long)v_block, nr, bhs, flags,
@@ -973,7 +976,18 @@ int ocfs2_read_virt_blocks(struct inode *inode, u64 v_block, int nr,
 	}
 
 	while (done < nr) {
-		down_read(&OCFS2_I(inode)->ip_alloc_sem);
+		retries = 0;
+		while (retries < max_retries) {
+			if (down_read_trylock(&OCFS2_I(inode)->ip_alloc_sem))
+				break; // Lock acquired
+			msleep(retry_delay_ms);
+			retries++;
+		}
+		if (retries == max_retries) {
+			rc = -EAGAIN;
+			mlog(ML_ERROR, "Cannot acquire lock\n");
+			break;
+		}
 		rc = ocfs2_extent_map_get_blocks(inode, v_block + done,
 						 &p_block, &p_count, NULL);
 		up_read(&OCFS2_I(inode)->ip_alloc_sem);
-- 
2.46.0