Re: [PATCH] cifs: Fix cifsInodeInfo lock_sem deadlock when reconnect occurs

[Ronnie Sahlberg <lsahlber@redhat.com>]

Reviewed-by me

----- Original Message -----
From: "Dave Wysochanski" <dwysocha@redhat.com>
To: linux-cifs@vger.kernel.org
Sent: Thursday, 24 October, 2019 12:08:22 PM
Subject: [PATCH] cifs: Fix cifsInodeInfo lock_sem deadlock when reconnect occurs

There's a deadlock that is possible and can easily be seen with
a test where multiple readers open/read/close of the same file
and a disruption occurs causing reconnect.  The deadlock is due
a reader thread inside cifs_strict_readv calling down_read and
obtaining lock_sem, and then after reconnect inside
cifs_reopen_file calling down_read a second time.  If in
between the two down_read calls, a down_write comes from
another process, deadlock occurs.

        CPU0                    CPU1
        ----                    ----
cifs_strict_readv()
 down_read(&cifsi->lock_sem);
                               _cifsFileInfo_put
                                  OR
                               cifs_new_fileinfo
                                down_write(&cifsi->lock_sem);
cifs_reopen_file()
 down_read(&cifsi->lock_sem);

Fix the above by changing all down_write(lock_sem) calls to
down_write_trylock(lock_sem)/msleep() loop, which in turn
makes the second down_read call benign since it will never
block behind the writer while holding lock_sem.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Suggested-by: Ronnie Sahlberg <lsahlber@redhat.com>
---
 fs/cifs/cifsglob.h |  5 +++++
 fs/cifs/file.c     | 24 ++++++++++++++++--------
 fs/cifs/smb2file.c |  3 ++-
 3 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/fs/cifs/cifsglob.h b/fs/cifs/cifsglob.h
index 50dfd9049370..2c4a7adbcb4e 100644
--- a/fs/cifs/cifsglob.h
+++ b/fs/cifs/cifsglob.h
@@ -1391,6 +1391,11 @@ struct cifs_writedata {
 struct cifsInodeInfo {
 	bool can_cache_brlcks;
 	struct list_head llist;	/* locks helb by this inode */
+	/*
+	 * NOTE: Some code paths call down_read(lock_sem) twice, so
+	 * we must always use use down_write_trylock()/msleep() loop
+	 * to avoid deadlock.
+	 */
 	struct rw_semaphore lock_sem;	/* protect the fields above */
 	/* BB add in lists for dirty pages i.e. write caching info for oplock */
 	struct list_head openFileList;
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 5ad15de2bb4f..52454df5ae39 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -306,7 +306,8 @@ struct cifsFileInfo *
 	INIT_LIST_HEAD(&fdlocks->locks);
 	fdlocks->cfile = cfile;
 	cfile->llist = fdlocks;
-	down_write(&cinode->lock_sem);
+	while (!down_write_trylock(&cinode->lock_sem))
+		msleep(125);
 	list_add(&fdlocks->llist, &cinode->llist);
 	up_write(&cinode->lock_sem);
 
@@ -464,7 +465,8 @@ void _cifsFileInfo_put(struct cifsFileInfo *cifs_file, bool wait_oplock_handler)
 	 * Delete any outstanding lock records. We'll lose them when the file
 	 * is closed anyway.
 	 */
-	down_write(&cifsi->lock_sem);
+	while (!down_write_trylock(&cifsi->lock_sem))
+		msleep(125);
 	list_for_each_entry_safe(li, tmp, &cifs_file->llist->locks, llist) {
 		list_del(&li->llist);
 		cifs_del_lock_waiters(li);
@@ -1027,7 +1029,8 @@ int cifs_closedir(struct inode *inode, struct file *file)
 cifs_lock_add(struct cifsFileInfo *cfile, struct cifsLockInfo *lock)
 {
 	struct cifsInodeInfo *cinode = CIFS_I(d_inode(cfile->dentry));
-	down_write(&cinode->lock_sem);
+	while (!down_write_trylock(&cinode->lock_sem))
+		msleep(125);
 	list_add_tail(&lock->llist, &cfile->llist->locks);
 	up_write(&cinode->lock_sem);
 }
@@ -1049,7 +1052,8 @@ int cifs_closedir(struct inode *inode, struct file *file)
 
 try_again:
 	exist = false;
-	down_write(&cinode->lock_sem);
+	while (!down_write_trylock(&cinode->lock_sem))
+		msleep(125);
 
 	exist = cifs_find_lock_conflict(cfile, lock->offset, lock->length,
 					lock->type, lock->flags, &conf_lock,
@@ -1072,7 +1076,8 @@ int cifs_closedir(struct inode *inode, struct file *file)
 					(lock->blist.next == &lock->blist));
 		if (!rc)
 			goto try_again;
-		down_write(&cinode->lock_sem);
+		while (!down_write_trylock(&cinode->lock_sem))
+			msleep(125);
 		list_del_init(&lock->blist);
 	}
 
@@ -1125,7 +1130,8 @@ int cifs_closedir(struct inode *inode, struct file *file)
 		return rc;
 
 try_again:
-	down_write(&cinode->lock_sem);
+	while (!down_write_trylock(&cinode->lock_sem))
+		msleep(125);
 	if (!cinode->can_cache_brlcks) {
 		up_write(&cinode->lock_sem);
 		return rc;
@@ -1331,7 +1337,8 @@ struct lock_to_push {
 	int rc = 0;
 
 	/* we are going to update can_cache_brlcks here - need a write access */
-	down_write(&cinode->lock_sem);
+	while (!down_write_trylock(&cinode->lock_sem))
+		msleep(125);
 	if (!cinode->can_cache_brlcks) {
 		up_write(&cinode->lock_sem);
 		return rc;
@@ -1522,7 +1529,8 @@ struct lock_to_push {
 	if (!buf)
 		return -ENOMEM;
 
-	down_write(&cinode->lock_sem);
+	while (!down_write_trylock(&cinode->lock_sem))
+		msleep(125);
 	for (i = 0; i < 2; i++) {
 		cur = buf;
 		num = 0;
diff --git a/fs/cifs/smb2file.c b/fs/cifs/smb2file.c
index e6a1fc72018f..61f6cc8d9cc9 100644
--- a/fs/cifs/smb2file.c
+++ b/fs/cifs/smb2file.c
@@ -145,7 +145,8 @@
 
 	cur = buf;
 
-	down_write(&cinode->lock_sem);
+	while (!down_write_trylock(&cinode->lock_sem))
+		msleep(125);
 	list_for_each_entry_safe(li, tmp, &cfile->llist->locks, llist) {
 		if (flock->fl_start > li->offset ||
 		    (flock->fl_start + length) <
-- 
1.8.3.1