How to list processes locking file?

lslocks, from the util-linux package, does exactly this.

In the MODE column, processes waiting for a lock will be marked with a *.

Two possibilities: lsof (my preference) or lslk (specifically for file locks):

[root@policyServer ~]# lslk | grep "master.lock"
SRC          PID   DEV  INUM   SZ TY M   ST WH  END LEN NAME
master      1650 253,0 12423   33  w 0    0  0    0   0 /var/lib/postfix/master.lock

[root@policyServer ~]# lsof | grep "master.lock"
master     1650      root   10uW     REG              253,0       33      12423 /var/lib/postfix/master.lock

Output of lslk is self-expanatory but lsof puts the lock description in the "FD" column (which is 10uW above). From the man page:

The mode character is followed by one of these lock characters, describing the type of lock applied to the file:

N for a Solaris NFS lock of unknown type;
r for read lock on part of the file;
R for a read lock on the entire file;
w for a write lock on part of the file;
W for a write lock on the entire file;
u for a read and write lock of any length;
U for a lock of unknown type;
x for an SCO OpenServer Xenix lock on part      of the file;
X for an SCO OpenServer Xenix lock on the      entire file;
                       space if there is no lock.

So the "FD" column of lsof above breaks down to:

10 The literal descriptor of this open file. What's linked to by /proc/1650/fd/10

u File is open for reading and writing

W program has a write lock on the file.

lsof can help to see the list of file. here is way to see the locked files.

sudo lsof /var/lib/dpkg/lock