IB/umad: BKL is not needed for ib_umad_open()

Remove explicit lock_kernel() calls and document why the code is safe.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/drivers/infiniband/core/user_mad.c b/drivers/infiniband/core/user_mad.c
index 1fc1794..ef6dd82 100644 (file)
--- a/drivers/infiniband/core/user_mad.c
+++ b/drivers/infiniband/core/user_mad.c
@@ -47,7 +47,6 @@
 #include <linux/kref.h>
 #include <linux/compat.h>
 #include <linux/semaphore.h>
-#include <linux/smp_lock.h>
 
 #include <asm/uaccess.h>
 
@@ -778,23 +777,33 @@ static long ib_umad_compat_ioctl(struct file *filp, unsigned int cmd,
 }
 #endif
 
+/*
+ * ib_umad_open() does not need the BKL:
+ *
+ *  - umad_port[] accesses are protected by port_lock, the
+ *    ib_umad_port structures are properly reference counted, and
+ *    everything else is purely local to the file being created, so
+ *    races against other open calls are not a problem;
+ *  - the ioctl method does not affect any global state outside of the
+ *    file structure being operated on;
+ *  - the port is added to umad_port[] as the last part of module
+ *    initialization so the open method will either immediately run
+ *    -ENXIO, or all required initialization will be done.
+ */
 static int ib_umad_open(struct inode *inode, struct file *filp)
 {
        struct ib_umad_port *port;
        struct ib_umad_file *file;
        int ret = 0;
 
-       lock_kernel();
        spin_lock(&port_lock);
        port = umad_port[iminor(inode) - IB_UMAD_MINOR_BASE];
        if (port)
                kref_get(&port->umad_dev->ref);
        spin_unlock(&port_lock);
 
-       if (!port) {
-               unlock_kernel();
+       if (!port)
                return -ENXIO;
-       }
 
        mutex_lock(&port->file_mutex);
 
@@ -823,7 +832,6 @@ static int ib_umad_open(struct inode *inode, struct file *filp)
 
 out:
        mutex_unlock(&port->file_mutex);
-       unlock_kernel();
        return ret;
 }