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>

1 files changed, 14 insertions, 6 deletions

diff --git a/drivers/infiniband/core/user_mad.c b/drivers/infiniband/core/user_mad.c
index 1fc17940a765..ef6dd825fee9 100644
--- 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;
 }