param: simple locking for sysfs-writable charp parameters

Since the writing to sysfs can free the old one, we need to block that
when we access the charp variables.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Dan Williams <dcbw@redhat.com>
Cc: John W. Linville <linville@tuxdriver.com>
Cc: Jing Huang <huangj@brocade.com>
Cc: James E.J. Bottomley <James.Bottomley@suse.de>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: user-mode-linux-devel@lists.sourceforge.net
Cc: libertas-dev@lists.infradead.org
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: linux-scsi@vger.kernel.org
Cc: linux-usb@vger.kernel.org

net/mac80211/rate.c

@@ -103,6 +103,7 @@ ieee80211_rate_control_ops_get(const char *name)
 	struct rate_control_ops *ops;
 	const char *alg_name;
 
+	kparam_block_sysfs_write(ieee80211_default_rc_algo);
 	if (!name)
 		alg_name = ieee80211_default_rc_algo;
 	else
@@ -120,6 +121,7 @@ ieee80211_rate_control_ops_get(const char *name)
 	/* try built-in one if specific alg requested but not found */
 	if (!ops && strlen(CONFIG_MAC80211_RC_DEFAULT))
 		ops = ieee80211_try_rate_control_ops_get(CONFIG_MAC80211_RC_DEFAULT);
+	kparam_unblock_sysfs_write(ieee80211_default_rc_algo);
 
 	return ops;
 }