[PATCH 1/2] PM / Usermodehelper: Introduce reference counting to solve usermodehelper_disabled race

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

This patch adds the necessary synchronization framework to fix the race
condition with the 'usermodehelper_disabled' flag, by implementing a
refcounting solution. Specifically, it introduces the pair get_usermodehelper()
and put_usermodehelper(), which can be used by the readers (those who want to
read the value of the usermodehelper_disabled flag, such as _request_firmware()
in this case). The writers (those who enable/disable usermodehelpers by
setting/resetting that flag) can use the pair umh_control_begin() and
umh_control_done().

The reason for using a refcounting solution and not just a plain mutex, is
that we don't want to unnecessarily serialize all users of request_firmware(),
which act as readers. But note that we cannot use reader-writer locks here
because the readers sleep (waiting for the firmware load from user-space),
and sleeping with spinlocks held is not allowed. So refcounting implemented
using mutex locks underneath, seems to be the best fit here.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

The refcounting solution implemented here is adapted from the one used in
the CPU hotplug infrastructure (kernel/cpu.c). If this patchset sounds
reasonable, I plan to make the refcounting generic (in a later patch) and
expose it via include/linux/refcount.h or something similar, and then use it
at these 2 places instead of duplicating code.

 include/linux/kmod.h |    2 ++
 kernel/kmod.c        |   64 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+), 0 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index b16f653..845fe3d 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -117,5 +117,7 @@ extern void usermodehelper_init(void);
 extern int usermodehelper_disable(void);
 extern void usermodehelper_enable(void);
 extern bool usermodehelper_is_disabled(void);
+extern void get_usermodehelper(void);
+extern void put_usermodehelper(void);
 
 #endif /* __LINUX_KMOD_H__ */
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 2142687..acb52af 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -49,6 +49,70 @@ static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;
 static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
 static DEFINE_SPINLOCK(umh_sysctl_lock);
 
+static struct {
+	struct task_struct *active_writer;
+	struct mutex lock; /* Synchronizes accesses to refcount, */
+	/*
+	 * Also blocks the new readers during an ongoing update to the
+	 * 'usermodehelper_disabled' flag.
+	 */
+	int refcount;
+} umhelper = {
+	.active_writer = NULL,
+	.lock = __MUTEX_INITIALIZER(umhelper.lock),
+	.refcount = 0,
+};
+
+void get_usermodehelper(void)
+{
+	might_sleep();
+	if (umhelper.active_writer == current)
+		return;
+	mutex_lock(&umhelper.lock);
+	umhelper.refcount++;
+	mutex_unlock(&umhelper.lock);
+}
+EXPORT_SYMBOL_GPL(get_usermodehelper);
+
+void put_usermodehelper(void)
+{
+	if (umhelper.active_writer == current)
+		return;
+	mutex_lock(&umhelper.lock);
+	if (!--umhelper.refcount && unlikely(umhelper.active_writer))
+		wake_up_process(umhelper.active_writer);
+	mutex_unlock(&umhelper.lock);
+}
+EXPORT_SYMBOL_GPL(put_usermodehelper);
+
+/*
+ * This ensures that enabling or disabling usermodehelpers can begin
+ * only when the refcount goes to zero.
+ *
+ * Note that during an ongoing usermodehelper enable/disable operation,
+ * the new readers, if any, will be blocked by umhelper.lock
+ */
+static void umh_control_begin(void)
+{
+	umhelper.active_writer = current;
+
+	for (;;) {
+		mutex_lock(&umhelper.lock);
+		if (likely(!umhelper.refcount))
+			break;
+		__set_current_state(TASK_UNINTERRUPTIBLE);
+		mutex_unlock(&umhelper.lock);
+		schedule();
+	}
+}
+
+static void umh_control_done(void)
+{
+	umhelper.active_writer = NULL;
+	mutex_unlock(&umhelper.lock);
+}
+
+
 #ifdef CONFIG_MODULES
 
 /*