[PATCH 5/6] kmod: preempt on kmod_umh_threads_get()

["Luis R. Rodriguez" <mcgrof@kernel.org>]

In theory it is possible multiple concurrent threads will try to
kmod_umh_threads_get() and as such atomic_inc(&kmod_concurrent) at
the same time, therefore enabling a small time during which we've
bumped kmod_concurrent but have not really enabled work. By using
preemption we mitigate this a bit.

Preemption is not needed when we kmod_umh_threads_put().

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
---
 kernel/kmod.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 563600fc9bb1..7ea11dbc7564 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -113,15 +113,35 @@ static int call_modprobe(char *module_name, int wait)
 
 static int kmod_umh_threads_get(void)
 {
+	int ret = 0;
+
+	/*
+	 * Disabling preemption makes sure that we are not rescheduled here
+	 *
+	 * Also preemption helps kmod_concurrent is not increased by mistake
+	 * for too long given in theory two concurrent threads could race on
+	 * atomic_inc() before we atomic_read() -- we know that's possible
+	 * and but we don't care, this is not used for object accounting and
+	 * is just a subjective threshold. The alternative is a lock.
+	 */
+	preempt_disable();
 	atomic_inc(&kmod_concurrent);
 	if (atomic_read(&kmod_concurrent) <= max_modprobes)
-		return 0;
+		goto out;
+
 	atomic_dec(&kmod_concurrent);
-	return -EBUSY;
+	ret = -EBUSY;
+out:
+	preempt_enable();
+	return ret;
 }
 
 static void kmod_umh_threads_put(void)
 {
+	/*
+	 * Preemption is not needed given once work is done we can
+	 * pace ourselves on our way out.
+	 */
 	atomic_dec(&kmod_concurrent);
 }
 
-- 
2.11.0