[RFC 06/10] kmod: provide sanity check on kmod_concurrent access

[Luis R. Rodriguez]

Only decrement *iff* we're possitive. Warn if we've hit
a situation where the counter is already 0 after we're done
with a modprobe call, this would tell us we have an unaccounted
counter access -- this in theory should not be possible as
only one routine controls the counter, however preemption is
one case that could trigger this situation. Avoid that situation
by disabling preemptiong while we access the counter.

Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
---
 kernel/kmod.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index ab38539f7e91..09cf35a2075a 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -113,16 +113,28 @@ static int call_modprobe(char *module_name, int wait)
 
 static int kmod_umh_threads_get(void)
 {
+	int ret = 0;
+
+	preempt_disable();
 	atomic_inc(&kmod_concurrent);
 	if (atomic_read(&kmod_concurrent) < max_modprobes)
-		return 0;
-	atomic_dec(&kmod_concurrent);
-	return -EBUSY;
+		goto out;
+
+	atomic_dec_if_positive(&kmod_concurrent);
+	ret = -EBUSY;
+out:
+	preempt_enable();
+	return 0;
 }
 
 static void kmod_umh_threads_put(void)
 {
-	atomic_dec(&kmod_concurrent);
+	int ret;
+
+	preempt_disable();
+	ret = atomic_dec_if_positive(&kmod_concurrent);
+	WARN_ON(ret < 0);
+	preempt_enable();
 }
 
 /**
-- 
2.10.1