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

Luis R. Rodriguez
2017-05-19T03:25:27Z

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