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-rw-r--r--kernel/kmod.c32
1 files changed, 24 insertions, 8 deletions
diff --git a/kernel/kmod.c b/kernel/kmod.c
index d910b63..81c6705 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -225,8 +225,8 @@ static int call_usermodehelper_exec_async(void *data)
spin_unlock_irq(&current->sighand->siglock);
/*
- * Our parent is keventd, which runs with elevated scheduling priority.
- * Avoid propagating that into the userspace child.
+ * Our parent is khelper which runs with elevated scheduling
+ * priority. Avoid propagating that into the userspace child.
*/
set_user_nice(current, 0);
@@ -267,7 +267,11 @@ out:
do_exit(0);
}
-/* Keventd can't block, but this (a child) can. */
+/*
+ * Handles UMH_WAIT_PROC. Our parent khelper can't wait for usermodehelper
+ * completion without blocking every other pending requests. That's why
+ * we use a kernel thread dedicated for that purpose.
+ */
static int call_usermodehelper_exec_sync(void *data)
{
struct subprocess_info *sub_info = data;
@@ -283,8 +287,8 @@ static int call_usermodehelper_exec_sync(void *data)
/*
* Normally it is bogus to call wait4() from in-kernel because
* wait4() wants to write the exit code to a userspace address.
- * But call_usermodehelper_exec_sync() always runs as keventd,
- * and put_user() to a kernel address works OK for kernel
+ * But call_usermodehelper_exec_sync() always runs as kernel
+ * thread and put_user() to a kernel address works OK for kernel
* threads, due to their having an mm_segment_t which spans the
* entire address space.
*
@@ -305,7 +309,19 @@ static int call_usermodehelper_exec_sync(void *data)
do_exit(0);
}
-/* This is run by khelper thread */
+/*
+ * This function doesn't strictly needs to be called asynchronously. But we
+ * need to create the usermodehelper kernel threads from a task that is affine
+ * to all CPUs (or nohz housekeeping ones) such that they inherit a widest
+ * affinity irrespective of call_usermodehelper() callers with possibly reduced
+ * affinity (eg: per-cpu workqueues). We don't want usermodehelper targets to
+ * contend any busy CPU.
+ * Khelper provides such wide affinity.
+ *
+ * Besides, khelper provides the privilege level that caller might not have to
+ * perform the usermodehelper request.
+ *
+ */
static void call_usermodehelper_exec_work(struct work_struct *work)
{
struct subprocess_info *sub_info =
@@ -533,8 +549,8 @@ EXPORT_SYMBOL(call_usermodehelper_setup);
* from interrupt context.
*
* Runs a user-space application. The application is started
- * asynchronously if wait is not set, and runs as a child of keventd.
- * (ie. it runs with full root capabilities).
+ * asynchronously if wait is not set, and runs as a child of khelper.
+ * (ie. it runs with full root capabilities and wide affinity).
*/
int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
{
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