kobject: don't block for each kobject_uevent
Currently kobject_uevent has somewhat unpredictable semantics. The
point is, since it may call a usermode helper and wait for it to execute
(UMH_WAIT_EXEC), it is impossible to say for sure what lock dependencies
it will introduce for the caller - strictly speaking it depends on what
fs the binary is located on and the set of locks fork may take. There
are quite a few kobject_uevent's users that do not take this into
account and call it with various mutexes taken, e.g. rtnl_mutex,
net_mutex, which might potentially lead to a deadlock.
Since there is actually no reason to wait for the usermode helper to
execute there, let's make kobject_uevent start the helper asynchronously
with the aid of the UMH_NO_WAIT flag.
Personally, I'm interested in this, because I really want kobject_uevent
to be called under the slab_mutex in the slub implementation as it used
to be some time ago, because it greatly simplifies synchronization and
automatically fixes a kmemcg-related race. However, there was a
deadlock detected on an attempt to call kobject_uevent under the
slab_mutex (see https://lkml.org/lkml/2012/1/14/45), which was reported
to be fixed by releasing the slab_mutex for kobject_uevent.
Unfortunately, there was no information about who exactly blocked on the
slab_mutex causing the usermode helper to stall, neither have I managed
to find this out or reproduce the issue.
BTW, this is not the first attempt to make kobject_uevent use
UMH_NO_WAIT. Previous one was made by commit f520360d93cd ("kobject:
don't block for each kobject_uevent"), but it was wrong (it passed
arguments allocated on stack to async thread) so it was reverted in
05f54c13cd0c ("Revert "kobject: don't block for each kobject_uevent".").
It targeted on speeding up the boot process though.
Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Greg KH <greg@kroah.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index 5f72767..4e3bd71 100644
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -124,6 +124,30 @@
return 0;
}
+static int init_uevent_argv(struct kobj_uevent_env *env, const char *subsystem)
+{
+ int len;
+
+ len = strlcpy(&env->buf[env->buflen], subsystem,
+ sizeof(env->buf) - env->buflen);
+ if (len >= (sizeof(env->buf) - env->buflen)) {
+ WARN(1, KERN_ERR "init_uevent_argv: buffer size too small\n");
+ return -ENOMEM;
+ }
+
+ env->argv[0] = uevent_helper;
+ env->argv[1] = &env->buf[env->buflen];
+ env->argv[2] = NULL;
+
+ env->buflen += len + 1;
+ return 0;
+}
+
+static void cleanup_uevent_env(struct subprocess_info *info)
+{
+ kfree(info->data);
+}
+
/**
* kobject_uevent_env - send an uevent with environmental data
*
@@ -301,11 +325,8 @@
/* call uevent_helper, usually only enabled during early boot */
if (uevent_helper[0] && !kobj_usermode_filter(kobj)) {
- char *argv [3];
+ struct subprocess_info *info;
- argv [0] = uevent_helper;
- argv [1] = (char *)subsystem;
- argv [2] = NULL;
retval = add_uevent_var(env, "HOME=/");
if (retval)
goto exit;
@@ -313,9 +334,18 @@
"PATH=/sbin:/bin:/usr/sbin:/usr/bin");
if (retval)
goto exit;
+ retval = init_uevent_argv(env, subsystem);
+ if (retval)
+ goto exit;
- retval = call_usermodehelper(argv[0], argv,
- env->envp, UMH_WAIT_EXEC);
+ retval = -ENOMEM;
+ info = call_usermodehelper_setup(env->argv[0], env->argv,
+ env->envp, GFP_KERNEL,
+ NULL, cleanup_uevent_env, env);
+ if (info) {
+ retval = call_usermodehelper_exec(info, UMH_NO_WAIT);
+ env = NULL; /* freed by cleanup_uevent_env */
+ }
}
exit: