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/include/linux/kobject.h b/include/linux/kobject.h
index 926afb6f6b5fd6959673dcec56915381bd541686..f896a33e8341e1e281ad6b19f5d75da0c4d70622 100644 (file)
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -119,6 +119,7 @@ struct kobj_type {
 };
 
 struct kobj_uevent_env {
+       char *argv[3];
        char *envp[UEVENT_NUM_ENVP];
        int envp_idx;
        char buf[UEVENT_BUFFER_SIZE];

diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
index 5f72767ddd9b8471a5237a0b39ef0865e176934b..4e3bd71bd9495cfbf95fd9a28adf5e18a71f4592 100644 (file)
--- a/lib/kobject_uevent.c
+++ b/lib/kobject_uevent.c
@@ -124,6 +124,30 @@ static int kobj_usermode_filter(struct kobject *kobj)
        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 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
 
        /* 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 @@ int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
                                        "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: