proc/kcore: don't grab lock for memory hotplug notifier

The memory hotplug notifier kcore_callback() only needs kclist_lock to
prevent races with __kcore_update_ram(), but we can easily eliminate that
race by using an atomic xchg() in __kcore_update_ram().  This is
preparation for converting kclist_lock to an rwsem.

Link: http://lkml.kernel.org/r/0a4bc89f4dbde8b5b2ea309f7b4fb6a85fe29df2.1531953780.git.osandov@fb.com
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Bhupesh Sharma <bhsharma@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: James Morse <james.morse@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index b0b9a76f28d6397acffb0005a9ce1f048f6ea72b..e83f15a4f66de6df361bba792a7012fc8bef15c3 100644 (file)
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -118,7 +118,7 @@ static void __kcore_update_ram(struct list_head *list)
        LIST_HEAD(garbage);
 
        write_lock(&kclist_lock);
-       if (kcore_need_update) {
+       if (xchg(&kcore_need_update, 0)) {
                list_for_each_entry_safe(pos, tmp, &kclist_head, list) {
                        if (pos->type == KCORE_RAM
                                || pos->type == KCORE_VMEMMAP)
@@ -127,7 +127,6 @@ static void __kcore_update_ram(struct list_head *list)
                list_splice_tail(list, &kclist_head);
        } else
                list_splice(list, &garbage);
-       kcore_need_update = 0;
        proc_root_kcore->size = get_kcore_size(&nphdr, &size);
        write_unlock(&kclist_lock);
 
@@ -593,9 +592,8 @@ static int __meminit kcore_callback(struct notifier_block *self,
        switch (action) {
        case MEM_ONLINE:
        case MEM_OFFLINE:
-               write_lock(&kclist_lock);
                kcore_need_update = 1;
-               write_unlock(&kclist_lock);
+               break;
        }
        return NOTIFY_OK;
 }