sanitize vfsmount refcounting changes

Instead of splitting refcount between (per-cpu) mnt_count
and (SMP-only) mnt_longrefs, make all references contribute
to mnt_count again and keep track of how many are longterm
ones.

Accounting rules for longterm count:
	* 1 for each fs_struct.root.mnt
	* 1 for each fs_struct.pwd.mnt
	* 1 for having non-NULL ->mnt_ns
	* decrement to 0 happens only under vfsmount lock exclusive

That allows nice common case for mntput() - since we can't drop the
final reference until after mnt_longterm has reached 0 due to the rules
above, mntput() can grab vfsmount lock shared and check mnt_longterm.
If it turns out to be non-zero (which is the common case), we know
that this is not the final mntput() and can just blindly decrement
percpu mnt_count.  Otherwise we grab vfsmount lock exclusive and
do usual decrement-and-check of percpu mnt_count.

For fs_struct.c we have mnt_make_longterm() and mnt_make_shortterm();
namespace.c uses the latter in places where we don't already hold
vfsmount lock exclusive and opencodes a few remaining spots where
we need to manipulate mnt_longterm.

Note that we mostly revert the code outside of fs/namespace.c back
to what we used to have; in particular, normal code doesn't need
to care about two kinds of references, etc.  And we get to keep
the optimization Nick's variant had bought us...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 24 insertions, 11 deletions

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index 68ca487bedb1..78b519c13536 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -4,6 +4,19 @@
 #include <linux/path.h>
 #include <linux/slab.h>
 #include <linux/fs_struct.h>
+#include "internal.h"
+
+static inline void path_get_longterm(struct path *path)
+{
+        path_get(path);
+        mnt_make_longterm(path->mnt);
+}
+
+static inline void path_put_longterm(struct path *path)
+{
+        mnt_make_shortterm(path->mnt);
+        path_put(path);
+}
 
 /*
  * Replace the fs->{rootmnt,root} with {mnt,dentry}. Put the old values.
@@ -17,11 +30,11 @@ void set_fs_root(struct fs_struct *fs, struct path *path)
         write_seqcount_begin(&fs->seq);
         old_root = fs->root;
         fs->root = *path;
-        path_get_long(path);
+        path_get_longterm(path);
         write_seqcount_end(&fs->seq);
         spin_unlock(&fs->lock);
         if (old_root.dentry)
-                path_put_long(&old_root);
+                path_put_longterm(&old_root);
 }
 
 /*
@@ -36,12 +49,12 @@ void set_fs_pwd(struct fs_struct *fs, struct path *path)
         write_seqcount_begin(&fs->seq);
         old_pwd = fs->pwd;
         fs->pwd = *path;
-        path_get_long(path);
+        path_get_longterm(path);
         write_seqcount_end(&fs->seq);
         spin_unlock(&fs->lock);
 
         if (old_pwd.dentry)
-                path_put_long(&old_pwd);
+                path_put_longterm(&old_pwd);
 }
 
 void chroot_fs_refs(struct path *old_root, struct path *new_root)
@@ -59,13 +72,13 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root)
                         write_seqcount_begin(&fs->seq);
                         if (fs->root.dentry == old_root->dentry
                             && fs->root.mnt == old_root->mnt) {
-                                path_get_long(new_root);
+                                path_get_longterm(new_root);
                                 fs->root = *new_root;
                                 count++;
                         }
                         if (fs->pwd.dentry == old_root->dentry
                             && fs->pwd.mnt == old_root->mnt) {
-                                path_get_long(new_root);
+                                path_get_longterm(new_root);
                                 fs->pwd = *new_root;
                                 count++;
                         }
@@ -76,13 +89,13 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root)
         } while_each_thread(g, p);
         read_unlock(&tasklist_lock);
         while (count--)
-                path_put_long(old_root);
+                path_put_longterm(old_root);
 }
 
 void free_fs_struct(struct fs_struct *fs)
 {
-        path_put_long(&fs->root);
-        path_put_long(&fs->pwd);
+        path_put_longterm(&fs->root);
+        path_put_longterm(&fs->pwd);
         kmem_cache_free(fs_cachep, fs);
 }
 
@@ -118,9 +131,9 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
 
                 spin_lock(&old->lock);
                 fs->root = old->root;
-                path_get_long(&fs->root);
+                path_get_longterm(&fs->root);
                 fs->pwd = old->pwd;
-                path_get_long(&fs->pwd);
+                path_get_longterm(&fs->pwd);
                 spin_unlock(&old->lock);
         }
         return fs;