fasync: re-organize fasync entry insertion to allow it under a spinlock

You currently cannot use "fasync_helper()" in an atomic environment to
insert a new fasync entry, because it will need to allocate the new
"struct fasync_struct".

Yet fcntl_setlease() wants to call this under lock_flocks(), which is in
the process of being converted from the BKL to a spinlock.

In order to fix this, this abstracts out the actual fasync list
insertion and the fasync allocations into functions of their own, and
teaches fs/locks.c to pre-allocate the fasync_struct entry.  That way
the actual list insertion can happen while holding the required
spinlock.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bfields@redhat.com: rebase on top of my changes to Arnd's patch]
Tested-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

3 files changed, 72 insertions, 17 deletions

diff --git a/fs/fcntl.c b/fs/fcntl.c
index f8cc34f542c3..dcdbc6f5c33b 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -640,7 +640,7 @@ static void fasync_free_rcu(struct rcu_head *head)
  * match the state "is the filp on a fasync list".
  *
  */
-static int fasync_remove_entry(struct file *filp, struct fasync_struct **fapp)
+int fasync_remove_entry(struct file *filp, struct fasync_struct **fapp)
 {
         struct fasync_struct *fa, **fp;
         int result = 0;
@@ -666,21 +666,28 @@ static int fasync_remove_entry(struct file *filp, struct fasync_struct **fapp)
         return result;
 }
 
+struct fasync_struct *fasync_alloc(void)
+{
+        return kmem_cache_alloc(fasync_cache, GFP_KERNEL);
+}
+
 /*
- * Add a fasync entry. Return negative on error, positive if
- * added, and zero if did nothing but change an existing one.
- *
- * NOTE! It is very important that the FASYNC flag always
- * match the state "is the filp on a fasync list".
+ * NOTE! This can be used only for unused fasync entries:
+ * entries that actually got inserted on the fasync list
+ * need to be released by rcu - see fasync_remove_entry.
  */
-static int fasync_add_entry(int fd, struct file *filp, struct fasync_struct **fapp)
+void fasync_free(struct fasync_struct *new)
 {
-        struct fasync_struct *new, *fa, **fp;
-        int result = 0;
+        kmem_cache_free(fasync_cache, new);
+}
 
-        new = kmem_cache_alloc(fasync_cache, GFP_KERNEL);
-        if (!new)
-                return -ENOMEM;
+/*
+ * Insert a new entry into the fasync list.  Return the pointer to the
+ * old one if we didn't use the new one.
+ */
+struct fasync_struct *fasync_insert_entry(int fd, struct file *filp, struct fasync_struct **fapp, struct fasync_struct *new)
+{
+        struct fasync_struct *fa, **fp;
 
         spin_lock(&filp->f_lock);
         spin_lock(&fasync_lock);
@@ -691,8 +698,6 @@ static int fasync_add_entry(int fd, struct file *filp, struct fasync_struct **fa
                 spin_lock_irq(&fa->fa_lock);
                 fa->fa_fd = fd;
                 spin_unlock_irq(&fa->fa_lock);
-
-                kmem_cache_free(fasync_cache, new);
                 goto out;
         }
 
@@ -702,13 +707,42 @@ static int fasync_add_entry(int fd, struct file *filp, struct fasync_struct **fa
         new->fa_fd = fd;
         new->fa_next = *fapp;
         rcu_assign_pointer(*fapp, new);
-        result = 1;
         filp->f_flags |= FASYNC;
 
 out:
         spin_unlock(&fasync_lock);
         spin_unlock(&filp->f_lock);
-        return result;
+        return fa;
+}
+
+/*
+ * Add a fasync entry. Return negative on error, positive if
+ * added, and zero if did nothing but change an existing one.
+ *
+ * NOTE! It is very important that the FASYNC flag always
+ * match the state "is the filp on a fasync list".
+ */
+static int fasync_add_entry(int fd, struct file *filp, struct fasync_struct **fapp)
+{
+        struct fasync_struct *new;
+
+        new = fasync_alloc();
+        if (!new)
+                return -ENOMEM;
+
+        /*
+         * fasync_insert_entry() returns the old (update) entry if
+         * it existed.
+         *
+         * So free the (unused) new entry and return 0 to let the
+         * caller know that we didn't add any new fasync entries.
+         */
+        if (fasync_insert_entry(fd, filp, fapp, new)) {
+                fasync_free(new);
+                return 0;
+        }
+
+        return 1;
 }
 
 /*
diff --git a/fs/locks.c b/fs/locks.c
index 0391d2ff5a4e..85fd9ce1abae 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1505,6 +1505,7 @@ EXPORT_SYMBOL_GPL(vfs_setlease);
 int fcntl_setlease(unsigned int fd, struct file *filp, long arg)
 {
         struct file_lock *fl;
+        struct fasync_struct *new;
         struct inode *inode = filp->f_path.dentry->d_inode;
         int error;
 
@@ -1512,12 +1513,25 @@ int fcntl_setlease(unsigned int fd, struct file *filp, long arg)
         if (IS_ERR(fl))
                 return PTR_ERR(fl);
 
+        new = fasync_alloc();
+        if (!new) {
+                locks_free_lock(fl);
+                return -ENOMEM;
+        }
         lock_flocks();
         error = __vfs_setlease(filp, arg, &fl);
         if (error || arg == F_UNLCK)
                 goto out_unlock;
 
-        error = fasync_helper(fd, filp, 1, &fl->fl_fasync);
+        /*
+         * fasync_insert_entry() returns the old entry if any.
+         * If there was no old entry, then it used 'new' and
+         * inserted it into the fasync list. Clear new so that
+         * we don't release it here.
+         */
+        if (!fasync_insert_entry(fd, filp, &fl->fl_fasync, new))
+                new = NULL;
+
         if (error < 0) {
                 /* remove lease just inserted by setlease */
                 fl->fl_type = F_UNLCK | F_INPROGRESS;
@@ -1529,6 +1543,8 @@ int fcntl_setlease(unsigned int fd, struct file *filp, long arg)
         error = __f_setown(filp, task_pid(current), PIDTYPE_PID, 0);
 out_unlock:
         unlock_flocks();
+        if (new)
+                fasync_free(new);
         return error;
 }
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8d7de08ab546..56285e5e1de4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1302,6 +1302,11 @@ struct fasync_struct {
 
 /* SMP safe fasync helpers: */
 extern int fasync_helper(int, struct file *, int, struct fasync_struct **);
+extern struct fasync_struct *fasync_insert_entry(int, struct file *, struct fasync_struct **, struct fasync_struct *);
+extern int fasync_remove_entry(struct file *, struct fasync_struct **);
+extern struct fasync_struct *fasync_alloc(void);
+extern void fasync_free(struct fasync_struct *);
+
 /* can be called from interrupts */
 extern void kill_fasync(struct fasync_struct **, int, int);