+static int __init init_mnt_writers(void)
+{
+ int cpu;
+ for_each_possible_cpu(cpu) {
+ struct mnt_writer *writer = &per_cpu(mnt_writers, cpu);
+ spin_lock_init(&writer->lock);
+ lockdep_set_class(&writer->lock, &writer->lock_class);
+ writer->count = 0;
+ }
+ return 0;
+}
+fs_initcall(init_mnt_writers);
+
+static void unlock_mnt_writers(void)
+{
+ int cpu;
+ struct mnt_writer *cpu_writer;
+
+ for_each_possible_cpu(cpu) {
+ cpu_writer = &per_cpu(mnt_writers, cpu);
+ spin_unlock(&cpu_writer->lock);
+ }
+}
+
+static inline void __clear_mnt_count(struct mnt_writer *cpu_writer)
+{
+ if (!cpu_writer->mnt)
+ return;
+ /*
+ * This is in case anyone ever leaves an invalid,
+ * old ->mnt and a count of 0.
+ */
+ if (!cpu_writer->count)
+ return;
+ atomic_add(cpu_writer->count, &cpu_writer->mnt->__mnt_writers);
+ cpu_writer->count = 0;
+}
+ /*
+ * must hold cpu_writer->lock
+ */
+static inline void use_cpu_writer_for_mount(struct mnt_writer *cpu_writer,
+ struct vfsmount *mnt)
+{
+ if (cpu_writer->mnt == mnt)
+ return;
+ __clear_mnt_count(cpu_writer);
+ cpu_writer->mnt = mnt;
+}
+
+/*
+ * Most r/o checks on a fs are for operations that take
+ * discrete amounts of time, like a write() or unlink().
+ * We must keep track of when those operations start
+ * (for permission checks) and when they end, so that
+ * we can determine when writes are able to occur to
+ * a filesystem.
+ */
+/**
+ * mnt_want_write - get write access to a mount
+ * @mnt: the mount on which to take a write
+ *
+ * This tells the low-level filesystem that a write is
+ * about to be performed to it, and makes sure that
+ * writes are allowed before returning success. When
+ * the write operation is finished, mnt_drop_write()
+ * must be called. This is effectively a refcount.
+ */
+int mnt_want_write(struct vfsmount *mnt)
+{
+ int ret = 0;
+ struct mnt_writer *cpu_writer;
+
+ cpu_writer = &get_cpu_var(mnt_writers);
+ spin_lock(&cpu_writer->lock);
+ if (__mnt_is_readonly(mnt)) {
+ ret = -EROFS;
+ goto out;
+ }
+ use_cpu_writer_for_mount(cpu_writer, mnt);
+ cpu_writer->count++;
+out:
+ spin_unlock(&cpu_writer->lock);
+ put_cpu_var(mnt_writers);
+ return ret;
+}
+EXPORT_SYMBOL_GPL(mnt_want_write);
+
+static void lock_mnt_writers(void)
+{
+ int cpu;
+ struct mnt_writer *cpu_writer;
+
+ for_each_possible_cpu(cpu) {
+ cpu_writer = &per_cpu(mnt_writers, cpu);
+ spin_lock(&cpu_writer->lock);
+ __clear_mnt_count(cpu_writer);
+ cpu_writer->mnt = NULL;
+ }
+}
+
+/*
+ * These per-cpu write counts are not guaranteed to have
+ * matched increments and decrements on any given cpu.
+ * A file open()ed for write on one cpu and close()d on
+ * another cpu will imbalance this count. Make sure it
+ * does not get too far out of whack.
+ */
+static void handle_write_count_underflow(struct vfsmount *mnt)
+{
+ if (atomic_read(&mnt->__mnt_writers) >=
+ MNT_WRITER_UNDERFLOW_LIMIT)
+ return;
+ /*
+ * It isn't necessary to hold all of the locks
+ * at the same time, but doing it this way makes
+ * us share a lot more code.
+ */
+ lock_mnt_writers();
+ /*
+ * vfsmount_lock is for mnt_flags.
+ */
+ spin_lock(&vfsmount_lock);
+ /*
+ * If coalescing the per-cpu writer counts did not
+ * get us back to a positive writer count, we have
+ * a bug.
+ */
+ if ((atomic_read(&mnt->__mnt_writers) < 0) &&
+ !(mnt->mnt_flags & MNT_IMBALANCED_WRITE_COUNT)) {
+ WARN(1, KERN_DEBUG "leak detected on mount(%p) writers "
+ "count: %d\n",
+ mnt, atomic_read(&mnt->__mnt_writers));
+ /* use the flag to keep the dmesg spam down */
+ mnt->mnt_flags |= MNT_IMBALANCED_WRITE_COUNT;
+ }
+ spin_unlock(&vfsmount_lock);
+ unlock_mnt_writers();
+}
+
+/**
+ * mnt_drop_write - give up write access to a mount
+ * @mnt: the mount on which to give up write access
+ *
+ * Tells the low-level filesystem that we are done
+ * performing writes to it. Must be matched with
+ * mnt_want_write() call above.
+ */
+void mnt_drop_write(struct vfsmount *mnt)
+{
+ int must_check_underflow = 0;
+ struct mnt_writer *cpu_writer;
+
+ cpu_writer = &get_cpu_var(mnt_writers);
+ spin_lock(&cpu_writer->lock);
+
+ use_cpu_writer_for_mount(cpu_writer, mnt);
+ if (cpu_writer->count > 0) {
+ cpu_writer->count--;
+ } else {
+ must_check_underflow = 1;
+ atomic_dec(&mnt->__mnt_writers);
+ }
+
+ spin_unlock(&cpu_writer->lock);
+ /*
+ * Logically, we could call this each time,
+ * but the __mnt_writers cacheline tends to
+ * be cold, and makes this expensive.
+ */
+ if (must_check_underflow)
+ handle_write_count_underflow(mnt);
+ /*
+ * This could be done right after the spinlock
+ * is taken because the spinlock keeps us on
+ * the cpu, and disables preemption. However,
+ * putting it here bounds the amount that
+ * __mnt_writers can underflow. Without it,
+ * we could theoretically wrap __mnt_writers.
+ */
+ put_cpu_var(mnt_writers);
+}
+EXPORT_SYMBOL_GPL(mnt_drop_write);
+
+static int mnt_make_readonly(struct vfsmount *mnt)
+{
+ int ret = 0;
+
+ lock_mnt_writers();
+ /*
+ * With all the locks held, this value is stable
+ */
+ if (atomic_read(&mnt->__mnt_writers) > 0) {
+ ret = -EBUSY;
+ goto out;
+ }
+ /*
+ * nobody can do a successful mnt_want_write() with all
+ * of the counts in MNT_DENIED_WRITE and the locks held.
+ */
+ spin_lock(&vfsmount_lock);
+ if (!ret)
+ mnt->mnt_flags |= MNT_READONLY;
+ spin_unlock(&vfsmount_lock);
+out:
+ unlock_mnt_writers();
+ return ret;
+}
+
+static void __mnt_unmake_readonly(struct vfsmount *mnt)
+{
+ spin_lock(&vfsmount_lock);
+ mnt->mnt_flags &= ~MNT_READONLY;
+ spin_unlock(&vfsmount_lock);
+}
+
+void simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)