2 files changed, 106 insertions, 183 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index b94ad3d685ff..22ae06ad751d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -131,10 +131,20 @@ struct vfsmount *alloc_vfsmnt(const char *name)
                 INIT_LIST_HEAD(&mnt->mnt_share);
                 INIT_LIST_HEAD(&mnt->mnt_slave_list);
                 INIT_LIST_HEAD(&mnt->mnt_slave);
-                atomic_set(&mnt->__mnt_writers, 0);
+#ifdef CONFIG_SMP
+                mnt->mnt_writers = alloc_percpu(int);
+                if (!mnt->mnt_writers)
+                        goto out_free_devname;
+#else
+                mnt->mnt_writers = 0;
+#endif
         }
         return mnt;
 
+#ifdef CONFIG_SMP
+out_free_devname:
+        kfree(mnt->mnt_devname);
+#endif
 out_free_id:
         mnt_free_id(mnt);
 out_free_cache:
@@ -171,65 +181,38 @@ int __mnt_is_readonly(struct vfsmount *mnt)
 }
 EXPORT_SYMBOL_GPL(__mnt_is_readonly);
 
-struct mnt_writer {
-        /*
-         * If holding multiple instances of this lock, they
-         * must be ordered by cpu number.
-         */
-        spinlock_t lock;
-        struct lock_class_key lock_class; /* compiles out with !lockdep */
-        unsigned long count;
-        struct vfsmount *mnt;
-} ____cacheline_aligned_in_smp;
-static DEFINE_PER_CPU(struct mnt_writer, mnt_writers);
+static inline void inc_mnt_writers(struct vfsmount *mnt)
+{
+#ifdef CONFIG_SMP
+        (*per_cpu_ptr(mnt->mnt_writers, smp_processor_id()))++;
+#else
+        mnt->mnt_writers++;
+#endif
+}
 
-static int __init init_mnt_writers(void)
+static inline void dec_mnt_writers(struct vfsmount *mnt)
 {
-        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;
+#ifdef CONFIG_SMP
+        (*per_cpu_ptr(mnt->mnt_writers, smp_processor_id()))--;
+#else
+        mnt->mnt_writers--;
+#endif
 }
-fs_initcall(init_mnt_writers);
 
-static void unlock_mnt_writers(void)
+static unsigned int count_mnt_writers(struct vfsmount *mnt)
 {
+#ifdef CONFIG_SMP
+        unsigned int count = 0;
         int cpu;
-        struct mnt_writer *cpu_writer;
 
         for_each_possible_cpu(cpu) {
-                cpu_writer = &per_cpu(mnt_writers, cpu);
-                spin_unlock(&cpu_writer->lock);
+                count += *per_cpu_ptr(mnt->mnt_writers, cpu);
         }
-}
 
-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;
+        return count;
+#else
+        return mnt->mnt_writers;
+#endif
 }
 
 /*
@@ -253,75 +236,34 @@ static inline void use_cpu_writer_for_mount(struct mnt_writer *cpu_writer,
 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);
+        preempt_disable();
+        inc_mnt_writers(mnt);
+        /*
+         * The store to inc_mnt_writers must be visible before we pass
+         * MNT_WRITE_HOLD loop below, so that the slowpath can see our
+         * incremented count after it has set MNT_WRITE_HOLD.
+         */
+        smp_mb();
+        while (mnt->mnt_flags & MNT_WRITE_HOLD)
+                cpu_relax();
+        /*
+         * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
+         * be set to match its requirements. So we must not load that until
+         * MNT_WRITE_HOLD is cleared.
+         */
+        smp_rmb();
         if (__mnt_is_readonly(mnt)) {
+                dec_mnt_writers(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);
+        preempt_enable();
         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
@@ -332,37 +274,9 @@ static void handle_write_count_underflow(struct vfsmount *mnt)
  */
 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);
+        preempt_disable();
+        dec_mnt_writers(mnt);
+        preempt_enable();
 }
 EXPORT_SYMBOL_GPL(mnt_drop_write);
 
@@ -370,24 +284,41 @@ static int mnt_make_readonly(struct vfsmount *mnt)
 {
         int ret = 0;
 
-        lock_mnt_writers();
+        spin_lock(&vfsmount_lock);
+        mnt->mnt_flags |= MNT_WRITE_HOLD;
         /*
-         * With all the locks held, this value is stable
+         * After storing MNT_WRITE_HOLD, we'll read the counters. This store
+         * should be visible before we do.
          */
-        if (atomic_read(&mnt->__mnt_writers) > 0) {
-                ret = -EBUSY;
-                goto out;
-        }
+        smp_mb();
+
         /*
-         * nobody can do a successful mnt_want_write() with all
-         * of the counts in MNT_DENIED_WRITE and the locks held.
+         * With writers on hold, if this value is zero, then there are
+         * definitely no active writers (although held writers may subsequently
+         * increment the count, they'll have to wait, and decrement it after
+         * seeing MNT_READONLY).
+         *
+         * It is OK to have counter incremented on one CPU and decremented on
+         * another: the sum will add up correctly. The danger would be when we
+         * sum up each counter, if we read a counter before it is incremented,
+         * but then read another CPU's count which it has been subsequently
+         * decremented from -- we would see more decrements than we should.
+         * MNT_WRITE_HOLD protects against this scenario, because
+         * mnt_want_write first increments count, then smp_mb, then spins on
+         * MNT_WRITE_HOLD, so it can't be decremented by another CPU while
+         * we're counting up here.
          */
-        spin_lock(&vfsmount_lock);
-        if (!ret)
+        if (count_mnt_writers(mnt) > 0)
+                ret = -EBUSY;
+        else
                 mnt->mnt_flags |= MNT_READONLY;
+        /*
+         * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
+         * that become unheld will see MNT_READONLY.
+         */
+        smp_wmb();
+        mnt->mnt_flags &= ~MNT_WRITE_HOLD;
         spin_unlock(&vfsmount_lock);
-out:
-        unlock_mnt_writers();
         return ret;
 }
 
@@ -410,6 +341,9 @@ void free_vfsmnt(struct vfsmount *mnt)
 {
         kfree(mnt->mnt_devname);
         mnt_free_id(mnt);
+#ifdef CONFIG_SMP
+        free_percpu(mnt->mnt_writers);
+#endif
         kmem_cache_free(mnt_cache, mnt);
 }
 
@@ -604,38 +538,18 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
 
 static inline void __mntput(struct vfsmount *mnt)
 {
-        int cpu;
         struct super_block *sb = mnt->mnt_sb;
         /*
-         * We don't have to hold all of the locks at the
-         * same time here because we know that we're the
-         * last reference to mnt and that no new writers
-         * can come in.
-         */
-        for_each_possible_cpu(cpu) {
-                struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
-                spin_lock(&cpu_writer->lock);
-                if (cpu_writer->mnt != mnt) {
-                        spin_unlock(&cpu_writer->lock);
-                        continue;
-                }
-                atomic_add(cpu_writer->count, &mnt->__mnt_writers);
-                cpu_writer->count = 0;
-                /*
-                 * Might as well do this so that no one
-                 * ever sees the pointer and expects
-                 * it to be valid.
-                 */
-                cpu_writer->mnt = NULL;
-                spin_unlock(&cpu_writer->lock);
-        }
-        /*
          * This probably indicates that somebody messed
          * up a mnt_want/drop_write() pair.  If this
          * happens, the filesystem was probably unable
          * to make r/w->r/o transitions.
          */
-        WARN_ON(atomic_read(&mnt->__mnt_writers));
+        /*
+         * atomic_dec_and_lock() used to deal with ->mnt_count decrements
+         * provides barriers, so count_mnt_writers() below is safe.  AV
+         */
+        WARN_ON(count_mnt_writers(mnt));
         dput(mnt->mnt_root);
         free_vfsmnt(mnt);
         deactivate_super(sb);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 51f55f903aff..ac49c1f8e5c0 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -30,7 +30,7 @@ struct mnt_namespace;
 #define MNT_STRICTATIME 0x80
 
 #define MNT_SHRINKABLE  0x100
-#define MNT_IMBALANCED_WRITE_COUNT      0x200 /* just for debugging */
+#define MNT_WRITE_HOLD  0x200
 
 #define MNT_SHARED      0x1000  /* if the vfsmount is a shared mount */
 #define MNT_UNBINDABLE  0x2000  /* if the vfsmount is a unbindable mount */
@@ -65,13 +65,22 @@ struct vfsmount {
         int mnt_expiry_mark;            /* true if marked for expiry */
         int mnt_pinned;
         int mnt_ghosts;
-        /*
-         * This value is not stable unless all of the mnt_writers[] spinlocks
-         * are held, and all mnt_writer[]s on this mount have 0 as their ->count
-         */
-        atomic_t __mnt_writers;
+#ifdef CONFIG_SMP
+        int *mnt_writers;
+#else
+        int mnt_writers;
+#endif
 };
 
+static inline int *get_mnt_writers_ptr(struct vfsmount *mnt)
+{
+#ifdef CONFIG_SMP
+        return mnt->mnt_writers;
+#else
+        return &mnt->mnt_writers;
+#endif
+}
+
 static inline struct vfsmount *mntget(struct vfsmount *mnt)
 {
         if (mnt)