4 files changed, 87 insertions, 51 deletions

diff --git a/fs/ubifs/gc.c b/fs/ubifs/gc.c
index d0f3dac29081..13f1019c859f 100644
--- a/fs/ubifs/gc.c
+++ b/fs/ubifs/gc.c
@@ -344,6 +344,12 @@ int ubifs_garbage_collect_leb(struct ubifs_info *c, struct ubifs_lprops *lp)
                 if (err)
                         goto out;
 
+                /* Allow for races with TNC */
+                c->gced_lnum = lnum;
+                smp_wmb();
+                c->gc_seq += 1;
+                smp_wmb();
+
                 if (c->gc_lnum == -1) {
                         c->gc_lnum = lnum;
                         err = LEB_RETAINED;
diff --git a/fs/ubifs/misc.h b/fs/ubifs/misc.h
index 87dabf9fe742..87ced4c74a61 100644
--- a/fs/ubifs/misc.h
+++ b/fs/ubifs/misc.h
@@ -325,4 +325,21 @@ static inline struct timespec ubifs_current_time(struct inode *inode)
                 current_fs_time(inode->i_sb) : CURRENT_TIME_SEC;
 }
 
+/**
+ * ubifs_tnc_lookup - look up a file-system node.
+ * @c: UBIFS file-system description object
+ * @key: node key to lookup
+ * @node: the node is returned here
+ *
+ * This function look up and reads node with key @key. The caller has to make
+ * sure the @node buffer is large enough to fit the node. Returns zero in case
+ * of success, %-ENOENT if the node was not found, and a negative error code in
+ * case of failure.
+ */
+static inline int ubifs_tnc_lookup(struct ubifs_info *c,
+                                   const union ubifs_key *key, void *node)
+{
+        return ubifs_tnc_locate(c, key, node, NULL, NULL);
+}
+
 #endif /* __UBIFS_MISC_H__ */
diff --git a/fs/ubifs/tnc.c b/fs/ubifs/tnc.c
index 4fbc5921688f..7da209ab9378 100644
--- a/fs/ubifs/tnc.c
+++ b/fs/ubifs/tnc.c
@@ -506,7 +506,7 @@ static int fallible_read_node(struct ubifs_info *c, const union ubifs_key *key,
                 if (keys_cmp(c, key, &node_key) != 0)
                         ret = 0;
         }
-        if (ret == 0)
+        if (ret == 0 && c->replaying)
                 dbg_mnt("dangling branch LEB %d:%d len %d, key %s",
                         zbr->lnum, zbr->offs, zbr->len, DBGKEY(key));
         return ret;
@@ -1382,50 +1382,39 @@ static int lookup_level0_dirty(struct ubifs_info *c, const union ubifs_key *key,
 }
 
 /**
- * ubifs_tnc_lookup - look up a file-system node.
+ * maybe_leb_gced - determine if a LEB may have been garbage collected.
  * @c: UBIFS file-system description object
- * @key: node key to lookup
- * @node: the node is returned here
+ * @lnum: LEB number
+ * @gc_seq1: garbage collection sequence number
  *
- * This function look up and reads node with key @key. The caller has to make
- * sure the @node buffer is large enough to fit the node. Returns zero in case
- * of success, %-ENOENT if the node was not found, and a negative error code in
- * case of failure.
+ * This function determines if @lnum may have been garbage collected since
+ * sequence number @gc_seq1. If it may have been then %1 is returned, otherwise
+ * %0 is returned.
  */
-int ubifs_tnc_lookup(struct ubifs_info *c, const union ubifs_key *key,
-                     void *node)
+static int maybe_leb_gced(struct ubifs_info *c, int lnum, int gc_seq1)
 {
-        int found, n, err;
-        struct ubifs_znode *znode;
-        struct ubifs_zbranch zbr, *zt;
-
-        mutex_lock(&c->tnc_mutex);
-        found = ubifs_lookup_level0(c, key, &znode, &n);
-        if (!found) {
-                err = -ENOENT;
-                goto out;
-        } else if (found < 0) {
-                err = found;
-                goto out;
-        }
-        zt = &znode->zbranch[n];
-        if (is_hash_key(c, key)) {
-                /*
-                 * In this case the leaf node cache gets used, so we pass the
-                 * address of the zbranch and keep the mutex locked
-                 */
-                err = tnc_read_node_nm(c, zt, node);
-                goto out;
-        }
-        zbr = znode->zbranch[n];
-        mutex_unlock(&c->tnc_mutex);
-
-        err = ubifs_tnc_read_node(c, &zbr, node);
-        return err;
+        int gc_seq2, gced_lnum;
 
-out:
-        mutex_unlock(&c->tnc_mutex);
-        return err;
+        gced_lnum = c->gced_lnum;
+        smp_rmb();
+        gc_seq2 = c->gc_seq;
+        /* Same seq means no GC */
+        if (gc_seq1 == gc_seq2)
+                return 0;
+        /* Different by more than 1 means we don't know */
+        if (gc_seq1 + 1 != gc_seq2)
+                return 1;
+        /*
+         * We have seen the sequence number has increased by 1. Now we need to
+         * be sure we read the right LEB number, so read it again.
+         */
+        smp_rmb();
+        if (gced_lnum != c->gced_lnum)
+                return 1;
+        /* Finally we can check lnum */
+        if (gced_lnum == lnum)
+                return 1;
+        return 0;
 }
 
 /**
@@ -1436,16 +1425,19 @@ out:
  * @lnum: LEB number is returned here
  * @offs: offset is returned here
  *
- * This function is the same as 'ubifs_tnc_lookup()' but it returns the node
- * location also. See 'ubifs_tnc_lookup()'.
+ * This function look up and reads node with key @key. The caller has to make
+ * sure the @node buffer is large enough to fit the node. Returns zero in case
+ * of success, %-ENOENT if the node was not found, and a negative error code in
+ * case of failure. The node location can be returned in @lnum and @offs.
  */
 int ubifs_tnc_locate(struct ubifs_info *c, const union ubifs_key *key,
                      void *node, int *lnum, int *offs)
 {
-        int found, n, err;
+        int found, n, err, safely = 0, gc_seq1;
         struct ubifs_znode *znode;
         struct ubifs_zbranch zbr, *zt;
 
+again:
         mutex_lock(&c->tnc_mutex);
         found = ubifs_lookup_level0(c, key, &znode, &n);
         if (!found) {
@@ -1456,24 +1448,43 @@ int ubifs_tnc_locate(struct ubifs_info *c, const union ubifs_key *key,
                 goto out;
         }
         zt = &znode->zbranch[n];
+        if (lnum) {
+                *lnum = zt->lnum;
+                *offs = zt->offs;
+        }
         if (is_hash_key(c, key)) {
                 /*
                  * In this case the leaf node cache gets used, so we pass the
                  * address of the zbranch and keep the mutex locked
                  */
-                *lnum = zt->lnum;
-                *offs = zt->offs;
                 err = tnc_read_node_nm(c, zt, node);
                 goto out;
         }
+        if (safely) {
+                err = ubifs_tnc_read_node(c, zt, node);
+                goto out;
+        }
+        /* Drop the TNC mutex prematurely and race with garbage collection */
         zbr = znode->zbranch[n];
+        gc_seq1 = c->gc_seq;
         mutex_unlock(&c->tnc_mutex);
 
-        *lnum = zbr.lnum;
-        *offs = zbr.offs;
+        if (ubifs_get_wbuf(c, zbr.lnum)) {
+                /* We do not GC journal heads */
+                err = ubifs_tnc_read_node(c, &zbr, node);
+                return err;
+        }
 
-        err = ubifs_tnc_read_node(c, &zbr, node);
-        return err;
+        err = fallible_read_node(c, key, &zbr, node);
+        if (maybe_leb_gced(c, zbr.lnum, gc_seq1)) {
+                /*
+                 * The node may have been GC'ed out from under us so try again
+                 * while keeping the TNC mutex locked.
+                 */
+                safely = 1;
+                goto again;
+        }
+        return 0;
 
 out:
         mutex_unlock(&c->tnc_mutex);
diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
index d7f706f7a302..7828d69ca4f8 100644
--- a/fs/ubifs/ubifs.h
+++ b/fs/ubifs/ubifs.h
@@ -1028,6 +1028,8 @@ struct ubifs_mount_opts {
  * @sbuf: a buffer of LEB size used by GC and replay for scanning
  * @idx_gc: list of index LEBs that have been garbage collected
  * @idx_gc_cnt: number of elements on the idx_gc list
+ * @gc_seq: incremented for every non-index LEB garbage collected
+ * @gced_lnum: last non-index LEB that was garbage collected
  *
  * @infos_list: links all 'ubifs_info' objects
  * @umount_mutex: serializes shrinker and un-mount
@@ -1257,6 +1259,8 @@ struct ubifs_info {
         void *sbuf;
         struct list_head idx_gc;
         int idx_gc_cnt;
+        volatile int gc_seq;
+        volatile int gced_lnum;
 
         struct list_head infos_list;
         struct mutex umount_mutex;
@@ -1451,8 +1455,6 @@ int ubifs_save_dirty_idx_lnums(struct ubifs_info *c);
 /* tnc.c */
 int ubifs_lookup_level0(struct ubifs_info *c, const union ubifs_key *key,
                         struct ubifs_znode **zn, int *n);
-int ubifs_tnc_lookup(struct ubifs_info *c, const union ubifs_key *key,
-                     void *node);
 int ubifs_tnc_lookup_nm(struct ubifs_info *c, const union ubifs_key *key,
                         void *node, const struct qstr *nm);
 int ubifs_tnc_locate(struct ubifs_info *c, const union ubifs_key *key,