cifs: avoid starvation when refreshing dfs cache

When refreshing the DFS cache, keep SMB2 IOCTL calls as much outside
critical sections as possible and avoid read/write starvation when
getting new DFS referrals by using broken or slow connections.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

diff --git a/fs/cifs/dfs_cache.c b/fs/cifs/dfs_cache.c
index d9c2c75..775dbc7 100644 (file)
--- a/fs/cifs/dfs_cache.c
+++ b/fs/cifs/dfs_cache.c
@@ -554,6 +554,8 @@ static void remove_oldest_entry_locked(void)
        struct cache_entry *ce;
        struct cache_entry *to_del = NULL;
 
+       WARN_ON(!rwsem_is_locked(&htable_rw_lock));
+
        for (i = 0; i < CACHE_HTABLE_SIZE; i++) {
                struct hlist_head *l = &cache_htable[i];
 
@@ -583,7 +585,13 @@ static int add_cache_entry_locked(struct dfs_info3_param *refs, int numrefs)
        struct cache_entry *ce;
        unsigned int hash;
 
-       convert_delimiter(refs[0].path_name, '\\');
+       WARN_ON(!rwsem_is_locked(&htable_rw_lock));
+
+       if (atomic_read(&cache_count) >= CACHE_MAX_ENTRIES) {
+               cifs_dbg(FYI, "%s: reached max cache size (%d)\n", __func__, CACHE_MAX_ENTRIES);
+               remove_oldest_entry_locked();
+       }
+
        rc = cache_entry_hash(refs[0].path_name, strlen(refs[0].path_name), &hash);
        if (rc)
                return rc;
@@ -605,6 +613,8 @@ static int add_cache_entry_locked(struct dfs_info3_param *refs, int numrefs)
        hlist_add_head(&ce->hlist, &cache_htable[hash]);
        dump_ce(ce);
 
+       atomic_inc(&cache_count);
+
        return 0;
 }
 
@@ -719,16 +729,13 @@ void dfs_cache_destroy(void)
 }
 
 /* Update a cache entry with the new referral in @refs */
-static int update_cache_entry_locked(const char *path, const struct dfs_info3_param *refs,
+static int update_cache_entry_locked(struct cache_entry *ce, const struct dfs_info3_param *refs,
                                     int numrefs)
 {
        int rc;
-       struct cache_entry *ce;
        char *s, *th = NULL;
 
-       ce = lookup_cache_entry(path);
-       if (IS_ERR(ce))
-               return PTR_ERR(ce);
+       WARN_ON(!rwsem_is_locked(&htable_rw_lock));
 
        if (ce->tgthint) {
                s = ce->tgthint->name;
@@ -750,18 +757,28 @@ static int update_cache_entry_locked(const char *path, const struct dfs_info3_pa
 static int get_dfs_referral(const unsigned int xid, struct cifs_ses *ses, const char *path,
                            struct dfs_info3_param **refs, int *numrefs)
 {
+       int rc;
+       int i;
+
        cifs_dbg(FYI, "%s: get an DFS referral for %s\n", __func__, path);
 
+       *refs = NULL;
+       *numrefs = 0;
+
        if (!ses || !ses->server || !ses->server->ops->get_dfs_refer)
                return -EOPNOTSUPP;
        if (unlikely(!cache_cp))
                return -EINVAL;
 
-       *refs = NULL;
-       *numrefs = 0;
+       rc =  ses->server->ops->get_dfs_refer(xid, ses, path, refs, numrefs, cache_cp,
+                                             NO_MAP_UNI_RSVD);
+       if (!rc) {
+               struct dfs_info3_param *ref = *refs;
 
-       return ses->server->ops->get_dfs_refer(xid, ses, path, refs, numrefs, cache_cp,
-                                              NO_MAP_UNI_RSVD);
+               for (i = 0; i < *numrefs; i++)
+                       convert_delimiter(ref[i].path_name, '\\');
+       }
+       return rc;
 }
 
 /*
@@ -807,18 +824,11 @@ static int cache_refresh_path(const unsigned int xid, struct cifs_ses *ses, cons
        dump_refs(refs, numrefs);
 
        if (!newent) {
-               rc = update_cache_entry_locked(path, refs, numrefs);
+               rc = update_cache_entry_locked(ce, refs, numrefs);
                goto out_unlock;
        }
 
-       if (atomic_read(&cache_count) >= CACHE_MAX_ENTRIES) {
-               cifs_dbg(FYI, "%s: reached max cache size (%d)\n", __func__, CACHE_MAX_ENTRIES);
-               remove_oldest_entry_locked();
-       }
-
        rc = add_cache_entry_locked(refs, numrefs);
-       if (!rc)
-               atomic_inc(&cache_count);
 
 out_unlock:
        up_write(&htable_rw_lock);
@@ -1313,15 +1323,43 @@ static void refresh_mounts(struct cifs_ses **sessions)
 
        list_for_each_entry_safe(tcon, ntcon, &tcons, ulist) {
                const char *path = tcon->dfs_path + 1;
+               struct cache_entry *ce;
+               struct dfs_info3_param *refs = NULL;
+               int numrefs = 0;
+               bool needs_refresh = false;
                int rc = 0;
 
                list_del_init(&tcon->ulist);
+
                ses = find_ipc_from_server_path(sessions, path);
-               if (!IS_ERR(ses)) {
-                       xid = get_xid();
-                       cache_refresh_path(xid, ses, path);
-                       free_xid(xid);
+               if (IS_ERR(ses))
+                       goto next_tcon;
+
+               down_read(&htable_rw_lock);
+               ce = lookup_cache_entry(path);
+               needs_refresh = IS_ERR(ce) || cache_entry_expired(ce);
+               up_read(&htable_rw_lock);
+
+               if (!needs_refresh)
+                       goto next_tcon;
+
+               xid = get_xid();
+               rc = get_dfs_referral(xid, ses, path, &refs, &numrefs);
+               free_xid(xid);
+
+               /* Create or update a cache entry with the new referral */
+               if (!rc) {
+                       down_write(&htable_rw_lock);
+                       ce = lookup_cache_entry(path);
+                       if (IS_ERR(ce))
+                               add_cache_entry_locked(refs, numrefs);
+                       else if (cache_entry_expired(ce))
+                               update_cache_entry_locked(ce, refs, numrefs);
+                       up_write(&htable_rw_lock);
                }
+
+next_tcon:
+               free_dfs_info_array(refs, numrefs);
                cifs_put_tcon(tcon);
        }
 }
@@ -1331,40 +1369,67 @@ static void refresh_cache(struct cifs_ses **sessions)
        int i;
        struct cifs_ses *ses;
        unsigned int xid;
-       int rc;
+       char *ref_paths[CACHE_MAX_ENTRIES];
+       int count = 0;
+       struct cache_entry *ce;
 
        /*
-        * Refresh all cached entries.
+        * Refresh all cached entries.  Get all new referrals outside critical section to avoid
+        * starvation while performing SMB2 IOCTL on broken or slow connections.
+
         * The cache entries may cover more paths than the active mounts
         * (e.g. domain-based DFS referrals or multi tier DFS setups).
         */
-       down_write(&htable_rw_lock);
+       down_read(&htable_rw_lock);
        for (i = 0; i < CACHE_HTABLE_SIZE; i++) {
-               struct cache_entry *ce;
                struct hlist_head *l = &cache_htable[i];
 
                hlist_for_each_entry(ce, l, hlist) {
-                       struct dfs_info3_param *refs = NULL;
-                       int numrefs = 0;
-
-                       if (hlist_unhashed(&ce->hlist) || !cache_entry_expired(ce))
+                       if (count == ARRAY_SIZE(ref_paths))
+                               goto out_unlock;
+                       if (hlist_unhashed(&ce->hlist) || !cache_entry_expired(ce) ||
+                           IS_ERR(find_ipc_from_server_path(sessions, ce->path)))
                                continue;
+                       ref_paths[count++] = kstrdup(ce->path, GFP_ATOMIC);
+               }
+       }
 
-                       ses = find_ipc_from_server_path(sessions, ce->path);
-                       if (IS_ERR(ses))
-                               continue;
+out_unlock:
+       up_read(&htable_rw_lock);
 
-                       xid = get_xid();
-                       rc = get_dfs_referral(xid, ses, ce->path, &refs, &numrefs);
-                       free_xid(xid);
+       for (i = 0; i < count; i++) {
+               char *path = ref_paths[i];
+               struct dfs_info3_param *refs = NULL;
+               int numrefs = 0;
+               int rc = 0;
 
-                       if (!rc)
-                               update_cache_entry_locked(ce->path, refs, numrefs);
+               if (!path)
+                       continue;
 
-                       free_dfs_info_array(refs, numrefs);
+               ses = find_ipc_from_server_path(sessions, path);
+               if (IS_ERR(ses))
+                       goto next_referral;
+
+               xid = get_xid();
+               rc = get_dfs_referral(xid, ses, path, &refs, &numrefs);
+               free_xid(xid);
+
+               if (!rc) {
+                       down_write(&htable_rw_lock);
+                       ce = lookup_cache_entry(path);
+                       /*
+                        * We need to re-check it because other tasks might have it deleted or
+                        * updated.
+                        */
+                       if (!IS_ERR(ce) && cache_entry_expired(ce))
+                               update_cache_entry_locked(ce, refs, numrefs);
+                       up_write(&htable_rw_lock);
                }
+
+next_referral:
+               kfree(path);
+               free_dfs_info_array(refs, numrefs);
        }
-       up_write(&htable_rw_lock);
 }
 
 /*