NFS: nfs_lock_and_join_requests and nfs_scan_commit_list can deadlock

Since the commit list is not ordered, it is possible for nfs_scan_commit_list
to hold a request that nfs_lock_and_join_requests() is waiting for, while
at the same time trying to grab a request that nfs_lock_and_join_requests
already holds.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

1 files changed, 11 insertions, 4 deletions

diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index ae26775b5448..c3f627b08ec6 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1028,21 +1028,28 @@ int
 nfs_scan_commit_list(struct list_head *src, struct list_head *dst,
                      struct nfs_commit_info *cinfo, int max)
 {
-        struct nfs_page *req;
+        struct nfs_page *req, *tmp;
         int ret = 0;
 
-        while(!list_empty(src)) {
-                req = list_first_entry(src, struct nfs_page, wb_list);
+restart:
+        list_for_each_entry_safe(req, tmp, src, wb_list) {
                 kref_get(&req->wb_kref);
                 if (!nfs_lock_request(req)) {
                         int status;
+
+                        /* Prevent deadlock with nfs_lock_and_join_requests */
+                        if (!list_empty(dst)) {
+                                nfs_release_request(req);
+                                continue;
+                        }
+                        /* Ensure we make progress to prevent livelock */
                         mutex_unlock(&NFS_I(cinfo->inode)->commit_mutex);
                         status = nfs_wait_on_request(req);
                         nfs_release_request(req);
                         mutex_lock(&NFS_I(cinfo->inode)->commit_mutex);
                         if (status < 0)
                                 break;
-                        continue;
+                        goto restart;
                 }
                 nfs_request_remove_commit_list(req, cinfo);
                 nfs_list_add_request(req, dst);