[PATCH] Take i_mutex in splice_from_pipe()

The splice_actor may be calling ->prepare_write() and ->commit_write(). We
want i_mutex on the inode being written to before calling those so that we
don't race i_size changes.

The double locking behavior is done elsewhere in splice.c, and if we
eventually want _nolock variants of generic_file_splice_write(), fs modules
might have to replicate the nasty locking code. We introduce
inode_double_lock() and inode_double_unlock() to consolidate the locking
rules into one set of functions.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 47 insertions, 13 deletions

diff --git a/fs/inode.c b/fs/inode.c
index d9a21d122926..26cdb115ce67 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -1306,6 +1306,42 @@ void wake_up_inode(struct inode *inode)
         wake_up_bit(&inode->i_state, __I_LOCK);
 }
 
+/*
+ * We rarely want to lock two inodes that do not have a parent/child
+ * relationship (such as directory, child inode) simultaneously. The
+ * vast majority of file systems should be able to get along fine
+ * without this. Do not use these functions except as a last resort.
+ */
+void inode_double_lock(struct inode *inode1, struct inode *inode2)
+{
+        if (inode1 == NULL || inode2 == NULL || inode1 == inode2) {
+                if (inode1)
+                        mutex_lock(&inode1->i_mutex);
+                else if (inode2)
+                        mutex_lock(&inode2->i_mutex);
+                return;
+        }
+
+        if (inode1 < inode2) {
+                mutex_lock_nested(&inode1->i_mutex, I_MUTEX_PARENT);
+                mutex_lock_nested(&inode2->i_mutex, I_MUTEX_CHILD);
+        } else {
+                mutex_lock_nested(&inode2->i_mutex, I_MUTEX_PARENT);
+                mutex_lock_nested(&inode1->i_mutex, I_MUTEX_CHILD);
+        }
+}
+EXPORT_SYMBOL(inode_double_lock);
+
+void inode_double_unlock(struct inode *inode1, struct inode *inode2)
+{
+        if (inode1)
+                mutex_unlock(&inode1->i_mutex);
+
+        if (inode2 && inode2 != inode1)
+                mutex_unlock(&inode2->i_mutex);
+}
+EXPORT_SYMBOL(inode_double_unlock);
+
 static __initdata unsigned long ihash_entries;
 static int __init set_ihash_entries(char *str)
 {
diff --git a/fs/splice.c b/fs/splice.c
index a567010b62ac..c1072b6940c3 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -713,6 +713,7 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
 {
         int ret, do_wakeup, err;
         struct splice_desc sd;
+        struct inode *inode = out->f_mapping->host;
 
         ret = 0;
         do_wakeup = 0;
@@ -722,8 +723,13 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
         sd.file = out;
         sd.pos = *ppos;
 
-        if (pipe->inode)
-                mutex_lock(&pipe->inode->i_mutex);
+        /*
+         * The actor worker might be calling ->prepare_write and
+         * ->commit_write. Most of the time, these expect i_mutex to
+         * be held. Since this may result in an ABBA deadlock with
+         * pipe->inode, we have to order lock acquiry here.
+         */
+        inode_double_lock(inode, pipe->inode);
 
         for (;;) {
                 if (pipe->nrbufs) {
@@ -797,8 +803,7 @@ ssize_t splice_from_pipe(struct pipe_inode_info *pipe, struct file *out,
                 pipe_wait(pipe);
         }
 
-        if (pipe->inode)
-                mutex_unlock(&pipe->inode->i_mutex);
+        inode_double_unlock(inode, pipe->inode);
 
         if (do_wakeup) {
                 smp_mb();
@@ -1400,13 +1405,7 @@ static int link_pipe(struct pipe_inode_info *ipipe,
          * grabbing by inode address. Otherwise two different processes
          * could deadlock (one doing tee from A -> B, the other from B -> A).
          */
-        if (ipipe->inode < opipe->inode) {
-                mutex_lock_nested(&ipipe->inode->i_mutex, I_MUTEX_PARENT);
-                mutex_lock_nested(&opipe->inode->i_mutex, I_MUTEX_CHILD);
-        } else {
-                mutex_lock_nested(&opipe->inode->i_mutex, I_MUTEX_PARENT);
-                mutex_lock_nested(&ipipe->inode->i_mutex, I_MUTEX_CHILD);
-        }
+        inode_double_lock(ipipe->inode, opipe->inode);
 
         do {
                 if (!opipe->readers) {
@@ -1450,8 +1449,7 @@ static int link_pipe(struct pipe_inode_info *ipipe,
                 i++;
         } while (len);
 
-        mutex_unlock(&ipipe->inode->i_mutex);
-        mutex_unlock(&opipe->inode->i_mutex);
+        inode_double_unlock(ipipe->inode, opipe->inode);
 
         /*
          * If we put data in the output pipe, wakeup any potential readers.