[Qemu-devel] [PATCH 2/2] 9p-synth: use mutex on read-side

[Paolo Bonzini <pbonzini@redhat.com>]

Even with the fix in the previous patch, the lockless handling of paths
in 9p-synth is wrong.  Paths can outlive rcu_read_unlock arbitrarily
via the V9fsPath objects that 9p-synth creates.  This would require
a reference counting mechanism that is not there and is quite hard to
retrofit into V9fsPath.

It seems to me that this was a premature optimization, so replace
everything with a simple mutex.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/9pfs/virtio-9p-synth.c | 12 ++++++------
 qemu-thread.h             |  3 ---
 2 file modificati, 6 inserzioni(+), 9 rimozioni(-)

diff --git a/hw/9pfs/virtio-9p-synth.c b/hw/9pfs/virtio-9p-synth.c
index a91ebe1..426605e 100644
--- a/hw/9pfs/virtio-9p-synth.c
+++ b/hw/9pfs/virtio-9p-synth.c
@@ -59,7 +59,7 @@ static V9fsSynthNode *v9fs_add_dir_node(V9fsSynthNode *parent, int mode,
     }
     node->private = node;
     strncpy(node->name, name, sizeof(node->name));
-    QLIST_INSERT_HEAD_RCU(&parent->child, node, sibling);
+    QLIST_INSERT_HEAD(&parent->child, node, sibling);
     return node;
 }
 
@@ -133,7 +133,7 @@ int qemu_v9fs_synth_add_file(V9fsSynthNode *parent, int mode,
     node->attr->mode   = mode;
     node->private      = arg;
     strncpy(node->name, name, sizeof(node->name));
-    QLIST_INSERT_HEAD_RCU(&parent->child, node, sibling);
+    QLIST_INSERT_HEAD(&parent->child, node, sibling);
     ret = 0;
 err_out:
     qemu_mutex_unlock(&v9fs_synth_mutex);
@@ -229,7 +229,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry,
     int i = 0;
     V9fsSynthNode *node;
 
-    rcu_read_lock();
+    qemu_mutex_lock(&v9fs_synth_mutex);
     QLIST_FOREACH(node, &dir->child, sibling) {
         /* This is the off child of the directory */
         if (i == off) {
@@ -245,7 +245,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry,
     v9fs_synth_direntry(node, entry, off);
     *result = entry;
 out:
-    rcu_read_unlock();
+    qemu_mutex_unlock(&v9fs_synth_mutex);
     return 0;
 }
 
@@ -476,7 +476,7 @@ static int v9fs_synth_name_to_path(FsContext *ctx, V9fsPath *dir_path,
 
     }
 
-    rcu_read_lock();
+    qemu_mutex_lock(&v9fs_synth_mutex);
     if (!dir_path) {
         dir_node = &v9fs_synth_root;
     } else {
@@ -504,7 +504,7 @@ static int v9fs_synth_name_to_path(FsContext *ctx, V9fsPath *dir_path,
     memcpy(target->data, &node, sizeof(void *));
     target->size = sizeof(void *);
 err_out:
-    rcu_read_unlock();
+    qemu_mutex_unlock(&v9fs_synth_mutex);
     return ret;
 }
 
diff --git a/qemu-thread.h b/qemu-thread.h
index 05fdaaf..3c9715e 100644
--- a/qemu-thread.h
+++ b/qemu-thread.h
@@ -23,9 +23,6 @@ void qemu_mutex_lock(QemuMutex *mutex);
 int qemu_mutex_trylock(QemuMutex *mutex);
 void qemu_mutex_unlock(QemuMutex *mutex);
 
-#define rcu_read_lock() do { } while (0)
-#define rcu_read_unlock() do { } while (0)
-
 void qemu_cond_init(QemuCond *cond);
 void qemu_cond_destroy(QemuCond *cond);
 
-- 
1.7.11.2