[104/104] virtiofsd: Convert lo_destroy to take the lo->mutex lock itself

Commit Message

Dr. David Alan Gilbert Dec. 12, 2019, 4:39 p.m. UTC

From: "Dr. David Alan Gilbert" <[e-mail]>

lo_destroy was relying on some implicit knowledge of the locking;
we can avoid this if we create an unref_inode that doesn't take
the lock and then grab it for the whole of the lo_destroy.

Suggested-by: Vivek Goyal <[e-mail]>
Signed-off-by: Dr. David Alan Gilbert <[e-mail]>
---
 tools/virtiofsd/passthrough_ll.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

Comments

Philippe Mathieu-Daudé Jan. 17, 2020, 1:33 p.m. UTC | #1

On 12/12/19 5:39 PM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <[e-mail]>
> 
> lo_destroy was relying on some implicit knowledge of the locking;
> we can avoid this if we create an unref_inode that doesn't take
> the lock and then grab it for the whole of the lo_destroy.
> 
> Suggested-by: Vivek Goyal <[e-mail]>
> Signed-off-by: Dr. David Alan Gilbert <[e-mail]>
> ---
>   tools/virtiofsd/passthrough_ll.c | 31 +++++++++++++++++--------------
>   1 file changed, 17 insertions(+), 14 deletions(-)
> 
> diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> index 38f4948e61..c37f57157e 100644
> --- a/tools/virtiofsd/passthrough_ll.c
> +++ b/tools/virtiofsd/passthrough_ll.c
> @@ -1328,14 +1328,13 @@ static void lo_unlink(fuse_req_t req, fuse_ino_t parent, const char *name)
>       lo_inode_put(lo, &inode);
>   }
>   
> -static void unref_inode_lolocked(struct lo_data *lo, struct lo_inode *inode,
> -                                 uint64_t n)
> +/* To be called with lo->mutex held */
> +static void unref_inode(struct lo_data *lo, struct lo_inode *inode, uint64_t n)
>   {
>       if (!inode) {
>           return;
>       }
>   
> -    pthread_mutex_lock(&lo->mutex);
>       assert(inode->nlookup >= n);
>       inode->nlookup -= n;
>       if (!inode->nlookup) {
> @@ -1346,15 +1345,24 @@ static void unref_inode_lolocked(struct lo_data *lo, struct lo_inode *inode,
>           }
>           g_hash_table_destroy(inode->posix_locks);
>           pthread_mutex_destroy(&inode->plock_mutex);
> -        pthread_mutex_unlock(&lo->mutex);
>   
>           /* Drop our refcount from lo_do_lookup() */
>           lo_inode_put(lo, &inode);
> -    } else {
> -        pthread_mutex_unlock(&lo->mutex);
>       }
>   }
>   
> +static void unref_inode_lolocked(struct lo_data *lo, struct lo_inode *inode,
> +                                 uint64_t n)
> +{
> +    if (!inode) {
> +        return;
> +    }
> +
> +    pthread_mutex_lock(&lo->mutex);
> +    unref_inode(lo, inode, n);
> +    pthread_mutex_unlock(&lo->mutex);
> +}
> +
>   static void lo_forget_one(fuse_req_t req, fuse_ino_t ino, uint64_t nlookup)
>   {
>       struct lo_data *lo = lo_data(req);
> @@ -2441,13 +2449,7 @@ static void lo_destroy(void *userdata)
>   {
>       struct lo_data *lo = (struct lo_data *)userdata;
>   
> -    /*
> -     * Normally lo->mutex must be taken when traversing lo->inodes but
> -     * lo_destroy() is a serialized request so no races are possible here.
> -     *
> -     * In addition, we cannot acquire lo->mutex since unref_inode() takes it
> -     * too and this would result in a recursive lock.
> -     */
> +    pthread_mutex_lock(&lo->mutex);
>       while (true) {
>           GHashTableIter iter;
>           gpointer key, value;
> @@ -2458,8 +2460,9 @@ static void lo_destroy(void *userdata)
>           }
>   
>           struct lo_inode *inode = value;
> -        unref_inode_lolocked(lo, inode, inode->nlookup);
> +        unref_inode(lo, inode, inode->nlookup);
>       }
> +    pthread_mutex_unlock(&lo->mutex);
>   }
>   
>   static struct fuse_lowlevel_ops lo_oper = {
> 

Reviewed-by: Philippe Mathieu-Daudé <[e-mail]>

Patch

diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 38f4948e61..c37f57157e 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -1328,14 +1328,13 @@  static void lo_unlink(fuse_req_t req, fuse_ino_t parent, const char *name)
     lo_inode_put(lo, &inode);
 }
 
-static void unref_inode_lolocked(struct lo_data *lo, struct lo_inode *inode,
-                                 uint64_t n)
+/* To be called with lo->mutex held */
+static void unref_inode(struct lo_data *lo, struct lo_inode *inode, uint64_t n)
 {
     if (!inode) {
         return;
     }
 
-    pthread_mutex_lock(&lo->mutex);
     assert(inode->nlookup >= n);
     inode->nlookup -= n;
     if (!inode->nlookup) {
@@ -1346,15 +1345,24 @@  static void unref_inode_lolocked(struct lo_data *lo, struct lo_inode *inode,
         }
         g_hash_table_destroy(inode->posix_locks);
         pthread_mutex_destroy(&inode->plock_mutex);
-        pthread_mutex_unlock(&lo->mutex);
 
         /* Drop our refcount from lo_do_lookup() */
         lo_inode_put(lo, &inode);
-    } else {
-        pthread_mutex_unlock(&lo->mutex);
     }
 }
 
+static void unref_inode_lolocked(struct lo_data *lo, struct lo_inode *inode,
+                                 uint64_t n)
+{
+    if (!inode) {
+        return;
+    }
+
+    pthread_mutex_lock(&lo->mutex);
+    unref_inode(lo, inode, n);
+    pthread_mutex_unlock(&lo->mutex);
+}
+
 static void lo_forget_one(fuse_req_t req, fuse_ino_t ino, uint64_t nlookup)
 {
     struct lo_data *lo = lo_data(req);
@@ -2441,13 +2449,7 @@  static void lo_destroy(void *userdata)
 {
     struct lo_data *lo = (struct lo_data *)userdata;
 
-    /*
-     * Normally lo->mutex must be taken when traversing lo->inodes but
-     * lo_destroy() is a serialized request so no races are possible here.
-     *
-     * In addition, we cannot acquire lo->mutex since unref_inode() takes it
-     * too and this would result in a recursive lock.
-     */
+    pthread_mutex_lock(&lo->mutex);
     while (true) {
         GHashTableIter iter;
         gpointer key, value;
@@ -2458,8 +2460,9 @@  static void lo_destroy(void *userdata)
         }
 
         struct lo_inode *inode = value;
-        unref_inode_lolocked(lo, inode, inode->nlookup);
+        unref_inode(lo, inode, inode->nlookup);
     }
+    pthread_mutex_unlock(&lo->mutex);
 }
 
 static struct fuse_lowlevel_ops lo_oper = {