Re: [Qemu-devel] [PATCH v2 2/2] Replace rwlocks with RCU variants of interfaces.

[Harsh Bora <harsh@linux.vnet.ibm.com>]

On 10/14/2011 02:05 AM, Harsh Prateek Bora wrote:
> Use QLIST_INSERT_HEAD_RCU and rcu_read_lock/unlock instead of rwlocks.
> Use v9fs_synth_mutex as a write-only mutex to handle concurrent writers.

This patch is based on top of:
http://repo.or.cz/w/qemu/v9fs.git/commitdiff/d647a2337e6e010c9f733d1ff68f7f97b27c5f80
>
> Signed-off-by: Harsh Prateek Bora<harsh@linux.vnet.ibm.com>
> ---
>   hw/9pfs/virtio-9p-synth.c |   26 ++++++++++++--------------
>   1 files changed, 12 insertions(+), 14 deletions(-)
>
> diff --git a/hw/9pfs/virtio-9p-synth.c b/hw/9pfs/virtio-9p-synth.c
> index cbf74e4..567611e 100644
> --- a/hw/9pfs/virtio-9p-synth.c
> +++ b/hw/9pfs/virtio-9p-synth.c
> @@ -30,8 +30,7 @@ V9fsSynthNode v9fs_synth_root = {
>       .attr =&v9fs_synth_root.actual_attr,
>   };
>
> -/*FIXME!! should be converted to qemu_rwlock_t */
> -static pthread_rwlock_t v9fs_synth_mutex;
> +static QemuMutex  v9fs_synth_mutex;
>   static int v9fs_synth_node_count;
>   /* set to 1 when the synth fs is ready */
>   static int v9fs_synth_fs;
> @@ -79,7 +78,7 @@ int qemu_v9fs_synth_mkdir(V9fsSynthNode *parent, int mode,
>       if (!parent) {
>           parent =&v9fs_synth_root;
>       }
> -    pthread_rwlock_wrlock(&v9fs_synth_mutex);
> +    qemu_mutex_lock(&v9fs_synth_mutex);
>       QLIST_FOREACH(tmp,&parent->child, sibling) {
>           if (!strcmp(tmp->name, name)) {
>               ret = EEXIST;
> @@ -95,7 +94,7 @@ int qemu_v9fs_synth_mkdir(V9fsSynthNode *parent, int mode,
>       *result = node;
>       ret = 0;
>   err_out:
> -    pthread_rwlock_unlock(&v9fs_synth_mutex);
> +    qemu_mutex_unlock(&v9fs_synth_mutex);
>       return ret;
>   }
>
> @@ -116,7 +115,7 @@ int qemu_v9fs_synth_add_file(V9fsSynthNode *parent, int mode,
>           parent =&v9fs_synth_root;
>       }
>
> -    pthread_rwlock_wrlock(&v9fs_synth_mutex);
> +    qemu_mutex_lock(&v9fs_synth_mutex);
>       QLIST_FOREACH(tmp,&parent->child, sibling) {
>           if (!strcmp(tmp->name, name)) {
>               ret = EEXIST;
> @@ -134,10 +133,10 @@ 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(&parent->child, node, sibling);
> +    QLIST_INSERT_HEAD_RCU(&parent->child, node, sibling);
>       ret = 0;
>   err_out:
> -    pthread_rwlock_unlock(&v9fs_synth_mutex);
> +    qemu_mutex_unlock(&v9fs_synth_mutex);
>       return ret;
>   }
>
> @@ -230,7 +229,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry,
>       int i = 0;
>       V9fsSynthNode *node;
>
> -    pthread_rwlock_rdlock(&v9fs_synth_mutex);
> +    rcu_read_lock();
>       QLIST_FOREACH(node,&dir->child, sibling) {
>           /* This is the off child of the directory */
>           if (i == off) {
> @@ -238,7 +237,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry,
>           }
>           i++;
>       }
> -    pthread_rwlock_unlock(&v9fs_synth_mutex);
> +    rcu_read_unlock();
>       if (!node) {
>           /* end of directory */
>           *result = NULL;
> @@ -483,13 +482,14 @@ static int v9fs_synth_name_to_path(FsContext *ctx, V9fsPath *dir_path,
>           goto out;
>       }
>       /* search for the name in the childern */
> -    pthread_rwlock_rdlock(&v9fs_synth_mutex);
> +    rcu_read_lock();
>       QLIST_FOREACH(node,&dir_node->child, sibling) {
>           if (!strcmp(node->name, name)) {
>               break;
>           }
>       }
> -    pthread_rwlock_unlock(&v9fs_synth_mutex);
> +    rcu_read_unlock();
> +
>       if (!node) {
>           errno = ENOENT;
>           return -1;
> @@ -532,11 +532,9 @@ static ssize_t my_test_read(void *in_buf, int len, off_t offset, void *arg)
>   static int v9fs_synth_init(FsContext *ctx)
>   {
>       V9fsSynthNode *node;
> -    pthread_rwlockattr_t rwlockattr;
>
>       QLIST_INIT(&v9fs_synth_root.child);
> -    pthread_rwlockattr_init(&rwlockattr);
> -    pthread_rwlock_init(&v9fs_synth_mutex,&rwlockattr);
> +    qemu_mutex_init(&v9fs_synth_mutex);
>
>       /* Add "." and ".." entries for root */
>       v9fs_add_dir_node(&v9fs_synth_root, v9fs_synth_root.attr->mode,