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

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

On 08/08/2012 05:25 PM, Paolo Bonzini wrote:
> 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.

Hi Paolo,

The rcu_read_[un]lock() macros were added as no-ops (based on your 
inputs on #qemu) to replace reader-writer locks with RCU based locking 
as suggested while proposing QemuRWLock API for RW locks (See 
http://lists.gnu.org/archive/html/qemu-devel/2011-10/msg00192.html).

v9fs_synth_mutex was actually a pthread_rwlock_t earlier.
I am not sure if reader lock would be better than having a plain mutex 
for readers as well.

Aneesh, inputs ?

Also, if we are going forward with this change, we may also want to 
remove definition of QLIST_INSERT_HEAD_RCU, since the code being remove 
below is the only consumer of that macro.

regards,
Harsh

>
> 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);
>