Re: [Qemu-devel] [PATCH v4 04/12] migration: avoid concurrent invoke channel_close by different threads

[Dr. David Alan Gilbert]

* Lidong Chen (address@hidden) wrote:
> From: Lidong Chen <address@hidden>
> 
> The channel_close maybe invoked by different threads. For example, source
> qemu invokes qemu_fclose in main thread, migration thread and return path
> thread. Destination qemu invokes qemu_fclose in main thread, listen thread
> and COLO incoming thread.
> 
> Add a mutex in QEMUFile struct to avoid concurrent invoke channel_close.
> 
> Signed-off-by: Lidong Chen <address@hidden>
> ---
>  migration/qemu-file.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index 977b9ae..87d0f05 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -52,6 +52,7 @@ struct QEMUFile {
>      unsigned int iovcnt;
>  
>      int last_error;
> +    QemuMutex lock;

That could do with a comment saying what you're protecting

>  };
>  
>  /*
> @@ -96,6 +97,7 @@ QEMUFile *qemu_fopen_ops(void *opaque, const QEMUFileOps 
> *ops)
>  
>      f = g_new0(QEMUFile, 1);
>  
> +    qemu_mutex_init(&f->lock);
>      f->opaque = opaque;
>      f->ops = ops;
>      return f;
> @@ -328,7 +330,9 @@ int qemu_fclose(QEMUFile *f)
>      ret = qemu_file_get_error(f);
>  
>      if (f->ops->close) {
> +        qemu_mutex_lock(&f->lock);
>          int ret2 = f->ops->close(f->opaque);
> +        qemu_mutex_unlock(&f->lock);

OK, and at least for the RDMA code, if it calls
close a 2nd time, rioc->rdma is checked so it wont actually free stuff a
2nd time.

>          if (ret >= 0) {
>              ret = ret2;
>          }
> @@ -339,6 +343,7 @@ int qemu_fclose(QEMUFile *f)
>      if (f->last_error) {
>          ret = f->last_error;
>      }
> +    qemu_mutex_destroy(&f->lock);
>      g_free(f);

Hmm but that's not safe; if two things really do call qemu_fclose()
on the same structure they race here and can end up destroying the lock
twice, or doing f->lock  after the 1st one has already g_free(f).


So lets go back a step.
I think:
  a) There should always be a separate QEMUFile* for
     to_src_file and from_src_file - I don't see where you open
     the 2nd one; I don't see your implementation of
     f->ops->get_return_path.
  b) I *think* that while the different threads might all call
     fclose(), I think there should only ever be one qemu_fclose
     call for each direction on the QEMUFile.

But now we have two problems:
  If (a) is true then f->lock  is separate on each one so
   doesn't really protect if the two directions are closed
   at once. (Assuming (b) is true)

  If (a) is false and we actually share a single QEMUFile then
 that race at the end happens.

Dave


>      trace_qemu_file_fclose();
>      return ret;
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK