[Gluster-devel] aclaration about local stat structs

Anand Avati avati at zresearch.com
Mon Jan 21 00:28:14 UTC 2008

> I found on posix  writev that stbuf is local , but a ptr to it is returned
> to caller.

The callback path of transaction is not actually C stack unwinds. So passing
pointers from the stack to STACK_WIND and STACK_UNWIND is fine, as long as
the fops ahead do not store the pointer for 'later use'.

> posix_writev (call_frame_t *frame,
>               xlator_t *this,
>               fd_t *fd,
>               struct iovec *vector,
>               int32_t count,
>               off_t offset)
> {
>   int32_t op_ret;
>   int32_t op_errno;
>   int32_t _fd;
>   struct posix_private *priv = this->private;
>   data_t *pfd_data = dict_get (fd->ctx, this->name);
>   struct posix_fd *pfd;
>   struct stat stbuf = {0,};
> ...
> ...
>   STACK_UNWIND (frame, op_ret, op_errno, &stbuf);
> }
> Upper modules (for example my pet QUOTA) declare args as:
> static int32_t
> quota_writev_cbk (call_frame_t *frame,
>                   void *cookie,
>                   xlator_t *this,
>                   int32_t op_ret,
>                   int32_t op_errno,
>                   struct stat *buf)
> Shouldnt be stat declared as 'struct stat const *buf'?


also as stat is local to posix_writev can two or more threads hold a ponter
> to it while serving diferent requests and thus cause a race condition??

As I mentioned previously, no fops are supposed to store the pointer. they
can only immediately STACK_WIND/STACK_UNWIND with the pointer (essentially
meaning the pointer usage scope is within the same thread only).

We plan to capture all these nitty-gritty details in the hackers guide.
thanks for your interesting questions.


If I traveled to the end of the rainbow
As Dame Fortune did intend,
Murphy would be there to tell me
The pot's at the other end.

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