maria-developers team mailing list archive

[Sergey Vojtovich <svoj@xxxxxxxxxxx>]

Hi Sergei,

On Thu, May 07, 2015 at 01:04:24PM +0200, Sergei Golubchik wrote:
> Hi, Sergey!
> 
> On Mar 17, svoj@xxxxxxxxxxx wrote:
> > revision-id: 93e746afb2d1a23cb933291ad736a20858b5ac3e
> > parent(s): ccc7297fe94af1129c717f91d31fa075d54a0371
> > committer: Sergey Vojtovich
> > branch nick: mariadb
> > timestamp: 2015-03-17 19:49:04 +0400
> > message:
> > 
> > MDEV-7793 - Race condition between XA COMMIT/ROLLBACK and disconnect
> > 
> > XA COMMIT/ROLLBACK of XA transaction owned by different thread may access
> > freed memory if that thread disconnects at the same time.
> > 
> > Also concurrent XA COMMIT/ROLLBACK of recovered XA transaction were not
> > serialized properly.
> > 
> > diff --git a/sql/sql_class.cc b/sql/sql_class.cc
> > index 3071ba6..f18231d 100644
> > --- a/sql/sql_class.cc
> > +++ b/sql/sql_class.cc
> > @@ -5355,7 +5378,7 @@ bool xid_cache_insert(THD *thd, XID_STATE *xid_state)
> >    switch (res)
> >    {
> >    case 0:
> > -    xid_state->xid_cache_element->mark_initialized();
> > +    xid_state->xid_cache_element->set(XID_cache_element::ACQUIRED);
> 
> Why not mark_acquired() here?
mark_acquired() is intended to acquire RECOVERED elements. That is to allow
multiple threads to attempt to acquire the same XID. One thread will succeed,
others will get "not found".

> Can you be sure that no other thread has already acquired this
> xid_cache_element after you've inserted it?
Only RECOVERED elements can be acquired. This newly inserted element has
m_state == 0.

Thanks,
Sergey