Re: [PATCH] dlm: wait for outstanding SRCU callbacks to complete in exit paths

[Zqiang]

> 
> Hi,
> 
> On Wed, Sep 2, 2026 at 10:30 AM Zqiang <qiang.zhang@xxxxxxxxx> wrote:
> 
> > 
> > >
> >  > Hi,
> >  >
> >  > On Tue, Sep 1, 2026 at 5:19 AM Zqiang <qiang.zhang@xxxxxxxxx> wrote:
> >  >
> >  >
> >  > The dlm_lowcomms_exit() and dlm_midcomms_exit() iterate over the
> >  > srcu protected connection and node hash tables and hand each
> >  > element to call_srcu() for deferred freeing (connection_release()
> >  > and midcomms_node_release()). call_srcu() is asynchronous: the
> >  > callbacks are invoked only after an SRCU grace period, which may
> >  > happen after the exit function has already returned.
> >  >
> >  > These exit functions are reached from exit_dlm() on module unload.
> >  > Once they return, module teardown continues and the module text
> >  > may be unloaded while call_srcu() callbacks are still pending. When
> >  > such a callback finally runs, it executes freed module code and
> >  > touches the static SRCU domains that are being torn down, resulting
> >  > in a use-after-free.
> > 
> >  I should rephrase it:
> > 
> >  Due to missed call to the srcu_barrier(), when invoke cleanup_srcu_struct()
> > 
> Then there need to be a MUST call of srcu_barrier() before
> cleanup_srcu_struct() every time.

If there are some call_srcu() call, the srcu_barrier() should be called before cleanup_srcu_struct().
usually, when the cleanup_srcu_struct() run, we should ensure that there are no pending SRCU callbacks
or ongoing SRCU grace periods at this time.

there are another reason:

The midcomms_node_release() srcu callback can call call_rcu(), we must wait the all
call_rcu() to complete, because rcu_barrier() which in dlm_memory_exit() is required
to successfully intercept these RCU callbacks.

Thanks
Zqiang

> 
> I am confused here.
> 
> - Alex
>