>
> >
> > Hi,
> >
> > On Tue, Sep 1, 2026 at 5:19 AM Zqiang <[email protected]> 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()
and find there are still pending srcu callback, the WARN_ON() will trigger.
from kernel/rcu/srcutree.c in cleanup_srcu_struct():
// Call srcu_barrier() before this cleanup_srcu_struct()
// to avoid triggering this WARN_ON().
if (WARN_ON(timer_delete_sync(&sdp->delay_work) &&
rcu_segcblist_n_cbs(&sdp->srcu_cblist)) &&
rcu_cpu_beenfullyonline(sdp->cpu))
queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
flush_work(&sdp->work);
Thanks
Zqiang
> >
> > I thought again about this and in my opinion this is not possible as
> > it is already being handled by DEFINE_STATIC_SRCU() with a cleanup
> > handling when the module is unloaded.
> >
> When the moudle unload, the srcu_module_going() will call
> cleanup_srcu_struct()
> and free_percpu(ssp->sda) to release resource. but we not call srcu_barrier(),
> the srcu_barrier() should be called before cleanup_srcu_struct().
>
> >
> > I know that srcu subsystem does a lot of magic with modules init/exit
> > functionality to call init_srcu_struct() and cleanup_srcu_struct().
> > See
> >
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/module/main.c?h=v7.3-rc1#n2711
> >
> > so this patch should be reverted. If they don't use a barrier there,
> > the fix should be in the SRCU subsystem, but I believe the current
> > SRCU implementation already handles this.
> >
> The srcu_barrier() need to be called, there are some description
> from Documentation/RCU/rcubarrier.rst:
>
> rcu_barrier()
> -------------
>
> This situation can be handled by the rcu_barrier() primitive. Rather
> than waiting for a grace period to elapse, rcu_barrier() waits for all
> outstanding RCU callbacks to complete. Please note that rcu_barrier()
> does **not** imply synchronize_rcu(), in particular, if there are no RCU
> callbacks queued anywhere, rcu_barrier() is within its rights to return
> immediately, without waiting for anything, let alone a grace period.
>
> Pseudo-code using rcu_barrier() is as follows:
>
> 1. Prevent any new RCU callbacks from being posted.
> 2. Execute rcu_barrier().
> 3. Allow the module to be unloaded.
>
> There is also an srcu_barrier() function for SRCU, and you of course
> must match the flavor of srcu_barrier() with that of call_srcu().
> If your module uses multiple srcu_struct structures, then it must also
> use multiple invocations of srcu_barrier() when unloading that module.
> For example, if it uses call_rcu(), call_srcu() on srcu_struct_1, and
> call_srcu() on srcu_struct_2, then the following three lines of code
> will be required when unloading::
>
> 1 rcu_barrier();
> 2 srcu_barrier(&srcu_struct_1);
> 3 srcu_barrier(&srcu_struct_2);
> ....
>
> Thanks
> Zqiang
>
> >
> > - Alex
> >
> >
> > Add an srcu_barrier() after the call_srcu() loop in each exit function
> > to wait for all outstanding callbacks of the respective SRCU domain to
> > complete before returning. In dlm_midcomms_exit() the barrier is issued
> > before dlm_lowcomms_exit() so that node callbacks are drained prior to
> > tearing down the lower layer.
> >
> > Signed-off-by: Zqiang <[email protected]>
> > ---
> > fs/dlm/lowcomms.c | 1 +
> > fs/dlm/midcomms.c | 1 +
> > 2 files changed, 2 insertions(+)
> >
> > diff --git a/fs/dlm/lowcomms.c b/fs/dlm/lowcomms.c
> > index 2aff1c7c17de..ea8353c4638d 100644
> > --- a/fs/dlm/lowcomms.c
> > +++ b/fs/dlm/lowcomms.c
> > @@ -1984,4 +1984,5 @@ void dlm_lowcomms_exit(void)
> > }
> > }
> > srcu_read_unlock(&connections_srcu, idx);
> > + srcu_barrier(&connections_srcu);
> > }
> > diff --git a/fs/dlm/midcomms.c b/fs/dlm/midcomms.c
> > index 8964164600d2..045431524494 100644
> > --- a/fs/dlm/midcomms.c
> > +++ b/fs/dlm/midcomms.c
> > @@ -1178,6 +1178,7 @@ void dlm_midcomms_exit(void)
> > }
> > }
> > srcu_read_unlock(&nodes_srcu, idx);
> > + srcu_barrier(&nodes_srcu);
> >
> > dlm_lowcomms_exit();
> > }
> > --
> > 2.17.1
> >
>
