Re: [BUG] srcu: false-positive WARN in cleanup_srcu_struct() after 78a38cbf6f20

["Zqiang" <qiang.zhang@linux.dev>]

> 
> On 8/27/26 08:53, Zqiang wrote:
> 
> > 
> > Based on your description below, the 5.srcu_barrier() did not intercept the callback of 4.call_srcu(),
> >  this means that 4.call_srcu() and 5.srcu_barrier() concurrent calls, or calls after 5.srcu_barrier().
> >  The combination of srcu_barrier() and cleanup_srcu_struct() typically occurs on the module exit path.
> >  srcu_barrier() ensures that all previously inserted callbacks complete，so therefore, theoretically,
> >  there shouldn't be any callbacks left to execute after we finish executing srcu_barrier().
> >  and of course, there are also shouldn't be any ongoing or newly started SRCU grace period and the
> >  WARN_ON() in cleanup_srcu_struct() is designed to detect it.
> >  If when the cleanup_srcu_struct() detects an incomplete SRCU grace period or any srcu callbacks
> >  that have not yet been executed, this is a risk that needs to be reported, even if the
> >  cleanup_srcu_struct() can intercept it internally.
> >  > Therefore, we should investigate this issue to ensure that 5.srcu_barrier() can intercept the step 4 callback.
> >  Thanks
> >  Zqiang
> > 
> Before 5. srcu_barrier() call, the 4. call_srcu() is finished and the callback was enqueued, and srcu_barrier() did intercept it by appending barrier cb after it. They can't be called concurrently because when call_srcu() is called inside kvm_io_bus_register_dev(), the kvm reference count is nonzero and kvm_destroy_vm() which calls srcu_barrier() could not be started.
>

Which tree is your test based on? (rcu tree or linux-next tree)

The step 4 call_srcu() is finished and then the step5 call srcu_barrier(), it failed to intercept it?
(theoretically, this shouldn't happen)

 
> In my debugging, all five __free_bus() callbacks enqueued by kvm_io_bus_register_dev() were invoked before the barrier cb, and only then the barrier cb is called and srcu_barrier() returns. So all the real callbacks were executed and practically there were no callbacks left to execute when srcu_barrier() returns, but a stale n_cbs > 0 is left because srcu_invoke_callbacks()'s invoking loop has not finished.
> 
> KVM logic correctly called srcu_barrier() and cleanup_srcu_struct() without calling call_srcu() in between. In my opinion, the root causes are as follows:
> 
> 1) for timer_delete_sync(): the srcu_gp_end() which ended the last grace period (from 4. ) arms sdp->delay_work to expire at jiffies + 1 even though the invoke work (queued in 3. ) is already queued, and cleanup_srcu_struct() runs before that one-jiffy timer expires, so timer_delete_sync() cancels that pending timer and returns true.
> 
> 2) for n_cbs: srcu_invoke_callbacks() calls rcu_segcblist_add_len(-len) only at the end of the work item, after the barrier callback has been invoked and srcu_barrier() has already returned, so cleanup_srcu_struct() observes a stale n_cbs > 0 while the cblist is physically empty.

because you say the 5.srcu_barrier() can not intercept setp4 call_srcu() enqueue callback func, 
that means that the step4 srcu callback shuold not be run, the cblist also not empty.

Thanks
Zqiang 

> 
> Thanks
> Sunho Park
>