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)
> 

Sorry, here I missed.

>  
> 
> > 
> > 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.

The step5 srcu_barrier() has been intercept the srcu callback which by step4 call_srcu() insert,
so at here, the srcu_barrier should not be return, and the step4 srcu callback should not be run.
the cleanup_srcu_struct() also should not be called.


Thanks
Zqiang


> >  
> >  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
> >
>