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

[Sunho Park <shpark061104@gmail.com>]

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.

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.

Thanks
Sunho Park