> > 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 completeļ¼so > > 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 >
