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Sunho Park
26f90559a7
The WARN_ON() added by commit78a38cbf6f("srcu: Queue sdp->work when the delay timer is successfully deleted") uses rcu_segcblist_n_cbs() to detect callbacks that srcu_barrier() failed to wait for. However, the ->len counter is decremented only at the end of srcu_invoke_callbacks(), after the invoking loop has finished. Since srcu_barrier() can return right after the barrier callback is invoked, cleanup_srcu_struct() can see a non-zero n_cbs even though the cblist is already physically empty, falsely triggering the WARN_ON() together with a still-pending delay_work timer. This can be triggered as follows, as seen in the syzbot report against kvm_destroy_vm() -> cleanup_srcu_struct(&kvm->srcu): 1. call_srcu(&kvm->srcu, &bus->rcu, __free_bus) starts SRCU grace period GP1. 2. GP1 ends: a delay timer is armed and sdp->work is queued, but sdp->work has not run yet. 3. Another call_srcu(&kvm->srcu, &bus->rcu, __free_bus) call invokes srcu_segcblist_advance(), which moves the GP1 callback to RCU_DONE_TAIL, and starts SRCU grace period GP2. 4. srcu_barrier() is called. It queues its barrier callback after the GP2 callback and waits for srcu_invoke_callbacks() to invoke it. 5. GP2 ends: another delay timer is armed, and the sdp->work queued in step 2 begins to run. Its srcu_invoke_callbacks() call invokes srcu_segcblist_advance() again, moving the GP2 and barrier callbacks to RCU_DONE_TAIL as well, and then invokes all of them. However, rcu_segcblist_add_len(), which updates srcu_cblist's ->len, has not run yet at this point. 6. srcu_barrier() returns once its callback has been invoked, and cleanup_srcu_struct() starts running. It finds the delay timer armed in step 5 still pending and srcu_cblist's ->len still non-zero (because step 5 has not reached rcu_segcblist_add_len() yet), and WARN_ON() fires even though every callback has actually been invoked. Use rcu_segcblist_empty(), which checks the actual head of the cblist, instead of rcu_segcblist_n_cbs(), which checks the racy ->len counter. Callbacks that have genuinely not been invoked yet still leave the list non-empty, so the WARN_ON() still catches callers that skip srcu_barrier() or queue callbacks after it. Link: https://lore.kernel.org/rcu/[email protected]/T/#t Reported-by: [email protected] Closes: https://syzkaller.appspot.com/bug?extid=d4faf7db59e11f6fd1ab Fixes:78a38cbf6f("srcu: Queue sdp->work when the delay timer is successfully deleted") Suggested-by: Zqiang <[email protected]> Signed-off-by: Sunho Park <[email protected]> Signed-off-by: Paul E. McKenney <[email protected]> Tested-by: Sean Christopherson <[email protected]>