Re: [PATCH 09/19] srcu: Fix WARN_ON() for rcu_segcblist_n_cbs() in cleanup_srcu_struct()

[Sean Christopherson <seanjc@google.com>]

On Fri, Sep 18, 2026, Paul E. McKenney wrote:
> From: Sunho Park <shpark061104@gmail.com>
> 
> The WARN_ON() added by commit 78a38cbf6f20 ("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/e6350377085ddd85d6ef00d8e9a67bd50c762d3c@linux.dev/T/#t
> Reported-by: syzbot+d4faf7db59e11f6fd1ab@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=d4faf7db59e11f6fd1ab
> Fixes: 78a38cbf6f20 ("srcu: Queue sdp->work when the delay timer is successfully deleted")
> Suggested-by: Zqiang <qiang.zhang@linux.dev>
> Signed-off-by: Sunho Park <shpark061104@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---

Is this going to be grabbed for 7.3?  It shows up relatively frequently in KVM.
Not so much that it's truly problematic/alarming, but enough that it definitely
stands out.

Tested-by: Sean Christopherson <seanjc@google.com>