Re: [Xen-devel] [PATCH v8 3/5] xen: don't process rcu callbacks when holding a rcu_read_lock()

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 26/03/2020 09:19, Juergen Gross wrote:
> Some keyhandlers are calling process_pending_softirqs() while holding
> a rcu_read_lock(). This is wrong, as process_pending_softirqs() might
> activate rcu calls which should not happen inside a rcu_read_lock().
> 
> For that purpose modify process_pending_softirqs() to not allow rcu
> callback processing when a rcu_read_lock() is being held.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> V3:
> - add RCU_SOFTIRQ to ignore in process_pending_softirqs_norcu()
>   (Roger Pau Monné)
> 
> V5:
> - block rcu processing depending on rch_read_lock() being held or not
>   (Jan Beulich)

Juergen,

Our BVT revealed a likely problem with this commit in that form.
Since 12509bbeb9e ("rwlocks: call preempt_disable() when taking a rwlock")
preemption is disabled after taking cpu_maps which will block RCU
callback processing inside rcu_barrier itself. This will result in
all system hang on boot after 540d4d60378 ("cpu: sync any remaining
RCU callbacks before CPU up/down").

Igor