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

["Jürgen Groß" <jgross@suse.com>]

On 27.03.20 00:24, Igor Druzhinin wrote:
> 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@suse.com>
>> Reviewed-by: Jan Beulich <jbeulich@suse.com>
>> ---
>> 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

Why would that block RCU callback processing?

RCU callbacks should be blocked only if a rcu lock is being held.

Did I miss something in my patches?

> all system hang on boot after 540d4d60378 ("cpu: sync any remaining
> RCU callbacks before CPU up/down").


Juergen