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

On 27.03.20 09:35, Jan Beulich wrote:
On 27.03.2020 09:10, Jürgen Groß wrote:
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@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
- add RCU_SOFTIRQ to ignore in process_pending_softirqs_norcu()
    (Roger Pau Monné)

- block rcu processing depending on rch_read_lock() being held or not
    (Jan Beulich)


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?

Igor, are you perhaps running without "rcu: add assertions to debug
build"? I think this actually fixes what you describe. Without it
rcu_barrier(), in its second loop, calling process_pending_softirqs(),
would cause the RCU softirq to not be invoked anymore with preemption
disabled. Of course the title of this change doesn't reflect this at

Right. This explains why I don't see the hang on my test system.

Jürgen, as an aside, while looking at the code again, I think the
comment near the end of process_pending_softirqs() would now rather
belong at its very beginning; should have spotted this while

Oh, indeed. Will send a patch.




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