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Re: [Xen-devel] [PATCH v6 3/4] xen/rcu: add assertions to debug build

On 17.03.20 15:36, Jan Beulich wrote:
On 13.03.2020 14:06, Juergen Gross wrote:
Xen's RCU implementation relies on no softirq handling taking place
while being in a RCU critical section. Add ASSERT()s in debug builds
in order to catch any violations.

For that purpose modify rcu_read_[un]lock() to use a dedicated percpu
counter additional to preempt_[en|dis]able() as this enables to test
that condition in __do_softirq() (ASSERT_NOT_IN_ATOMIC() is not
usable there due to __cpu_up() calling process_pending_softirqs()
while holding the cpu hotplug lock).

While at it switch the rcu_read_[un]lock() implementation to static
inline functions instead of macros.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one remark:

@@ -91,16 +114,23 @@ typedef struct _rcu_read_lock rcu_read_lock_t;
   * will be deferred until the outermost RCU read-side critical section
   * completes.
- * It is illegal to block while in an RCU read-side critical section.
+ * It is illegal to process softirqs while in an RCU read-side critical 

The latest with the re-added preempt_disable(), wouldn't this better
say "... to process softirqs or block ..."?

I can add this, but OTOH blocking without processing softirqs is not
possible, as there is no other (legal) way to enter the scheduler.


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