Re: [PATCH] xfrm: policy: Read seqcount outside of rcu-read side in xfrm_policy_lookup_bytype

On Fri, May 28, 2021, Varad Gautam wrote: ...

Thead 1 (xfrm_hash_resize) Thread 2 (xfrm_policy_lookup_bytype)

		rcu_read_lock();

mutex_lock(&hash_resize_mutex); read_seqcount_begin(&xfrm_policy_hash_generation); mutex_lock(&hash_resize_mutex); // block xfrm_bydst_resize(); synchronize_rcu(); // block <RCU stalls in xfrm_policy_lookup_bytype>

...

Fixes: a7c44247f70 ("xfrm: policy: make xfrm_policy_lookup_bytype lockless")

Minor note: the 'Fixes' commit should be 77cc278f7b20 ("xfrm: policy: Use sequence counters with associated lock") instead.

The reason read_seqcount_begin() is emitting a mutex_lock() on PREEMPT_RT is because of the s/seqcount_t/seqcount_mutex_t/ change.

Kind regards,

-- Ahmed S. Darwish Linutronix GmbH