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

On 5/28/21 5:11 PM, Ahmed S. Darwish wrote:

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

quoted

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>

...

quoted

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.

Thanks, corrected it in v2.

Kind regards,

--
Ahmed S. Darwish
Linutronix GmbH

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