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Date: Tue, 2 Aug 2022 08:37:31 -0700 From: Jakub Kicinski <kuba@...nel.org> To: Maxim Mikityanskiy <maximmi@...dia.com> Cc: "davem@...emloft.net" <davem@...emloft.net>, Tariq Toukan <tariqt@...dia.com>, Gal Pressman <gal@...dia.com>, "john.fastabend@...il.com" <john.fastabend@...il.com>, "pabeni@...hat.com" <pabeni@...hat.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "edumazet@...gle.com" <edumazet@...gle.com>, Saeed Mahameed <saeedm@...dia.com>, Boris Pismenny <borisp@...dia.com> Subject: Re: [PATCH net-next] net/tls: Use RCU API to access tls_ctx->netdev On Tue, 2 Aug 2022 12:03:32 +0000 Maxim Mikityanskiy wrote: > > For cases like this where we don't actually hold onto the object, just > > take a peek at the address of it we can save a handful of LoC by using > > rcu_access_pointer(). > > The documentation of rcu_access_pointer says it shouldn't be used on > the update side, because we lose lockdep protection: > > --cut-- > > Although rcu_access_pointer() may also be used in cases > where update-side locks prevent the value of the pointer from changing, > you should instead use rcu_dereference_protected() for this use case. I think what this is trying to say is to not use the rcu_access_pointer() as a hack against lockdep: lock(writer_lock); /* no need for rcu_dereference() because we have writer lock */ ptr = rcu_access_pointer(obj->ptr); ptr->something = 1; unlock(writer_lock); It's still perfectly fine to use access_pointer as intended on the write side, which is just checking the value of the pointer, not deferencing it: lock(writer_lock); if (rcu_access_pointer(obj->ptr) == target) so_something(obj); unlock(writer_lock);
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