Messages in this thread | | | Date | Wed, 8 Nov 2023 10:07:58 +0100 | From | Sabrina Dubroca <> | Subject | Re: Missing a write memory barrier in tls_init() |
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2023-11-07, 18:53:24 -0800, Jakub Kicinski wrote: > On Tue, 7 Nov 2023 23:45:46 +0100 Sabrina Dubroca wrote: > > Wouldn't it be enough to just move the rcu_assign_pointer after ctx is > > fully initialized, ie just before update_sk_prot? also clearer wrt > > RCU. > > I'm not sure, IIUC rcu_assign_pointer() is equivalent to > WRITE_ONCE() on any sane architecture, it depends on address > dependencies to provide ordering.
Not what the doc says:
/** * rcu_assign_pointer() - assign to RCU-protected pointer [...] * Inserts memory barriers on architectures that require them * (which is most of them), and also prevents the compiler from * reordering the code that initializes the structure after the pointer * assignment. [...] */
And it uses smp_store_release (unless writing NULL).
rcu_dereference is the one that usually doesn't contain a barrier:
/** * rcu_dereference_check() - rcu_dereference with debug checking [...] * Inserts memory barriers on architectures that require them * (currently only the Alpha), prevents the compiler from refetching * (and from merging fetches), and, more importantly, documents exactly * which pointers are protected by RCU and checks that the pointer is * annotated as __rcu. */
> Since here we care about > ctx->sk_prot being updated, when changes to sk->sk_prot > are visible there is no super-obvious address dependency. > > There may be one. But to me at least it isn't an obvious > "RCU used right will handle this" case.
Ok, I think you're right. Looking at smp_store_release used by rcu_assign_pointer:
#define __smp_store_release(p, v) \ do { \ compiletime_assert_atomic_type(*p); \ barrier(); \ WRITE_ONCE(*p, v); \ } while (0)
it's only going to make sure ctx->sk_proto is set when ctx is visible, and not guarantee that ctx is visible whenever sk->sk_prot has been switched over.
-- Sabrina
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