Am 9/25/2024 um 1:36 PM schrieb Mathieu Desnoyers:
On 2024-09-25 12:06, Jonas Oberhauser wrote:
Am 9/25/2024 um 8:35 AM schrieb Mathieu Desnoyers:
On 2024-09-25 07:57, Jonas Oberhauser wrote:
Hi Mathieu,
I haven't read your code in detail but it seems to me you have an
ABA bug: as I explained elsewhere, you could read the same pointer
after ABA but you don't synchronize with the newer store that gave
you node2, leaving you to speculatively read stale values through
*ctx->hp.
(I am assuming here that ctx->hp is essentially an out parameter
used to let the caller know which node got protected).
The following change should fix it:
cmm_barrier();
- node2 = uatomic_load(node_p, CMM_RELAXED); /* Load A */
+ node2 = rcu_dereference(*node_p); /* Load A */
I don't think this fixes it, because IIRC rcu_dereference relies on
the address dependency (which we don't have here) to provide ordering.
I would recommend either:
- ctx->hp = node;
+ ctx->hp = node2;
which fixes the problem under the perhaps too weak assumption that the
compiler doesn't use its knowledge that node==node2 to just undo this
fix, or more strictly,
As stated in Documentation/RCU/rcu_dereference.rst from the Linux
kernel, comparing the result of rcu_dereference against another
non-NULL pointer is discouraged, as you rightly point out.
+ ctx->hp = READ_ONCE(node2);
which I believe makes sure that the value of node2 is used.
I am not entirely sure this extra READ_ONCE() would be sufficient
to prevent the compiler from making assumptions about the content
of node2 and thus use the result of the first load (node) instead.
It would also not suffice to prevent the CPU from speculatively
using the result of the first load to perform dependent loads AFAIU.
The reason I think it should be sufficient is that it forces the
compiler to assume that since the comparison, the value of node2 might
have changed.
Therefore, simply using the value of node at that point should be
unsound from the compiler's POV.
But I'm not a compiler expert... So I definitely support uneasiness
about this construct :))
jonas
