Re: [PATCH net-next] tcp: make smp_rmb() conditional in tcp_poll()

[Simon Horman <horms@kernel.org>]

On Sun, Sep 13, 2026 at 12:32:24PM +0000, Eric Dumazet wrote:
> Commit a4d258036ed9 ("tcp: Fix race in tcp_poll") added smp_rmb() in
> tcp_poll() and smp_wmb() in tcp_reset() (now tcp_done_with_error())
> to ensure that if tcp_poll() observed socket closure, it would also
> observe sk->sk_err.
> 
> Currently, tcp_poll() unconditionally executes smp_rmb() at the end
> of every invocation, which on weakly-ordered architectures such as ARM64
> emits a memory barrier instruction (dmb ishld) on the poll fast path,
> even for healthy, active sockets.
> 
> However, tcp_poll() only needs this barrier if socket closure has been
> observed, to ensure that the error code set by tcp_done_with_error()
> before socket closure is visible before returning EPOLLERR.
> 
> Move smp_rmb() inside the conditional block handling socket closure
> (shutdown == SHUTDOWN_MASK || state == TCP_CLOSE). For healthy
> connected sockets in epoll, tcp_poll() avoids the barrier entirely.
> 
> Signed-off-by: Eric Dumazet <edumazet@google.com>

Reviewed-by: Simon Horman <horms@kernel.org>