linux-kernel - Re: Question about the barrier() in hlist_nulls_for_each_entry

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Message-ID: <CAEXW_YS_raHUrvVAFPpnhL2PRH0hkcqT=1hD+gQOg_cMLkGrjQ@mail.gmail.com>
Date:   Fri, 21 Jul 2023 13:14:40 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Alan Huang <mmpgouride@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "roman.gushchin@...ux.dev" <roman.gushchin@...ux.dev>
Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()

On Fri, Jul 21, 2023 at 11:59 AM David Laight <David.Laight@...lab.com> wrote:
>
> ....
> > Right, it shouldn't need to cache. To Eric's point it might be risky to remove
> > the barrier() and someone needs to explain that issue first (or IMO there needs
> > to be another tangible reason like performance etc). Anyway, FWIW I wrote a
> > simple program and I am not seeing the head->first cached with the pattern you
> > shared above:
> >
> > #include <stdlib.h>
> >
> > #define READ_ONCE(x) (*(volatile typeof(x) *)&(x))
> > #define barrier() __asm__ __volatile__("": : :"memory")
> >
> > typedef struct list_head {
> >      int first;
> >      struct list_head *next;
> > } list_head;
> >
> > int main() {
> >      list_head *head = (list_head *)malloc(sizeof(list_head));
> >      head->first = 1;
> >      head->next = 0;
> >
> >      READ_ONCE(head->first);
> >      barrier();
> >      READ_ONCE(head->first);
> >
> >      free(head);
> >      return 0;
> > }
>
> You probably need to try harder to generate the error.
> It probably has something to do code surrounding the
> sk_nulls_for_each_rcu() in the ca065d0c^ version of udp.c.
>
> That patch removes the retry loop - and probably breaks udp receive.
> The issue is that sockets can be moved between the 'hash2' chains
> (eg by connect()) without being freed.

I was just replying to Alan's question on the behavior of READ_ONCE()
since I myself recently got surprised by compiler optimizations
related to it. I haven't looked into the actual UDP code.

 - Joel