Re: [PATCH nf] x_tables: Properly close read section with read_seqcount_retry

[Florian Westphal <fw@strlen.de>]

Sean Tranchetti <stranche@codeaurora.org> wrote:
> xtables uses a modified version of a seqcount lock for synchronization
> between replacing private table information and the packet processing
> path (i.e. XX_do_table). The main modification is in the "write"
> semantics, where instead of adding 1 for each write, the write side will
> add 1 only if it finds no other writes ongoing, and adds 1 again (thereby
> clearing the LSB) when it finishes.
> 
> This allows the read side to avoid calling read_seqcount_begin() in a loop
> if a write is detected, since the value is guaranteed to only increment
> once all writes have completed. As such, it is only necessary to check if
> the initial value of the sequence count has changed to inform the reader
> that all writes are finished.
> 
> However, the current check for the changed value uses the wrong API;
> raw_seqcount_read() is protected by smp_rmb() in the same way as
> read_seqcount_begin(), making it appropriate for ENTERING read-side
> critical sections, but not exiting them. For that, read_seqcount_rety()
> must be used to ensure the proper barrier placement for synchronization
> with xt_write_recseq_end() (itself modeled after write_seqcount_end()).

[..]

> diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
> index af22dbe..39f1f2b 100644
> --- a/net/netfilter/x_tables.c
> +++ b/net/netfilter/x_tables.c
> @@ -1404,7 +1404,7 @@ xt_replace_table(struct xt_table *table,
>  			do {
>  				cond_resched();
>  				cpu_relax();
> -			} while (seq == raw_read_seqcount(s));
> +			} while (!read_seqcount_retry(s, seq));

I'm fine with this change but AFAIU this is just a cleanup since
this part isn't a read-sequence as no  'shared state' is accessed/read between
the seqcount begin and the do{}while. smb_rmb placement should not matter here.

Did I miss anything?

Thanks.