Re: inet: frags: Turn fqdir->dead into an int for old Alphas

[Eric Dumazet <eric.dumazet@gmail.com>]

On 6/7/19 7:09 AM, Herbert Xu wrote:
> On Tue, Jun 04, 2019 at 09:04:55AM -0700, Linus Torvalds wrote:
>>
>> In fact, the alpha port was always subtly buggy exactly because of the
>> "byte write turns into a read-and-masked-write", even if I don't think
>> anybody ever noticed (we did fix cases where people _did_ notice,
>> though, and we might still have some cases where we use 'int' for
>> booleans because of alpha issues.).
> 
> This is in fact a real bug in the code in question that no amount
> of READ_ONCE/WRITE_ONCE would have caught.  The field fqdir->dead is
> declared as boolean so writing to it is not atomic (on old Alphas).
> 
> I don't think it currently matters because padding would ensure
> that it is in fact 64 bits long.  However, should someone add another
> char/bool/bitfield in this struct in future it could become an issue.
> 
> So let's fix it.


There is common knowledge among us programmers that bit fields
(or bool) sharing a common 'word' need to be protected
with a common lock.

Converting all bit fields to plain int/long would be quite a waste of memory.

In this case, fqdir_exit() is called right before the whole
struct fqdir is dismantled, and the only cpu that could possibly
change the thing is ourself, and we are going to start an RCU grace period.

Note that first cache line in 'struct fqdir' is read-only.
Only ->dead field is flipped to one at exit time.

Your patch would send a strong signal to programmers to not even try using
bit fields.

Do we really want that ?

> 
> ---8<--
> The field fqdir->dead is meant to be written (and read) atomically.
> As old Alpha CPUs can't write a single byte atomically, we need at
> least an int for it to work.
> 
> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
> 
> diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
> index e91b79ad4e4a..8c458fba74ad 100644
> --- a/include/net/inet_frag.h
> +++ b/include/net/inet_frag.h
> @@ -14,7 +14,9 @@ struct fqdir {
>  	int			max_dist;
>  	struct inet_frags	*f;
>  	struct net		*net;
> -	bool			dead;
> +
> +	/* We can't use boolean because this needs atomic writes. */
> +	int			dead;
>  
>  	struct rhashtable       rhashtable ____cacheline_aligned_in_smp;
>  
> diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
> index 35e9784fab4e..05aa7c145817 100644
> --- a/net/ipv4/inet_fragment.c
> +++ b/net/ipv4/inet_fragment.c
> @@ -193,7 +193,7 @@ void fqdir_exit(struct fqdir *fqdir)
>  {
>  	fqdir->high_thresh = 0; /* prevent creation of new frags */
>  
> -	fqdir->dead = true;
> +	fqdir->dead = 1;
>  
>  	/* call_rcu is supposed to provide memory barrier semantics,
>  	 * separating the setting of fqdir->dead with the destruction
>