Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
From: Herbert Xu Date: Mon, 19 Nov 2018 12:06:34 +0800 > On Mon, Nov 19, 2018 at 11:56:35AM +0800, Herbert Xu wrote: >> >> I take that back. Because of your shift which cancels out the >> shift in NULLS_MARKER, it would appear that this should work just >> fine with RHT_NULLS_MARRKER(0), no? IOW, it would appear that >> >> RHT_NULLS_MARKER(0) = RHT_NULLS_MARKER(RHT_NULLS_MARKER(0)) > > My emails to Neil are bouncing: > > ne...@suse.com > host smtp.glb1.softwaregrp.com [15.124.2.87] > SMTP error from remote mail server after RCPT TO:: > 550 Cannot process address Yeah this just started happening 2 days ago.
Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
From: Herbert Xu Date: Mon, 19 Nov 2018 12:06:34 +0800 > On Mon, Nov 19, 2018 at 11:56:35AM +0800, Herbert Xu wrote: >> >> I take that back. Because of your shift which cancels out the >> shift in NULLS_MARKER, it would appear that this should work just >> fine with RHT_NULLS_MARRKER(0), no? IOW, it would appear that >> >> RHT_NULLS_MARKER(0) = RHT_NULLS_MARKER(RHT_NULLS_MARKER(0)) > > My emails to Neil are bouncing: > > ne...@suse.com > host smtp.glb1.softwaregrp.com [15.124.2.87] > SMTP error from remote mail server after RCPT TO:: > 550 Cannot process address Yeah this just started happening 2 days ago.
Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
On Mon, Nov 19, 2018 at 11:56:35AM +0800, Herbert Xu wrote: > > I take that back. Because of your shift which cancels out the > shift in NULLS_MARKER, it would appear that this should work just > fine with RHT_NULLS_MARRKER(0), no? IOW, it would appear that > > RHT_NULLS_MARKER(0) = RHT_NULLS_MARKER(RHT_NULLS_MARKER(0)) My emails to Neil are bouncing: ne...@suse.com host smtp.glb1.softwaregrp.com [15.124.2.87] SMTP error from remote mail server after RCPT TO:: 550 Cannot process address Cheers, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
On Mon, Nov 19, 2018 at 11:56:35AM +0800, Herbert Xu wrote: > > I take that back. Because of your shift which cancels out the > shift in NULLS_MARKER, it would appear that this should work just > fine with RHT_NULLS_MARRKER(0), no? IOW, it would appear that > > RHT_NULLS_MARKER(0) = RHT_NULLS_MARKER(RHT_NULLS_MARKER(0)) My emails to Neil are bouncing: ne...@suse.com host smtp.glb1.softwaregrp.com [15.124.2.87] SMTP error from remote mail server after RCPT TO:: 550 Cannot process address Cheers, -- Email: Herbert Xu Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
On Mon, Nov 19, 2018 at 11:54:15AM +0800, Herbert Xu wrote:
>
> > >> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> > >> index 30526afa8343..852ffa5160f1 100644
> > >> --- a/lib/rhashtable.c
> > >> +++ b/lib/rhashtable.c
> > >> @@ -1179,8 +1179,7 @@ struct rhash_head __rcu **rht_bucket_nested(const
> > >> struct bucket_table *tbl,
> > >> unsigned int hash)
> > >> {
> > >> const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
> > >> -static struct rhash_head __rcu *rhnull =
> > >> -(struct rhash_head __rcu *)NULLS_MARKER(0);
> > >> +static struct rhash_head __rcu *rhnull;
> > >
> > > I don't understand why you can't continue to do NULLS_MARKER(0) or
> > > RHT_NULLS_MARKER(0).
> >
> > Because then the test
> >
> > + } while (he != RHT_NULLS_MARKER(head));
> >
> > in __rhashtable_lookup() would always succeed, and it would loop
> > forever.
>
> This change is only necessary because of your shifting change
> above, which AFAICS adds no real benefit.
I take that back. Because of your shift which cancels out the
shift in NULLS_MARKER, it would appear that this should work just
fine with RHT_NULLS_MARRKER(0), no? IOW, it would appear that
RHT_NULLS_MARKER(0) = RHT_NULLS_MARKER(RHT_NULLS_MARKER(0))
Thanks,
--
Email: Herbert Xu
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
On Mon, Nov 19, 2018 at 11:54:15AM +0800, Herbert Xu wrote:
>
> > >> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> > >> index 30526afa8343..852ffa5160f1 100644
> > >> --- a/lib/rhashtable.c
> > >> +++ b/lib/rhashtable.c
> > >> @@ -1179,8 +1179,7 @@ struct rhash_head __rcu **rht_bucket_nested(const
> > >> struct bucket_table *tbl,
> > >> unsigned int hash)
> > >> {
> > >> const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
> > >> -static struct rhash_head __rcu *rhnull =
> > >> -(struct rhash_head __rcu *)NULLS_MARKER(0);
> > >> +static struct rhash_head __rcu *rhnull;
> > >
> > > I don't understand why you can't continue to do NULLS_MARKER(0) or
> > > RHT_NULLS_MARKER(0).
> >
> > Because then the test
> >
> > + } while (he != RHT_NULLS_MARKER(head));
> >
> > in __rhashtable_lookup() would always succeed, and it would loop
> > forever.
>
> This change is only necessary because of your shifting change
> above, which AFAICS adds no real benefit.
I take that back. Because of your shift which cancels out the
shift in NULLS_MARKER, it would appear that this should work just
fine with RHT_NULLS_MARRKER(0), no? IOW, it would appear that
RHT_NULLS_MARKER(0) = RHT_NULLS_MARKER(RHT_NULLS_MARKER(0))
Thanks,
--
Email: Herbert Xu
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
On Fri, Nov 16, 2018 at 05:59:19PM +1100, NeilBrown wrote:
>
> NULLS_MARKER assumes a hash value in which the bottom bits are most
> likely to be unique. To convert this to a pointer which certainly not
> valid, it shifts left by 1 and sets the lsb.
> We aren't passing a hash value, but are passing an address instead.
> In this case the bottom 2 bits are certain to be 0, and the top bit
> could contain valuable information (on a 32bit system).
> The best way to turn a pointer into a certainly-invalid pointer
> is to just set the lsb. By shifting right by one, we discard an
> uninteresting bit, preserve all the interesting bits, and effectively
> just set the lsb.
>
> I could add a comment explaining that if you like.
The top-bit is most likely to be fixed and offer no real value.
> >> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> >> index 30526afa8343..852ffa5160f1 100644
> >> --- a/lib/rhashtable.c
> >> +++ b/lib/rhashtable.c
> >> @@ -1179,8 +1179,7 @@ struct rhash_head __rcu **rht_bucket_nested(const
> >> struct bucket_table *tbl,
> >>unsigned int hash)
> >> {
> >>const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
> >> - static struct rhash_head __rcu *rhnull =
> >> - (struct rhash_head __rcu *)NULLS_MARKER(0);
> >> + static struct rhash_head __rcu *rhnull;
> >
> > I don't understand why you can't continue to do NULLS_MARKER(0) or
> > RHT_NULLS_MARKER(0).
>
> Because then the test
>
> + } while (he != RHT_NULLS_MARKER(head));
>
> in __rhashtable_lookup() would always succeed, and it would loop
> forever.
This change is only necessary because of your shifting change
above, which AFAICS adds no real benefit.
Cheers,
--
Email: Herbert Xu
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
Re: [PATCH] rhashtable: detect when object movement between tables might have invalidated a lookup
On Fri, Nov 16, 2018 at 05:59:19PM +1100, NeilBrown wrote:
>
> NULLS_MARKER assumes a hash value in which the bottom bits are most
> likely to be unique. To convert this to a pointer which certainly not
> valid, it shifts left by 1 and sets the lsb.
> We aren't passing a hash value, but are passing an address instead.
> In this case the bottom 2 bits are certain to be 0, and the top bit
> could contain valuable information (on a 32bit system).
> The best way to turn a pointer into a certainly-invalid pointer
> is to just set the lsb. By shifting right by one, we discard an
> uninteresting bit, preserve all the interesting bits, and effectively
> just set the lsb.
>
> I could add a comment explaining that if you like.
The top-bit is most likely to be fixed and offer no real value.
> >> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> >> index 30526afa8343..852ffa5160f1 100644
> >> --- a/lib/rhashtable.c
> >> +++ b/lib/rhashtable.c
> >> @@ -1179,8 +1179,7 @@ struct rhash_head __rcu **rht_bucket_nested(const
> >> struct bucket_table *tbl,
> >>unsigned int hash)
> >> {
> >>const unsigned int shift = PAGE_SHIFT - ilog2(sizeof(void *));
> >> - static struct rhash_head __rcu *rhnull =
> >> - (struct rhash_head __rcu *)NULLS_MARKER(0);
> >> + static struct rhash_head __rcu *rhnull;
> >
> > I don't understand why you can't continue to do NULLS_MARKER(0) or
> > RHT_NULLS_MARKER(0).
>
> Because then the test
>
> + } while (he != RHT_NULLS_MARKER(head));
>
> in __rhashtable_lookup() would always succeed, and it would loop
> forever.
This change is only necessary because of your shifting change
above, which AFAICS adds no real benefit.
Cheers,
--
Email: Herbert Xu
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
