[RFC PATCH 00/16] Replace smp_read_barrier_depends() with lockless_derefrence()

[RFC PATCH 00/16] Replace smp_read_barrier_depends() with lockless_derefrence()

[Pranith Kumar]

Recently lockless_dereference() was added which can be used in place of
hard-coding smp_read_barrier_depends(). 

http://lkml.iu.edu/hypermail/linux/kernel/1410.3/04561.html

The following series tries to do this.

There are still some hard-coded locations which I was not sure how to replace
with. I will send in separate patches/questions regarding them.

Pranith Kumar (16):
  crypto: caam - Remove unnecessary smp_read_barrier_depends()
  doc: memory-barriers.txt: Document use of lockless_dereference()
  drivers: dma: Replace smp_read_barrier_depends() with
    lockless_dereference()
  dcache: Replace smp_read_barrier_depends() with lockless_dereference()
  overlayfs: Replace smp_read_barrier_depends() with
    lockless_dereference()
  assoc_array: Replace smp_read_barrier_depends() with
    lockless_dereference()
  hyperv: Replace smp_read_barrier_depends() with lockless_dereference()
  rcupdate: Replace smp_read_barrier_depends() with
    lockless_dereference()
  percpu: Replace smp_read_barrier_depends() with lockless_dereference()
  perf: Replace smp_read_barrier_depends() with lockless_dereference()
  seccomp: Replace smp_read_barrier_depends() with
    lockless_dereference()
  task_work: Replace smp_read_barrier_depends() with
    lockless_dereference()
  ksm: Replace smp_read_barrier_depends() with lockless_dereference()
  slab: Replace smp_read_barrier_depends() with lockless_dereference()
  netfilter: Replace smp_read_barrier_depends() with
    lockless_dereference()
  rxrpc: Replace smp_read_barrier_depends() with lockless_dereference()

 Documentation/memory-barriers.txt |  2 +-
 drivers/crypto/caam/jr.c          |  3 ---
 drivers/dma/ioat/dma_v2.c         |  3 +--
 drivers/dma/ioat/dma_v3.c         |  3 +--
 fs/dcache.c                       |  7 ++-----
 fs/overlayfs/super.c              |  4 +---
 include/linux/assoc_array_priv.h  | 11 +++++++----
 include/linux/hyperv.h            |  9 ++++-----
 include/linux/percpu-refcount.h   |  4 +---
 include/linux/rcupdate.h          | 10 +++++-----
 kernel/events/core.c              |  3 +--
 kernel/events/uprobes.c           |  8 ++++----
 kernel/seccomp.c                  |  7 +++----
 kernel/task_work.c                |  3 +--
 lib/assoc_array.c                 |  7 -------
 mm/ksm.c                          |  7 +++----
 mm/slab.h                         |  6 +++---
 net/ipv4/netfilter/arp_tables.c   |  3 +--
 net/ipv4/netfilter/ip_tables.c    |  3 +--
 net/ipv6/netfilter/ip6_tables.c   |  3 +--
 net/rxrpc/ar-ack.c                | 22 +++++++++-------------
 security/keys/keyring.c           |  6 ------
 22 files changed, 50 insertions(+), 84 deletions(-)

-- 
1.9.1

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[PATCH 13/16] ksm: Replace smp_read_barrier_depends() with lockless_dereference()

[Pranith Kumar]

Recently lockless_dereference() was added which can be used in place of
hard-coding smp_read_barrier_depends(). The following PATCH makes the change.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 mm/ksm.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index d247efa..a67de79 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -542,15 +542,14 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
 	expected_mapping = (void *)stable_node +
 				(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
 again:
-	kpfn = ACCESS_ONCE(stable_node->kpfn);
-	page = pfn_to_page(kpfn);
-
 	/*
 	 * page is computed from kpfn, so on most architectures reading
 	 * page->mapping is naturally ordered after reading node->kpfn,
 	 * but on Alpha we need to be more careful.
 	 */
-	smp_read_barrier_depends();
+	kpfn = lockless_dereference(stable_node->kpfn);
+	page = pfn_to_page(kpfn);
+
 	if (ACCESS_ONCE(page->mapping) != expected_mapping)
 		goto stale;
 
-- 
1.9.1

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[PATCH 14/16] slab: Replace smp_read_barrier_depends() with lockless_dereference()

[Pranith Kumar]

Recently lockless_dereference() was added which can be used in place of
hard-coding smp_read_barrier_depends(). The following PATCH makes the change.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
---
 mm/slab.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/slab.h b/mm/slab.h
index 3347fd7..1cf40054 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -209,15 +209,15 @@ cache_from_memcg_idx(struct kmem_cache *s, int idx)
 
 	rcu_read_lock();
 	params = rcu_dereference(s->memcg_params);
-	cachep = params->memcg_caches[idx];
-	rcu_read_unlock();
 
 	/*
 	 * Make sure we will access the up-to-date value. The code updating
 	 * memcg_caches issues a write barrier to match this (see
 	 * memcg_register_cache()).
 	 */
-	smp_read_barrier_depends();
+	cachep = lockless_dereference(params->memcg_caches[idx]);
+	rcu_read_unlock();
+
 	return cachep;
 }
 
-- 
1.9.1

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Re: [RFC PATCH 00/16] Replace smp_read_barrier_depends() with lockless_derefrence()

[Paul E. McKenney]

On Thu, Nov 13, 2014 at 02:24:06PM -0500, Pranith Kumar wrote:
> Recently lockless_dereference() was added which can be used in place of
> hard-coding smp_read_barrier_depends(). 
> 
> http://lkml.iu.edu/hypermail/linux/kernel/1410.3/04561.html
> 
> The following series tries to do this.
> 
> There are still some hard-coded locations which I was not sure how to replace
> with. I will send in separate patches/questions regarding them.

Thank you for taking this on!  Some questions and comments in response
to the individual patches.

							Thanx, Paul

> Pranith Kumar (16):
>   crypto: caam - Remove unnecessary smp_read_barrier_depends()
>   doc: memory-barriers.txt: Document use of lockless_dereference()
>   drivers: dma: Replace smp_read_barrier_depends() with
>     lockless_dereference()
>   dcache: Replace smp_read_barrier_depends() with lockless_dereference()
>   overlayfs: Replace smp_read_barrier_depends() with
>     lockless_dereference()
>   assoc_array: Replace smp_read_barrier_depends() with
>     lockless_dereference()
>   hyperv: Replace smp_read_barrier_depends() with lockless_dereference()
>   rcupdate: Replace smp_read_barrier_depends() with
>     lockless_dereference()
>   percpu: Replace smp_read_barrier_depends() with lockless_dereference()
>   perf: Replace smp_read_barrier_depends() with lockless_dereference()
>   seccomp: Replace smp_read_barrier_depends() with
>     lockless_dereference()
>   task_work: Replace smp_read_barrier_depends() with
>     lockless_dereference()
>   ksm: Replace smp_read_barrier_depends() with lockless_dereference()
>   slab: Replace smp_read_barrier_depends() with lockless_dereference()
>   netfilter: Replace smp_read_barrier_depends() with
>     lockless_dereference()
>   rxrpc: Replace smp_read_barrier_depends() with lockless_dereference()
> 
>  Documentation/memory-barriers.txt |  2 +-
>  drivers/crypto/caam/jr.c          |  3 ---
>  drivers/dma/ioat/dma_v2.c         |  3 +--
>  drivers/dma/ioat/dma_v3.c         |  3 +--
>  fs/dcache.c                       |  7 ++-----
>  fs/overlayfs/super.c              |  4 +---
>  include/linux/assoc_array_priv.h  | 11 +++++++----
>  include/linux/hyperv.h            |  9 ++++-----
>  include/linux/percpu-refcount.h   |  4 +---
>  include/linux/rcupdate.h          | 10 +++++-----
>  kernel/events/core.c              |  3 +--
>  kernel/events/uprobes.c           |  8 ++++----
>  kernel/seccomp.c                  |  7 +++----
>  kernel/task_work.c                |  3 +--
>  lib/assoc_array.c                 |  7 -------
>  mm/ksm.c                          |  7 +++----
>  mm/slab.h                         |  6 +++---
>  net/ipv4/netfilter/arp_tables.c   |  3 +--
>  net/ipv4/netfilter/ip_tables.c    |  3 +--
>  net/ipv6/netfilter/ip6_tables.c   |  3 +--
>  net/rxrpc/ar-ack.c                | 22 +++++++++-------------
>  security/keys/keyring.c           |  6 ------
>  22 files changed, 50 insertions(+), 84 deletions(-)
> 
> -- 
> 1.9.1
> 

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Re: [PATCH 13/16] ksm: Replace smp_read_barrier_depends() with lockless_dereference()

[Hugh Dickins]

On Thu, 13 Nov 2014, Pranith Kumar wrote:

> Recently lockless_dereference() was added which can be used in place of
> hard-coding smp_read_barrier_depends(). The following PATCH makes the change.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>

Sorry, I don't think your patch is buggy, but I do think it
makes this tricky piece of code harder to follow, not easier.

It is certainly not a standard use of lockless_dereference() (kpfn
is not a pointer), and it both hides and moves where the barrier is.
And then at the end of the function, there's still explicit barriers
and ACCESS_ONCE comparison with kpfn, which this makes more obscure.

Unless you are actually fixing a bug (I don't pretend to have
tested this on Alpha, and I can get barriers wrong as we all do),
or smp_read_barrier_depends() is about to be withdrawn from use,
I'd rather say NAK to this patch.

Hugh

> ---
>  mm/ksm.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index d247efa..a67de79 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -542,15 +542,14 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
>  	expected_mapping = (void *)stable_node +
>  				(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
>  again:
> -	kpfn = ACCESS_ONCE(stable_node->kpfn);
> -	page = pfn_to_page(kpfn);
> -
>  	/*
>  	 * page is computed from kpfn, so on most architectures reading
>  	 * page->mapping is naturally ordered after reading node->kpfn,
>  	 * but on Alpha we need to be more careful.
>  	 */
> -	smp_read_barrier_depends();
> +	kpfn = lockless_dereference(stable_node->kpfn);
> +	page = pfn_to_page(kpfn);
> +
>  	if (ACCESS_ONCE(page->mapping) != expected_mapping)
>  		goto stale;
>  
> -- 
> 1.9.1

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