Re: [PATCH updated] libnvdimm/nvdimm/flush: Allow architecture to override the flush barrier

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Jun 30, 2020 at 5:48 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
>
>
> Update patch.
>
> From 1e6aa6c4182e14ec5d6bf878ae44c3f69ebff745 Mon Sep 17 00:00:00 2001
> From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> Date: Tue, 12 May 2020 20:58:33 +0530
> Subject: [PATCH] libnvdimm/nvdimm/flush: Allow architecture to override the
>  flush barrier
>
> Architectures like ppc64 provide persistent memory specific barriers
> that will ensure that all stores for which the modifications are
> written to persistent storage by preceding dcbfps and dcbstps
> instructions have updated persistent storage before any data
> access or data transfer caused by subsequent instructions is initiated.
> This is in addition to the ordering done by wmb()
>
> Update nvdimm core such that architecture can use barriers other than
> wmb to ensure all previous writes are architecturally visible for
> the platform buffer flush.

Looks good, after a few minor fixups below you can add:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

I'm expecting that these will be merged through the powerpc tree since
they mostly impact powerpc with only minor touches to libnvdimm.

> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> ---
>  Documentation/memory-barriers.txt | 14 ++++++++++++++
>  drivers/md/dm-writecache.c        |  2 +-
>  drivers/nvdimm/region_devs.c      |  8 ++++----
>  include/asm-generic/barrier.h     | 10 ++++++++++
>  4 files changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index eaabc3134294..340273a6b18e 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1935,6 +1935,20 @@ There are some more advanced barrier functions:
>       relaxed I/O accessors and the Documentation/DMA-API.txt file for more
>       information on consistent memory.
>
> + (*) pmem_wmb();
> +
> +     This is for use with persistent memory to ensure that stores for which
> +     modifications are written to persistent storage have updated the persistent
> +     storage.

I think this should be:

s/updated the persistent storage/reached a platform durability domain/

> +
> +     For example, after a non-temporal write to pmem region, we use pmem_wmb()
> +     to ensures that stores have updated the persistent storage. This ensures

s/ensures/ensure/

...and the same comment about "persistent storage" because pmem_wmb()
as implemented on x86 does not guarantee that the writes have reached
storage it ensures that writes have reached buffers / queues that are
within the ADR (platform persistence / durability) domain.

> +     that stores have updated persistent storage before any data access or
> +     data transfer caused by subsequent instructions is initiated. This is
> +     in addition to the ordering done by wmb().
> +
> +     For load from persistent memory, existing read memory barriers are sufficient
> +     to ensure read ordering.
>
>  ===============================
>  IMPLICIT KERNEL MEMORY BARRIERS
> diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
> index 74f3c506f084..00534fa4a384 100644
> --- a/drivers/md/dm-writecache.c
> +++ b/drivers/md/dm-writecache.c
> @@ -536,7 +536,7 @@ static void ssd_commit_superblock(struct dm_writecache *wc)
>  static void writecache_commit_flushed(struct dm_writecache *wc, bool wait_for_ios)
>  {
>         if (WC_MODE_PMEM(wc))
> -               wmb();
> +               pmem_wmb();
>         else
>                 ssd_commit_flushed(wc, wait_for_ios);
>  }
> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> index 4502f9c4708d..2333b290bdcf 100644
> --- a/drivers/nvdimm/region_devs.c
> +++ b/drivers/nvdimm/region_devs.c
> @@ -1206,13 +1206,13 @@ int generic_nvdimm_flush(struct nd_region *nd_region)
>         idx = this_cpu_add_return(flush_idx, hash_32(current->pid + idx, 8));
>
>         /*
> -        * The first wmb() is needed to 'sfence' all previous writes
> -        * such that they are architecturally visible for the platform
> -        * buffer flush.  Note that we've already arranged for pmem
> +        * The first arch_pmem_flush_barrier() is needed to 'sfence' all

One missed arch_pmem_flush_barrier() rename.

> +        * previous writes such that they are architecturally visible for
> +        * the platform buffer flush. Note that we've already arranged for pmem
>          * writes to avoid the cache via memcpy_flushcache().  The final
>          * wmb() ensures ordering for the NVDIMM flush write.
>          */
> -       wmb();
> +       pmem_wmb();
>         for (i = 0; i < nd_region->ndr_mappings; i++)
>                 if (ndrd_get_flush_wpq(ndrd, i, 0))
>                         writeq(1, ndrd_get_flush_wpq(ndrd, i, idx));
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index 2eacaf7d62f6..879d68faec1d 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -257,5 +257,15 @@ do {                                                                       \
>  })
>  #endif
>
> +/*
> + * pmem_barrier() ensures that all stores for which the modification

One missed pmem_barrier() conversion.

> + * are written to persistent storage by preceding instructions have
> + * updated persistent storage before any data  access or data transfer
> + * caused by subsequent instructions is initiated.
> + */
> +#ifndef pmem_wmb
> +#define pmem_wmb()     wmb()
> +#endif
> +
>  #endif /* !__ASSEMBLY__ */
>  #endif /* __ASM_GENERIC_BARRIER_H */
> --
> 2.26.2
>
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