[PATCH v2 12/23] nscd: Rewrite __nscd_cache_search using <concurrent_buffer.h>

[Carlos O'Donell]

On 3/20/26 4:42 PM, Florian Weimer wrote:
> This makes it somewhat clearer what is going on.  This still
> does not implement a proper software TM snapshot.

LGTM.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>   nscd/nscd_helper.c | 85 ++++++++++++++++++----------------------------
>   1 file changed, 33 insertions(+), 52 deletions(-)
> 
> diff --git a/nscd/nscd_helper.c b/nscd/nscd_helper.c
> index 2a60c3572e..9bd0e7818e 100644
> --- a/nscd/nscd_helper.c
> +++ b/nscd/nscd_helper.c
> @@ -41,6 +41,7 @@
>   #include "nscd-client.h"
>   #include "nscd-dbtype.h"
>   #include "nscd_proto.h"
> +#include "concurrent_buffer.h"
>   
>   /* Extra time we wait if the socket is still receiving data.  This
>      value is in milliseconds.  Note that the other side is nscd on the
> @@ -527,71 +528,51 @@ __nscd_cache_search (request_type type, const char *key, size_t keylen,
>   		     const struct mapped_database *mapped, size_t datalen)
>   {
>     unsigned long int hash = __nss_hash (key, keylen) % mapped->head->module;
> -  size_t datasize = mapped->datasize;
> +
> +  struct concurrent_buffer cb = cb_create (mapped->data, mapped->datasize);

OK. Create it for the mapping at the known size (no larger).

>   
>     ref_t trail = mapped->head->array[hash];
>     ref_t work = trail;
> -  size_t loop_cnt = datasize / (MINIMUM_HASHENTRY_SIZE
> -				+ offsetof (struct datahead, data) / 2);
> +  size_t loop_cnt = mapped->datasize / (MINIMUM_HASHENTRY_SIZE
> +					+ offsetof (struct datahead, data) / 2);

OK.

>     int tick = 0;
>   
> -  while (work != ENDREF && work + MINIMUM_HASHENTRY_SIZE <= datasize)
> +  while (work != ENDREF)

OK. Cleanup using the buffer no need to check for going past the end.

>       {
> -      struct hashentry *here = (struct hashentry *) (mapped->data + work);
> -      ref_t here_key, here_packet;
> -
> -      /* Although during garbage collection when moving struct hashentry
> -	 records around we first copy from old to new location and then
> -	 adjust pointer from previous hashentry to it, there is no barrier
> -	 between those memory writes!!! This is extremely risky on any
> -	 modern CPU which can reorder memory accesses very aggressively.
> -	 Check alignment, both as a partial consistency check and to avoid
> -	 crashes on targets which require atomic loads to be aligned.  */
> -      if ((uintptr_t) here & (__alignof__ (*here) - 1))
> -	return NULL;

OK. Removing the old comment :-)

> -
> -      if (type == here->type
> -	  && keylen == here->len
> -	  && (here_key = atomic_load_relaxed (&here->key)) + keylen <= datasize
> -	  && memcmp (key, mapped->data + here_key, keylen) == 0
> -	  && ((here_packet = atomic_load_relaxed (&here->packet))
> -	      + sizeof (struct datahead) <= datasize))
> +      /* First compare type and key.  */
> +      if (type == cb_field (&cb, work, struct hashentry, type)
> +	  && keylen == cb_field (&cb, work, struct hashentry, len))

OK.

>   	{
> -	  /* We found the entry.  Increment the appropriate counter.  */
> -	  struct datahead *dh
> -	    = (struct datahead *) (mapped->data + here_packet);
> -
> -	  if ((uintptr_t) dh & (__alignof__ (*dh) - 1))
> -	    return NULL;
> -
> -	  /* See whether we must ignore the entry or whether something
> -	     is wrong because garbage collection is in progress.  */
> -	  if (dh->usable
> -	      && here_packet + dh->allocsize <= datasize
> -	      && (here_packet + offsetof (struct datahead, data) + datalen
> -		  <= datasize))
> -	    return dh;
> +	  ref_t here_key = cb_field (&cb, work, struct hashentry, key);
> +	  if (cb_memeq (&cb, here_key, key, keylen))
> +	    {
> +	      /* Data found.  Now validate the data reference.  */
> +	      ref_t here_packet = cb_field (&cb, work,
> +					    struct hashentry, packet);
> +	      if ((here_packet & (__alignof__ (struct datahead) - 1)) == 0
> +		  && cb_field (&cb, here_packet, struct datahead, usable))
> +		{
> +		  size_t allocsize = cb_field (&cb, here_packet,
> +					       struct datahead, allocsize);
> +		  if (cb_available (&cb, here_packet, allocsize))
> +		      /* We found the entry.  */
> +		      return (struct datahead *) (mapped->data + here_packet);
> +		}
> +	    }

OK.

>   	}
>   
> -      work = atomic_load_relaxed (&here->next);
> +      work = cb_field (&cb, work, struct hashentry, next);
> +      if (cb_has_failed (&cb))
> +	return NULL;
> +
>         /* Prevent endless loops.  This should never happen but perhaps
> -	 the database got corrupted, accidentally or deliberately.  */
> +	 the database got corrupted, accidentally or deliberately.
> +	 Use both a loop count and a Floyd-style cycle detector
> +	 (work is the hare, and trail is the tortoise).  */

OK. Yes, Floyd-style.

>         if (work == trail || loop_cnt-- == 0)
>   	break;
>         if (tick)
> -	{
> -	  struct hashentry *trailelem;
> -	  trailelem = (struct hashentry *) (mapped->data + trail);
> -
> -	  /* We have to redo the checks.  Maybe the data changed.  */
> -	  if ((uintptr_t) trailelem & (__alignof__ (*trailelem) - 1))
> -	    return NULL;
> -
> -	  if (trail + MINIMUM_HASHENTRY_SIZE > datasize)
> -	    return NULL;
> -
> -	  trail = atomic_load_relaxed (&trailelem->next);
> -	}
> +	trail = cb_field (&cb, trail, struct hashentry, next);
>         tick = 1 - tick;
>       }
>   


-- 
Cheers,
Carlos.