Am 06.08.2015 um 17:54 schrieb Rainer Jung:
> I think the root cause is here (file subversion/libsvn_ra_svn/marshal.c):
>
> 1082 /* Allocate an APR array with room for (initially) 4 items.
> 1083 * We do this manually because lists are the most
> frequent protocol
> 1084 * element, often used to frame a single, optional value.
> We save
> 1085 * about 20% of total protocol handling time. */
> 1086 char *buffer = apr_palloc(pool, sizeof(apr_array_header_t)
> 1087 + 4 *
> sizeof(svn_ra_svn_item_t));
> 1088 svn_ra_svn_item_t *data
> 1089 = (svn_ra_svn_item_t *)(buffer +
> sizeof(apr_array_header_t));
> 1090
> 1091 item->kind = SVN_RA_SVN_LIST;
> 1092 item->u.list = (apr_array_header_t *)buffer;
>
> "buffer" is not specifically aligned, the array members in "item->u.list
> = (apr_array_header_t *)buffer" could be misaligned.
>
> The following (ugly) workaround fixes it for me:
>
> --- subversion/libsvn_ra_svn/marshal.c.kpdt_orig Fri Feb 13
> 12:17:40 2015
> +++ subversion/libsvn_ra_svn/marshal.c Thu Aug 6 17:46:58 2015
> @@ -1083,10 +1083,16 @@
> * We do this manually because lists are the most frequent protocol
> * element, often used to frame a single, optional value. We save
> * about 20% of total protocol handling time. */
> - char *buffer = apr_palloc(pool, sizeof(apr_array_header_t)
> +
> + /* Make sure the data part of the buffer has appropriate alignment
> + by prefixing it with a size that fits the needed
> apr_array_header_t
> + but is itself highly aligned. */
> + size_t offset = sizeof(apr_array_header_t) / 8 * 8;
> +
> + char *buffer = apr_palloc(pool, offset
> + 4 * sizeof(svn_ra_svn_item_t));
> svn_ra_svn_item_t *data
> - = (svn_ra_svn_item_t *)(buffer + sizeof(apr_array_header_t));
> + = (svn_ra_svn_item_t *)(buffer + offset);
>
> item->kind = SVN_RA_SVN_LIST;
> item->u.list = (apr_array_header_t *)buffer;
>
> But of course its a bit rough, because it would apply on all platforms,
> even if not needed. Also on some (future?) platforms, the alignment for
> 8 bytes might not always be correct.
>
> It's a bit tragic that this code part is prefixed with:
>
> * We do this manually because lists are the most frequent protocol
> * element, often used to frame a single, optional value. We save
> * about 20% of total protocol handling time. */
>
> and the trap is that doing it manually often is harder than expected.
> Switching to apr_array_make() would have not introduced this bug, but of
> course you did it for a reason.
The switch was introduced in r1485851. It is not part of 1.7 or 1.8.
Regards,
Rainer
Received on 2015-08-06 18:04:15 CEST