Stefan Fuhrmann wrote on Sun, Jun 26, 2011 at 23:00:24 +0200:
> On 11.06.2011 14:53, Daniel Shahaf wrote:
> >Stefan Fuhrmann wrote on Sat, Jun 11, 2011 at 10:13:27 +0200:
> >>On 10.06.2011 22:28, Daniel Shahaf wrote:
> >>>stefan2_at_apache.org wrote on Wed, May 25, 2011 at 22:20:25 -0000:
> >>>>Author: stefan2
> >>>>Date: Wed May 25 22:20:25 2011
> >>>>New Revision: 1127709
> >>>>
> >>>>URL: http://svn.apache.org/viewvc?rev=1127709&view=rev
> >>>>Log:
> >>>>Fix a pool usage issue: svn_cache__get_partial() may be called many
> >>>>times in a row. Thus, the internal pool used to construct keys should
> >>>>be cleared in this function as well from time to time.
> >>>>
> >>>>* subversion/libsvn_subr/cache-membuffer.c
> >>>> (svn_membuffer_cache_get_partial): regularly clear the internal scratch pool
> >>>>
> >>>>Modified:
> >>>> subversion/trunk/subversion/libsvn_subr/cache-membuffer.c
> >>>>
> >>>>Modified: subversion/trunk/subversion/libsvn_subr/cache-membuffer.c
> >>>>URL: http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_subr/cache-membuffer.c?rev=1127709&r1=1127708&r2=1127709&view=diff
> >>>>==============================================================================
> >>>>--- subversion/trunk/subversion/libsvn_subr/cache-membuffer.c (original)
> >>>>+++ subversion/trunk/subversion/libsvn_subr/cache-membuffer.c Wed May 25 22:20:25 2011
> >>>>@@ -1668,6 +1668,12 @@ svn_membuffer_cache_get_partial(void **v
> >>>>
> >>>> DEBUG_CACHE_MEMBUFFER_INIT_TAG
> >>>>
> >>>>+ if (++cache->alloc_counter> ALLOCATIONS_PER_POOL_CLEAR)
> >>>>+ {
> >>>>+ apr_pool_clear(cache->pool);
> >>>>+ cache->alloc_counter = 0;
> >>>>+ }
> >>>>+
> >>>Does this need to be guarded by a cache lock?
> >>>
> >>No. This happens in the outer / front-end code
> >>that merely adds a key prefix (combine_key below)
> >>before calling the shared cache object.
> >>
> >>All front-end operations assume single-threaded
> >>access, which should be o.k. for fs_t-local objects.
> >Okay, if that function is guaranteed (perhaps by API contract) not to
> >run concurrently to any other `front end' function (and in particular to
> >itself), then my concerns are resolved.
> >
> >(I was worried about accessing cache->pool and cache->alloc_counter from
> >multiple threads concurrently --- writer-writer or writer-reader ---
> >which might to undefined behaviour.)
> >
> >What guarantees the single-threaded access? I don't see it documented
> >in svn_cache.h (on the contrary, that one has explicit 'thread_safe'
> >parameters) and the code doesn't take a lock at at that point either.
> >
> I implemented optional thread-safety some days ago but it
> turned out to require / suggest a new API for svn_mutex_t
> structures (handles NULL, APR w/o threading etc.)
> Otherwise, there would be even more duplicated mutex
> handling code.
>
> However, to keep API churn low for 1.7 stabilization, I like
> to suggest the following: simply adding a thread_safe
> parameter to the membuffer cache constructor API and
> returning "not supported / implemented" if set to TRUE.
>
In other words, you're suggesting to have *all* membuffer caches declare
themselves as non-thread-safe in 1.7?
Assuming the cache users are fine with that, it solves the issue
I raised, and all these are private API's, so +1.
Also: creating the membuffer singleton respects the public API's config
singleton's SINGLE_THREADED parameter... so we'd have to somehow
"ignore" that parameter (and rely on FSFS-level mutexes?). Hmm.
> -- Stefan^2.
Received on 2011-06-27 13:55:38 CEST