Greg Stein wrote:
[...]
I gave up trying to do full reference counting semantics for
duplicates of apr_mmap_t, because I couldn't find a suitable
pool to own the locks that would be required in a threaded
implementation.
Not sure what you mean here. I don't quite understand how threading comes
into play. apr_file_t and apr_mmap_t can't be used by multiple threads,
AFAIK. And I don't think they should either...
When I tried to implement a traditional reference-counting solution
(all apr_mmap_t's duplicated from the same original share a reference
count, and the mmap'ed segment is destroyed only when the refcount
reaches zero), I ran into threading problems with mod_file_cache.
It's possible for two threads to be doing a dup of the same
cached apr_mmap_t at the same time--if they're both delivering the
same statically cached file, and they both end up having to call
mmap_setaside. To avoid the race condition here, we'd need a lock
around the reference count. The lock API requires that we create the
lock from a pool, but there isn't a really suitable pool from which
to allocate the lock, given that its lifetime isn't necessarily tied
to that of a request or connection.
Fortunately, this whole problem went away when I switched from the
reference-counted model to the transfer-of-ownership model. The latter
doesn't require any locking.
...
--- srclib/apr/mmap/win32/mmap.c 2001/06/27 22:07:24 1.6
+++ srclib/apr/mmap/win32/mmap.c 2001/11/15 05:45:58
@@ -62,10 +62,15 @@
#if APR_HAS_MMAP
-static apr_status_t mmap_cleanup(void *themmap)
+APR_DECLARE(apr_status_t) apr_mmap_cleanup(void *themmap)
{
Why is this APR_DECLARE'd now? And why the function rename?
This is an error left over from an earlier design approach that I tried
and abandoned. Thanks for catching that. I've attached a new version
of the patch that fixes it.
--Brian
Index: srclib/apr/include/apr_mmap.h
===================================================================
RCS file: /home/cvspublic/apr/include/apr_mmap.h,v
retrieving revision 1.25
diff -u -r1.25 apr_mmap.h
--- srclib/apr/include/apr_mmap.h 2001/08/18 15:15:46 1.25
+++ srclib/apr/include/apr_mmap.h 2001/11/16 04:22:14
@@ -111,6 +111,8 @@
void *mm;
/** The amount of data in the mmap */
apr_size_t size;
+ /** Whether this object is reponsible for doing the munmap */
+ int is_owner;
};
#if APR_HAS_MMAP || defined(DOXYGEN)
@@ -134,6 +136,19 @@
apr_file_t *file, apr_off_t offset,
apr_size_t size, apr_int32_t flag,
apr_pool_t *cntxt);
+
+/**
+ * Duplicate the specified MMAP.
+ * @param new_mmap The structure to duplicate into.
+ * @param old_mmap The file to duplicate.
+ * @param p The pool to use for the new file.
+ * @param transfer_ownership Whether responsibility for destroying
+ * the memory-mapped segment is transferred from old_mmap to new_mmap
+ */
+APR_DECLARE(apr_status_t) apr_mmap_dup(apr_mmap_t **new_mmap,
+ apr_mmap_t *old_mmap,
+ apr_pool_t *p,
+ int transfer_ownership);
/**
* Remove a mmap'ed.
Index: srclib/apr/mmap/unix/mmap.c
===================================================================
RCS file: /home/cvspublic/apr/mmap/unix/mmap.c,v
retrieving revision 1.36
diff -u -r1.36 mmap.c
--- srclib/apr/mmap/unix/mmap.c 2001/08/02 04:26:53 1.36
+++ srclib/apr/mmap/unix/mmap.c 2001/11/16 04:22:14
@@ -83,6 +83,11 @@
{
apr_mmap_t *mm = themmap;
int rv;
+
+ if (!mm->is_owner) {
+ return APR_SUCCESS;
+ }
+
#ifdef BEOS
rv = delete_area(mm->area);
@@ -159,10 +164,37 @@
(*new)->mm = mm;
(*new)->size = size;
(*new)->cntxt = cont;
+ (*new)->is_owner = 1;
/* register the cleanup... */
apr_pool_cleanup_register((*new)->cntxt, (void*)(*new), mmap_cleanup,
apr_pool_cleanup_null);
+ return APR_SUCCESS;
+}
+
+APR_DECLARE(apr_status_t) apr_mmap_dup(apr_mmap_t **new_mmap,
+ apr_mmap_t *old_mmap,
+ apr_pool_t *p,
+ int transfer_ownership)
+{
+ *new_mmap = (apr_mmap_t *)apr_pmemdup(p, old_mmap, sizeof(apr_mmap_t));
+ (*new_mmap)->cntxt = p;
+
+ /* The old_mmap can transfer ownership only if the old_mmap itself
+ * is an owner of the mmap'ed segment.
+ */
+ if (old_mmap->is_owner) {
+ if (transfer_ownership) {
+ (*new_mmap)->is_owner = 1;
+ old_mmap->is_owner = 0;
+ apr_pool_cleanup_kill(old_mmap->cntxt, old_mmap, mmap_cleanup);
+ }
+ else {
+ (*new_mmap)->is_owner = 0;
+ }
+ apr_pool_cleanup_register(p, *new_mmap, mmap_cleanup,
+ apr_pool_cleanup_null);
+ }
return APR_SUCCESS;
}
Index: srclib/apr/mmap/win32/mmap.c
===================================================================
RCS file: /home/cvspublic/apr/mmap/win32/mmap.c,v
retrieving revision 1.6
diff -u -r1.6 mmap.c
--- srclib/apr/mmap/win32/mmap.c 2001/06/27 22:07:24 1.6
+++ srclib/apr/mmap/win32/mmap.c 2001/11/16 04:22:14
@@ -66,6 +66,11 @@
{
apr_mmap_t *mm = themmap;
apr_status_t rv = 0;
+
+ if (!mm->is_owner) {
+ return APR_SUCCESS;
+ }
+
if (mm->mv) {
if (!UnmapViewOfFile(mm->mv))
{
@@ -155,10 +160,37 @@
(*new)->mm = (char*)((*new)->mv) + offset;
(*new)->size = size;
(*new)->cntxt = cont;
+ (*new)->is_owner = 1;
/* register the cleanup... */
apr_pool_cleanup_register((*new)->cntxt, (void*)(*new), mmap_cleanup,
apr_pool_cleanup_null);
+ return APR_SUCCESS;
+}
+
+APR_DECLARE(apr_status_t) apr_mmap_dup(apr_mmap_t **new_mmap,
+ apr_mmap_t *old_mmap,
+ apr_pool_t *p,
+ int transfer_ownership)
+{
+ *new_mmap = (apr_mmap_t *)apr_pmemdup(p, old_mmap, sizeof(apr_mmap_t));
+ (*new_mmap)->cntxt = p;
+
+ /* The old_mmap can transfer ownership only if the old_mmap itself
+ * is an owner of the mmap'ed segment.
+ */
+ if (old_mmap->is_owner) {
+ if (transfer_ownership) {
+ (*new_mmap)->is_owner = 1;
+ old_mmap->is_owner = 0;
+ apr_pool_cleanup_kill(old_mmap->cntxt, old_mmap, mmap_cleanup);
+ }
+ else {
+ (*new_mmap)->is_owner = 0;
+ }
+ apr_pool_cleanup_register(p, *new_mmap, mmap_cleanup,
+ apr_pool_cleanup_null);
+ }
return APR_SUCCESS;
}
Index: srclib/apr-util/buckets/apr_buckets_mmap.c
===================================================================
RCS file: /home/cvspublic/apr-util/buckets/apr_buckets_mmap.c,v
retrieving revision 1.43
diff -u -r1.43 apr_buckets_mmap.c
--- srclib/apr-util/buckets/apr_buckets_mmap.c 2001/09/22 22:36:07 1.43
+++ srclib/apr-util/buckets/apr_buckets_mmap.c 2001/11/16 04:22:14
@@ -121,24 +121,18 @@
{
apr_bucket_mmap *m = data->data;
apr_mmap_t *mm = m->mmap;
- char *base;
- void *addr;
+ apr_mmap_t *new_mm;
apr_status_t ok;
if (apr_pool_is_ancestor(mm->cntxt, p)) {
return APR_SUCCESS;
}
- ok = apr_mmap_offset(&addr, m->mmap, data->start);
+ ok = apr_mmap_dup(&new_mm, mm, p, 1);
if (ok != APR_SUCCESS) {
return ok;
}
-
- base = apr_palloc(p, data->length);
- memcpy(base, addr, data->length);
- data = apr_bucket_pool_make(data, base, data->length, p);
- mmap_destroy(m);
-
+ m->mmap = new_mm;
return APR_SUCCESS;
}
Index: modules/cache/mod_file_cache.c
===================================================================
RCS file: /home/cvspublic/httpd-2.0/modules/cache/mod_file_cache.c,v
retrieving revision 1.62
diff -u -r1.62 mod_file_cache.c
--- modules/cache/mod_file_cache.c 2001/08/11 04:04:12 1.62
+++ modules/cache/mod_file_cache.c 2001/11/16 04:22:15
@@ -333,8 +333,20 @@
#if APR_HAS_MMAP
apr_bucket *b;
apr_bucket_brigade *bb = apr_brigade_create(r->pool);
+ apr_mmap_t *mm;
- b = apr_bucket_mmap_create(file->mm, 0, (apr_size_t)file->finfo.size);
+ /* Create a copy of the apr_mmap_t that's marked as
+ * a "non-owner." The reason for this is that the
+ * mmap bucket setaside function might later try to
+ * transfer ownership of the mmap from a request
+ * pool to a parent pool. For this particular mmap,
+ * though, we want the cache to retain ownership so
+ * that it never gets munmapped. Thus we give the
+ * bucket code a non-owner copy to work with.
+ */
+ apr_mmap_dup(&mm, file->mm, r->pool, 0);
+
+ b = apr_bucket_mmap_create(mm, 0, (apr_size_t)file->finfo.size);
APR_BRIGADE_INSERT_TAIL(bb, b);
b = apr_bucket_eos_create();
APR_BRIGADE_INSERT_TAIL(bb, b);
