{
dsm_handle handle;
uint32 refcnt; /* 2+ = active, 1 = moribund, 0 = gone */
+ void *impl_private_pm_handle; /* only needed on Windows */
+ bool pinned;
} dsm_control_item;
/* Layout of the dynamic shared memory control segment. */
dsm_control->item[i].handle = seg->handle;
/* refcnt of 1 triggers destruction, so start at 2 */
dsm_control->item[i].refcnt = 2;
+ dsm_control->item[i].impl_private_pm_handle = NULL;
+ dsm_control->item[i].pinned = false;
seg->control_slot = i;
LWLockRelease(DynamicSharedMemoryControlLock);
return seg;
dsm_control->item[nitems].handle = seg->handle;
/* refcnt of 1 triggers destruction, so start at 2 */
dsm_control->item[nitems].refcnt = 2;
+ dsm_control->item[nitems].impl_private_pm_handle = NULL;
+ dsm_control->item[nitems].pinned = false;
seg->control_slot = nitems;
dsm_control->nitems++;
LWLockRelease(DynamicSharedMemoryControlLock);
/* If new reference count is 1, try to destroy the segment. */
if (refcnt == 1)
{
+ /* A pinned segment should never reach 1. */
+ Assert(!dsm_control->item[control_slot].pinned);
+
/*
* If we fail to destroy the segment here, or are killed before we
* finish doing so, the reference count will remain at 1, which
}
/*
- * Keep a dynamic shared memory segment until postmaster shutdown.
+ * Keep a dynamic shared memory segment until postmaster shutdown, or until
+ * dsm_unpin_segment is called.
*
- * This function should not be called more than once per segment;
- * on Windows, doing so will create unnecessary handles which will
- * consume system resources to no benefit.
+ * This function should not be called more than once per segment, unless the
+ * segment is explicitly unpinned with dsm_unpin_segment in between calls.
*
* Note that this function does not arrange for the current process to
* keep the segment mapped indefinitely; if that behavior is desired,
void
dsm_pin_segment(dsm_segment *seg)
{
+ void *handle;
+
/*
* Bump reference count for this segment in shared memory. This will
* ensure that even if there is no session which is attached to this
- * segment, it will remain until postmaster shutdown.
+ * segment, it will remain until postmaster shutdown or an explicit call
+ * to unpin.
*/
LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+ if (dsm_control->item[seg->control_slot].pinned)
+ elog(ERROR, "cannot pin a segment that is already pinned");
+ dsm_impl_pin_segment(seg->handle, seg->impl_private, &handle);
+ dsm_control->item[seg->control_slot].pinned = true;
dsm_control->item[seg->control_slot].refcnt++;
+ dsm_control->item[seg->control_slot].impl_private_pm_handle = handle;
LWLockRelease(DynamicSharedMemoryControlLock);
+}
+
+/*
+ * Unpin a dynamic shared memory segment that was previously pinned with
+ * dsm_pin_segment. This function should not be called unless dsm_pin_segment
+ * was previously called for this segment.
+ *
+ * The argument is a dsm_handle rather than a dsm_segment in case you want
+ * to unpin a segment to which you haven't attached. This turns out to be
+ * useful if, for example, a reference to one shared memory segment is stored
+ * within another shared memory segment. You might want to unpin the
+ * referenced segment before destroying the referencing segment.
+ */
+void
+dsm_unpin_segment(dsm_handle handle)
+{
+ uint32 control_slot = INVALID_CONTROL_SLOT;
+ bool destroy = false;
+ uint32 i;
- dsm_impl_pin_segment(seg->handle, seg->impl_private);
+ /* Find the control slot for the given handle. */
+ LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+ for (i = 0; i < dsm_control->nitems; ++i)
+ {
+ /* Skip unused slots. */
+ if (dsm_control->item[i].refcnt == 0)
+ continue;
+
+ /* If we've found our handle, we can stop searching. */
+ if (dsm_control->item[i].handle == handle)
+ {
+ control_slot = i;
+ break;
+ }
+ }
+
+ /*
+ * We should definitely have found the slot, and it should not already be
+ * in the process of going away, because this function should only be
+ * called on a segment which is pinned.
+ */
+ if (control_slot == INVALID_CONTROL_SLOT)
+ elog(ERROR, "cannot unpin unknown segment handle");
+ if (!dsm_control->item[control_slot].pinned)
+ elog(ERROR, "cannot unpin a segment that is not pinned");
+ Assert(dsm_control->item[control_slot].refcnt > 1);
+
+ /*
+ * Allow implementation-specific code to run. We have to do this before
+ * releasing the lock, because impl_private_pm_handle may get modified by
+ * dsm_impl_unpin_segment.
+ */
+ dsm_impl_unpin_segment(handle,
+ &dsm_control->item[control_slot].impl_private_pm_handle);
+
+ /* Note that 1 means no references (0 means unused slot). */
+ if (--dsm_control->item[control_slot].refcnt == 1)
+ destroy = true;
+ dsm_control->item[control_slot].pinned = false;
+
+ /* Now we can release the lock. */
+ LWLockRelease(DynamicSharedMemoryControlLock);
+
+ /* Clean up resources if that was the last reference. */
+ if (destroy)
+ {
+ void *junk_impl_private = NULL;
+ void *junk_mapped_address = NULL;
+ Size junk_mapped_size = 0;
+
+ /*
+ * For an explanation of how error handling works in this case, see
+ * comments in dsm_detach. Note that if we reach this point, the
+ * current process certainly does not have the segment mapped, because
+ * if it did, the reference count would have still been greater than 1
+ * even after releasing the reference count held by the pin. The fact
+ * that there can't be a dsm_segment for this handle makes it OK to
+ * pass the mapped size, mapped address, and private data as NULL
+ * here.
+ */
+ if (dsm_impl_op(DSM_OP_DESTROY, handle, 0, &junk_impl_private,
+ &junk_mapped_address, &junk_mapped_size, WARNING))
+ {
+ LWLockAcquire(DynamicSharedMemoryControlLock, LW_EXCLUSIVE);
+ Assert(dsm_control->item[control_slot].handle == handle);
+ Assert(dsm_control->item[control_slot].refcnt == 1);
+ dsm_control->item[control_slot].refcnt = 0;
+ LWLockRelease(DynamicSharedMemoryControlLock);
+ }
+ }
}
/*
#endif
/*
- * Implementation-specific actions that must be performed when a segment
- * is to be preserved until postmaster shutdown.
+ * Implementation-specific actions that must be performed when a segment is to
+ * be preserved even when no backend has it attached.
*
* Except on Windows, we don't need to do anything at all. But since Windows
* cleans up segments automatically when no references remain, we duplicate
* do anything to receive the handle; Windows transfers it automatically.
*/
void
-dsm_impl_pin_segment(dsm_handle handle, void *impl_private)
+dsm_impl_pin_segment(dsm_handle handle, void *impl_private,
+ void **impl_private_pm_handle)
{
switch (dynamic_shared_memory_type)
{
errmsg("could not duplicate handle for \"%s\": %m",
name)));
}
+
+ /*
+ * Here, we remember the handle that we created in the
+ * postmaster process. This handle isn't actually usable in
+ * any process other than the postmaster, but that doesn't
+ * matter. We're just holding onto it so that, if the segment
+ * is unpinned, dsm_impl_unpin_segment can close it.
+ */
+ *impl_private_pm_handle = hmap;
+ break;
+ }
+#endif
+ default:
+ break;
+ }
+}
+
+/*
+ * Implementation-specific actions that must be performed when a segment is no
+ * longer to be preserved, so that it will be cleaned up when all backends
+ * have detached from it.
+ *
+ * Except on Windows, we don't need to do anything at all. For Windows, we
+ * close the extra handle that dsm_impl_pin_segment created in the
+ * postmaster's process space.
+ */
+void
+dsm_impl_unpin_segment(dsm_handle handle, void **impl_private)
+{
+ switch (dynamic_shared_memory_type)
+ {
+#ifdef USE_DSM_WINDOWS
+ case DSM_IMPL_WINDOWS:
+ {
+ if (*impl_private &&
+ !DuplicateHandle(PostmasterHandle, *impl_private,
+ NULL, NULL, 0, FALSE,
+ DUPLICATE_CLOSE_SOURCE))
+ {
+ char name[64];
+
+ snprintf(name, 64, "%s.%u", SEGMENT_NAME_PREFIX, handle);
+ _dosmaperr(GetLastError());
+ ereport(ERROR,
+ (errcode_for_dynamic_shared_memory(),
+ errmsg("could not duplicate handle for \"%s\": %m",
+ name)));
+ }
+
+ *impl_private = NULL;
break;
}
#endif
projects / postgresql / commitdiff