This patch fixes a build failure in the NVPTX libgomp plugin after the
EPHEMERAL parameter was added to the GOMP_OFFLOAD_openacc_async_host2dev
plugin entry point.
So far the parameter is ignored, which is no change from the previous
behaviour but which I think may lead to a data race in some (presumably
rare) circumstances -- for reasons I describe in the FIXME comment added
to the function.
I will apply this to the openacc-gcc-9-branch shortly (to unbreak the
build), and investigate further.
Julian
ChangeLog
libgomp/
* plugin/plugin-nvptx.c (GOMP_OFFLOAD_openacc_async_host2dev):
Add EPHEMERAL parameter, and FIXME function comment.
---
libgomp/ChangeLog.openacc | 5 +++++
libgomp/plugin/plugin-nvptx.c | 13 ++++++++++++-
2 files changed, 17 insertions(+), 1 deletion(-)
diff --git a/libgomp/ChangeLog.openacc b/libgomp/ChangeLog.openacc
index 943a9e4a933..7813760e642 100644
--- a/libgomp/ChangeLog.openacc
+++ b/libgomp/ChangeLog.openacc
@@ -1,3 +1,8 @@
+2019-09-19 Julian Brown <[email protected]>
+
+ * plugin/plugin-nvptx.c (GOMP_OFFLOAD_openacc_async_host2dev):
+ Add EPHEMERAL parameter, and FIXME function comment.
+
2019-09-18 Tobias Burnus <[email protected]>
* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c: Fix dg-warning
diff --git a/libgomp/plugin/plugin-nvptx.c b/libgomp/plugin/plugin-nvptx.c
index 4beb3222e8f..452415e1879 100644
--- a/libgomp/plugin/plugin-nvptx.c
+++ b/libgomp/plugin/plugin-nvptx.c
@@ -1868,9 +1868,20 @@ GOMP_OFFLOAD_dev2dev (int ord, void *dst, const void
*src, size_t n)
return true;
}
+/* FIXME: It is unknown whether the cuMemcpyHtoDAsync API call caches source
+ data before the asynchronous copy takes place. Either way there is a data
+ race associated with ignoring the EPHEMERAL parameter here -- either if it
+ is TRUE (because we are copying uncached data that may disappear before the
+ async copy takes place) or if it is FALSE (because the source data may be
+ cached/snapshotted here before it is modified by an earlier async operation,
+ so stale data gets copied to the target).
+ Neither problem has been observed in practice, so far. */
+
bool
GOMP_OFFLOAD_openacc_async_host2dev (int ord, void *dst, const void *src,
- size_t n, struct goacc_asyncqueue *aq)
+ size_t n,
+ bool ephemeral __attribute__((unused)),
+ struct goacc_asyncqueue *aq)
{
if (!nvptx_attach_host_thread_to_device (ord)
|| !cuda_memcpy_sanity_check (src, dst, n))
--
2.22.0
