On 21/04/2012 23:08, Vlad wrote:

I use hwloc-1.4.1 stable on Red Hat 5 and am seeing a possible concurrency issue not covered by the "Thread Safety" guidelines:

- I start a small number (4) of threads, each of which does some work and periodically executes hwloc_get_last_cpu_location() with HWLOC_CPUBIND_PROCESS - occasionally, one or two of those threads will see the call fail with ENOSYS (even though the same call has already executed successfully a number of times)

These errors are transient and seem to occur only when some of the threads in the group are terminating. I've skimmed through the implementation in topology-linux.c and it seems plausible to me that the errors could be caused by failure to read /proc state "atomically" in the presence of concurrent thread starts/exits.

Of course, the latter is hard (impossible ?) to do because the state always changes and a snapshot can only be obtained with a single read() (which in turn would require knowing how many thread entries to expect in advance). However, returning ENOSYS in such cases does not seems intended but rather a flaw in retry logic. Similar issues may be present with other API methods that rely on hwloc_linux_foreach_proc_tid() orhwloc_linux_get_proc_tids().

Can you try the attached patch? It doesn't abort the loop immediately on per-tid errors anymore. This may work better when threads disappear. I don't remember if the retry logic was written while thinking about adding threads only or about adding and removing threads.

If the patch doesn't help, can you send your code to help debug things?

An alternative explanation could be that the retry logic is correct but the implementation relies on readdir(), which is documented to not be thread-safe: http://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html

I don't this can happen. Your threads should not be accessing the same DIR stream here.


diff --git a/src/topology-linux.c b/src/topology-linux.c
index e1f46cb..99a6381 100644
--- a/src/topology-linux.c
+++ b/src/topology-linux.c
@@ -475,7 +475,7 @@ hwloc_linux_foreach_proc_tid(hwloc_topology_t topology,
   char taskdir_path[128];
   DIR *taskdir;
   pid_t *tids, *newtids;
-  unsigned i, nr, newnr;
+  unsigned i, nr, newnr, failed;
   int err;

   if (pid)
@@ -497,11 +497,17 @@ hwloc_linux_foreach_proc_tid(hwloc_topology_t topology,

   /* apply the callback to all threads */
+  failed=0;
   for(i=0; i<nr; i++) {
     err = cb(topology, tids[i], data, i);
     if (err < 0)
-      goto out_with_tids;
+      failed++;
+  /* some may fail (if threads disappear), but some should succeed.
+   * if all failed, abort with the last errno.
+   */
+  if (failed == nr)
+    goto out_with_tids;

   /* re-read the list of thread and retry if it changed in the meantime */
   err = hwloc_linux_get_proc_tids(taskdir, &newnr, &newtids);

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