Greetings,
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() or hwloc_linux_get_proc_tids().
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
Regards,
Vlad
