OpenCL possible reason a clGetEventInfo would cause a segfault? - events

Question

I have a pretty complicated OpenCL app. It fires up 5 different contexts on 5 different GPUs, and executes the same kernel on all of them, splitting up the work into 1024 "chunks" to be processed.
Each time a kernel finishes, a result is checked for, and it's given a new chunk. Sometimes, when running, as the app is starting (very rarely mid-run) it will immediately segfault on the GetEventInfo call.
This is done in a loop using callbacks and clGetEventInfo calls to ensure something is finished before moving on to the next step.
GDB output:
(gdb) back
#0 0x00007fdc686ab525 in clGetEventInfo () from /usr/lib/libOpenCL.so.1
#1 0x00000000004018c1 in ready (event=0x26a00000267) at gputest.c:165
#2 0x0000000000404b5a in main (argc=9, argv=0x7fffdfe3b268) at gputest.c:544
The ready function:
int ready(cl_event event) {
int rdy;
if(!event)
return 0;
clGetEventInfo(event, CL_EVENT_COMMAND_EXECUTION_STATUS, sizeof(cl_int), &rdy, NULL);
if(rdy == CL_COMPLETE)
return 1;
return 0;
}
How the kernel is run, the event set, and checked. Some pseudocode inserted for brevity:
while(test if loop is complete) {
for(j = 0; j < GPUS; j++) {
if(gpu[j].waiting && loops < 9999) {
gpu[j].waiting = 0;
offset[j] = loops * 1024 * 1024;
loops++;
EC("kernel init", clEnqueueNDRangeKernel(queues[j], kernel_init[j], 1, &(offset[j]), &global_work_size, &work128, 0, NULL, &events[j]));
gpu[j].readsearch = events[j];
gpu[j].reading = 1;
}
}
for(j = 0; j < GPUS; j++) {
if(gpu[j].reading && ready(gpu[j].readsearch)) {
gpu[j].reading = 0;
gpu[j].waiting = 1;
// unrelated reporting other code here
}
}
}
Its pretty simple. There is more to the code, but it's unrelated. The ready/checking function is very simple. I even added debugging to the ready function to printf the event # to see what was happening when it crashed - nothing really. No pattern I could see.
What could be causing this?

Answer 1

Ugh. Found the problem. Since you cannot initialize values when you create/declare a struct, I was using some values uninitialized. I malloc'ed the gpu structs then just started using them. With if(gpu[x].reading &&...) being random data and completely uninitialized. So sometimes it was non-zero, which allowed the ready() function to fire off. Since the gpu[x].readsearch event was never set in the first place, clGetEventInfo bombed trying to use whatever was at the memory location.
This would be time number 482,847 that accidentally using uninitialized variables has burned me.