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ggml: fix backend split scheduler race condition (llama/26040) 

* ggml: fix backend split scheduler race condition

splits without input were running concurrently with other splits, while potentially reusing memory the other split is accessing

* only sync when split has no inputs
This commit is contained in:
Ruben Ortlam 2026-08-20 10:42:33 +02:00 committed by Georgi Gerganov
parent 2648a706e0
commit 13fa860764
1 changed files with 17 additions and 5 deletions
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22
ggml/src/ggml-backend.cpp
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@ -1601,11 +1601,23 @@ static enum ggml_status ggml_backend_sched_compute_splits(ggml_backend_sched_t s
std::vector<int32_t> ids;
std::vector<ggml_bitset_t> used_ids;
int prev_backend_id = -1;
for (int split_id = 0; split_id < sched->n_splits; split_id++) {
struct ggml_backend_sched_split * split = &splits[split_id];
int split_backend_id = split->backend_id;
ggml_backend_t split_backend = sched->backends[split_backend_id];
// ensure the previous split's async work has completed before we start
// this split, the allocator may have reused buffer regions across splits
if (split->n_inputs == 0 && prev_backend_id >= 0 && prev_backend_id != split_backend_id) {
if (sched->events[prev_backend_id][sched->cur_copy] != NULL) {
ggml_backend_event_synchronize(sched->events[prev_backend_id][sched->cur_copy]);
} else {
ggml_backend_synchronize(sched->backends[prev_backend_id]);
}
}
// copy the input tensors to the split backend
for (int input_id = 0; input_id < split->n_inputs; input_id++) {
ggml_backend_t input_backend = ggml_backend_sched_get_tensor_backend(sched, split->inputs[input_id]);
@ -1768,12 +1780,12 @@ static enum ggml_status ggml_backend_sched_compute_splits(ggml_backend_sched_t s
}
}
// record the event of this copy
if (split->n_inputs > 0) {
if (sched->events[split_backend_id][sched->cur_copy] != NULL) {
ggml_backend_event_record(sched->events[split_backend_id][sched->cur_copy], split_backend);
}
// record the event of this split
if (sched->events[split_backend_id][sched->cur_copy] != NULL) {
ggml_backend_event_record(sched->events[split_backend_id][sched->cur_copy], split_backend);
}
prev_backend_id = split_backend_id;
}
return GGML_STATUS_SUCCESS;