On 5/13/20 6:37 AM, Xiaoguang Wang wrote: > When IORING_SETUP_SQPOLL is enabled, io_ring_ctx_wait_and_kill() will wait > for sq thread to idle by busy loop: > while (ctx->sqo_thread && !wq_has_sleeper(&ctx->sqo_wait)) > cond_resched(); > Above codes are not friendly, indeed I think this busy loop will introduce a > cpu burst in current cpu, though it maybe short. > > In this patch, if ctx->refs is dying, we forbids sq_thread from submitting > sqes anymore, just discard leftover sqes. I don't think this really changes anything. What happens if: > @@ -6051,7 +6053,8 @@ static int io_sq_thread(void *data) > } > > mutex_lock(&ctx->uring_lock); > - ret = io_submit_sqes(ctx, to_submit, NULL, -1, true); > + if (likely(!percpu_ref_is_dying(&ctx->refs))) > + ret = io_submit_sqes(ctx, to_submit, NULL, -1, true); > mutex_unlock(&ctx->uring_lock); > timeout = jiffies + ctx->sq_thread_idle; You check for dying here, but that could change basically while you're checking it. So you're still submitting sqes with a ref that's going away. You've only reduced the window, you haven't eliminated it. -- Jens Axboe
