On Tuesday, October 25, 2016 6:29:45 PM CEST Binoy Jayan wrote: > On 25 October 2016 at 17:58, Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Tuesday, October 25, 2016 5:31:57 PM CEST Binoy Jayan wrote: > >> static int __hns_roce_cmd_mbox_wait(struct hns_roce_dev *hr_dev, u64 in_param, > >> u64 out_param, unsigned long in_modifier, > >> @@ -198,11 +218,12 @@ static int __hns_roce_cmd_mbox_wait(struct hns_roce_dev *hr_dev, u64 in_param, > >> struct hns_roce_cmdq *cmd = &hr_dev->cmd; > >> struct device *dev = &hr_dev->pdev->dev; > >> struct hns_roce_cmd_context *context; > >> - int ret = 0; > >> + int orig_free_head, ret = 0; > >> + > >> + wait_event(cmd->wq, (orig_free_head = atomic_free_node(cmd, -1)) != -1); > >> > >> spin_lock(&cmd->context_lock); > >> - WARN_ON(cmd->free_head < 0); > >> - context = &cmd->context[cmd->free_head]; > >> + context = &cmd->context[orig_free_head]; > >> context->token += cmd->token_mask + 1; > >> cmd->free_head = context->next; > >> spin_unlock(&cmd->context_lock); > >> > > > > You get the lock in atomic_free_node() and then again right after that. > > Why not combine the two and only take the lock inside of that > > function that returns a context? > > > Hi Arnd, > > I couldn't figure out a way to wait for a node to be free followed by > acquiring a lock > in an atomic fashion. If the lock is acquired after the wait_event, > there could be race > between the wait_event and acquiring the lock. If the lock is acquired > before the > wait_event, the process may goto sleep with the lock held which is not desired. > Could you suggest me of some way to circumvent this? Something like static struct hns_roce_cmd_context *hns_roce_try_get_context(struct hns_roce_cmdq *cmd) { struct hns_roce_cmd_context *context = NULL; spin_lock(&cmd->context_lock); if (cmd->free_head < 0) goto out; context = &cmd->context[cmd->free_head]; ... /* update free_head */ out: spin_unlock(&cmd->context_lock); return context; } ... static struct hns_roce_cmd_context *hns_roce_get_context(struct hns_roce_cmdq *cmd) { struct hns_roce_cmd_context *context; wait_event(cmd->wq, (context = hns_roce_try_get_context(cmd))); return context; } Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html