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SubjectRe: [PATCH v2 6/8] IB/hns: Replace counting semaphore event_sem with wait_event
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On Tuesday, October 25, 2016 6:29:45 PM CEST Binoy Jayan wrote:
> On 25 October 2016 at 17:58, Arnd Bergmann <arnd@arndb.de> 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

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