linux-kernel - Re: [PATCH v4 4/5] soc: qcom: rpmh-rsc: Simplify locking by eliminating the per-TCS lock

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Message-ID: <158769651085.135303.5206480555792176636@swboyd.mtv.corp.google.com>
Date:   Thu, 23 Apr 2020 19:48:30 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rafael.j.wysocki@...el.com
Cc:     mka@...omium.org, mkshah@...eaurora.org, evgreen@...omium.org,
        Douglas Anderson <dianders@...omium.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/5] soc: qcom: rpmh-rsc: Simplify locking by eliminating the per-TCS lock

Quoting Douglas Anderson (2020-04-22 14:55:02)
> The rpmh-rsc code had both a driver-level lock (sometimes referred to
> in comments as drv->lock) and a lock per-TCS.  The idea was supposed
> to be that there would be times where you could get by with just
> locking a TCS lock and therefor other RPMH users wouldn't be blocked.
> 
> The above didn't work out so well.
> 
> Looking at tcs_write() the bigger drv->lock was held for most of the
> function anyway.  Only the __tcs_buffer_write() and
> __tcs_set_trigger() calls were called without it the drv->lock.  It

without holding the drv->lock

> actually turns out that in tcs_write() we don't need to hold the
> drv->lock for those function calls anyway even if the per-TCS lock
> isn't there anymore. 

Why?

> Thus, from a tcs_write() point of view, the
> per-TCS lock was useless.
> 
> Looking at rpmh_rsc_write_ctrl_data(), only the per-TCS lock was held.
> It turns out, though, that this function already needs to be called
> with the equivalent of the drv->lock held anyway (we either need to
> hold drv->lock as we will in a future patch or we need to know no
> other CPUs could be running as happens today).  Specifically
> rpmh_rsc_write_ctrl_data() might be writing to a TCS that has been
> borrowed for writing an active transation but it never checks this.
> 
> Let's eliminate this extra overhead and avoid possible AB BA locking
> headaches.
> 
> Suggested-by: Maulik Shah <mkshah@...eaurora.org>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> 
> diff --git a/drivers/soc/qcom/rpmh-rsc.c b/drivers/soc/qcom/rpmh-rsc.c
> index e540e49fd61c..71cebe7fd452 100644
> --- a/drivers/soc/qcom/rpmh-rsc.c
> +++ b/drivers/soc/qcom/rpmh-rsc.c
> @@ -581,24 +575,19 @@ static int tcs_write(struct rsc_drv *drv, const struct tcs_request *msg)
>         if (IS_ERR(tcs))
>                 return PTR_ERR(tcs);
>  
> -       spin_lock_irqsave(&tcs->lock, flags);
> -       spin_lock(&drv->lock);
> +       spin_lock_irqsave(&drv->lock, flags);
>         /*
>          * The h/w does not like if we send a request to the same address,
>          * when one is already in-flight or being processed.
>          */
>         ret = check_for_req_inflight(drv, tcs, msg);
> -       if (ret) {
> -               spin_unlock(&drv->lock);
> -               goto done_write;
> -       }
> +       if (ret)
> +               goto err;

Nitpick: Usually 'goto err' is used for error paths, not unlock paths.
Use 'goto unlock' for that.

>  
> -       tcs_id = find_free_tcs(tcs);
> -       if (tcs_id < 0) {
> -               ret = tcs_id;
> -               spin_unlock(&drv->lock);
> -               goto done_write;
> -       }
> +       ret = find_free_tcs(tcs);
> +       if (ret < 0)
> +               goto err;
> +       tcs_id = ret;
>  
>         tcs->req[tcs_id - tcs->offset] = msg;
>         set_bit(tcs_id, drv->tcs_in_use);
> @@ -612,13 +601,21 @@ static int tcs_write(struct rsc_drv *drv, const struct tcs_request *msg)
>                 write_tcs_reg_sync(drv, RSC_DRV_CMD_WAIT_FOR_CMPL, tcs_id, 0);
>                 enable_tcs_irq(drv, tcs_id, true);
>         }
> -       spin_unlock(&drv->lock);
> +       spin_unlock_irqrestore(&drv->lock, flags);
>  
> +       /*
> +        * These two can be done after the lock is released because:
> +        * - We marked "tcs_in_use" under lock.
> +        * - Once "tcs_in_use" has been marked nobody else could be writing
> +        *   to these registers until the interrupt goes off.
> +        * - The interrupt can't go off until we trigger.

trigger via some function?

> +        */
>         __tcs_buffer_write(drv, tcs_id, 0, msg);
>         __tcs_set_trigger(drv, tcs_id, true);
>  
> -done_write:
> -       spin_unlock_irqrestore(&tcs->lock, flags);
> +       return 0;
> +err:
> +       spin_unlock_irqrestore(&drv->lock, flags);
>         return ret;
>  }
>