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SubjectRe: [RFT PATCH v2 08/10] drivers: qcom: rpmh-rsc: spin_lock_irqsave() for tcs_invalidate()
Hi,

On Wed, Mar 11, 2020 at 4:14 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> Auditing tcs_invalidate() made me worried. Specifically I saw that it
> used spin_lock(), not spin_lock_irqsave(). That always worries me.
>
> As I understand it, using spin_lock() is only valid in these
> situations:
>
> a) You know you are running in the interrupt handler (and all other
> users of the lock use the "irqsave" variant).
> b) You know that nobody using the lock is ever running in the
> interrupt handler.
> c) You know that someone else has always disabled interrupts before
> your code runs and thus the "irqsave" variant is pointless.
>
> From auditing the driver we look OK. ...except that there is one
> further corner case. If sometimes your code is called with IRQs
> disabled and sometimes it's not you will get in trouble if someone
> ever boots your board with "nosmp" (AKA in uniprocessor mode). In
> such a case if someone else has the lock (without disabling
> interrupts) and they get swapped out then your code (with interrupts
> disabled) might loop forever waiting for the spinlock.
>
> It's just safer to use the irqsave version, so let's do that. In
> future patches I believe tcs_invalidate() will always be called with
> interrupts off anyway.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> Changes in v2: None
>
> drivers/soc/qcom/rpmh-rsc.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/soc/qcom/rpmh-rsc.c b/drivers/soc/qcom/rpmh-rsc.c
> index ba489d18c20e..c82c734788b1 100644
> --- a/drivers/soc/qcom/rpmh-rsc.c
> +++ b/drivers/soc/qcom/rpmh-rsc.c
> @@ -218,9 +218,10 @@ static bool tcs_is_free(struct rsc_drv *drv, int tcs_id)
> static int tcs_invalidate(struct rsc_drv *drv, int type)
> {
> int m;
> + unsigned long flags;
> struct tcs_group *tcs = &drv->tcs[type];
>
> - spin_lock(&tcs->lock);
> + spin_lock_irqsave(&tcs->lock, flags);
> if (bitmap_empty(tcs->slots, MAX_TCS_SLOTS)) {
> spin_unlock(&tcs->lock);

Noticed a bug while doing a code review of:

https://lkml.kernel.org/r/1585244270-637-7-git-send-email-mkshah@codeaurora.org

...specifically my patch forgets to change the error case to
spin_unlock_irqrestore(). ...but perhaps if that other patch lands
when we can just remove the spinlocks from this function... I'll post
more in my reply to that other patch.


-Doug

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