From: Stephen Boyd <swboyd@chromium.org>
To: Andy Gross <agross@kernel.org>,
Bjorn Andersson <bjorn.andersson@linaro.org>,
Douglas Anderson <dianders@chromium.org>,
Maulik Shah <mkshah@codeaurora.org>
Cc: mka@chromium.org, Rajendra Nayak <rnayak@codeaurora.org>,
evgreen@chromium.org, Lina Iyer <ilina@codeaurora.org>,
Douglas Anderson <dianders@chromium.org>,
linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 09/10] drivers: qcom: rpmh-rsc: Caller handles tcs_invalidate() exclusivity
Date: Mon, 13 Apr 2020 16:03:04 -0700 [thread overview]
Message-ID: <158681898406.84447.2307984865936998508@swboyd.mtv.corp.google.com> (raw)
In-Reply-To: <20200413100321.v4.9.I07c1f70e0e8f2dc0004bd38970b4e258acdc773e@changeid>
Quoting Douglas Anderson (2020-04-13 10:04:14)
> Auditing tcs_invalidate() made me worried. Specifically I saw that it
> used spin_lock(), not spin_lock_irqsave(). That always worries me
> unless I can trace for sure that I'm in the interrupt handler or that
> someone else already disabled interrupts.
>
> Looking more at it, there is actually no reason for these locks
> anyway. Specifically the only reason you'd ever call
> rpmh_rsc_invalidate() is if you cared that the sleep/wake TCSes were
> empty. That means that they need to continue to be empty even after
> rpmh_rsc_invalidate() returns. The only way that can happen is if the
> caller already has done something to keep all other RPMH users out.
> It should be noted that even though the caller is only worried about
> making sleep/wake TCSes empty, they also need to worry about stopping
> active-only transfers if they need to handle the case where
> active-only transfers might borrow the wake TCS.
>
> At the moment rpmh_rsc_invalidate() is only called in PM code from the
> last CPU. If that later changes the caller will still need to solve
> the above problems themselves, so these locks will never be useful.
>
> Continuing to audit tcs_invalidate(), I found a bug. The function
> didn't properly check for a borrowed TCS if we hadn't recently written
> anything into the TCS. Specifically, if we've never written to the
> WAKE_TCS (or we've flushed it recently) then tcs->slots is empty.
> We'll early-out and we'll never call tcs_is_free().
>
> I thought about fixing this bug by either deleting the early check for
> bitmap_empty() or possibly only doing it if we knew we weren't on a
> TCS that could be borrowed. However, I think it's better to just
> delete the checks.
>
> As argued above it's up to the caller to make sure that all other
> users of RPMH are quiet before tcs_invalidate() is called. Since
> callers need to handle the zero-active-TCS case anyway that means they
> need to make sure that the active-only transfers are quiet before
> calling too. The one way tcs_invalidate() gets called today is
> through rpmh_rsc_cpu_pm_callback() which calls
> rpmh_rsc_ctrlr_is_busy() to handle this. When we have another path to
> get to tcs_invalidate() it will also need to come up with something
> similar and it won't need this extra check either. If we later find
> some code path that actually needs this check back in (and somehow
> manages to be race free) we can always add it back in.
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> Reviewed-by: Maulik Shah <mkshah@codeaurora.org>
> Tested-by: Maulik Shah <mkshah@codeaurora.org>
> ---
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
next prev parent reply other threads:[~2020-04-13 23:03 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-13 17:04 [PATCH v4 00/10] drivers: qcom: rpmh-rsc: Cleanup / add lots of comments Douglas Anderson
2020-04-13 17:04 ` [PATCH v4 01/10] drivers: qcom: rpmh-rsc: Clean code reading/writing TCS regs/cmds Douglas Anderson
2020-04-13 18:19 ` Joe Perches
2020-04-13 21:18 ` Doug Anderson
2020-04-13 21:33 ` Joe Perches
2020-04-14 17:43 ` Doug Anderson
2020-04-13 21:20 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 02/10] drivers: qcom: rpmh-rsc: Document the register layout better Douglas Anderson
2020-04-13 21:23 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 03/10] drivers: qcom: rpmh-rsc: Fold tcs_ctrl_write() into its single caller Douglas Anderson
2020-04-13 21:36 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 04/10] drivers: qcom: rpmh-rsc: Remove get_tcs_of_type() abstraction Douglas Anderson
2020-04-13 21:37 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 05/10] drivers: qcom: rpmh-rsc: Kill cmd_cache and find_match() with fire Douglas Anderson
2020-04-13 21:41 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 06/10] drivers: qcom: rpmh-rsc: A lot of comments Douglas Anderson
2020-04-13 21:58 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 07/10] drivers: qcom: rpmh-rsc: tcs_is_free() can just check tcs_in_use Douglas Anderson
2020-04-13 21:58 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 08/10] drivers: qcom: rpmh-rsc: Don't double-check rpmh payload Douglas Anderson
2020-04-13 23:02 ` Stephen Boyd
2020-04-13 17:04 ` [PATCH v4 09/10] drivers: qcom: rpmh-rsc: Caller handles tcs_invalidate() exclusivity Douglas Anderson
2020-04-13 23:03 ` Stephen Boyd [this message]
2020-04-13 17:04 ` [PATCH v4 10/10] drivers: qcom: rpmh-rsc: read_tcs_reg()/write_tcs_reg() are not for IRQ Douglas Anderson
2020-04-13 23:03 ` Stephen Boyd
2020-04-14 5:28 ` [PATCH v4 00/10] drivers: qcom: rpmh-rsc: Cleanup / add lots of comments Bjorn Andersson
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