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Sun, 19 Jan 2020 22:50:36 -0800 (PST) Date: Mon, 20 Jan 2020 14:50:34 +0800 From: chenqiwu To: Viresh Kumar Cc: mmayer@broadcom.com, rjw@rjwysocki.net, f.fainelli@gmail.com, bcm-kernel-feedback-list@broadcom.com, linux-pm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, chenqiwu Subject: Re: [PATCH v3] cpufreq: brcmstb-avs: fix imbalance of cpufreq policy refcount Message-ID: <20200120065034.GA5874@cqw-OptiPlex-7050> References: <1579417750-21984-1-git-send-email-qiwuchen55@gmail.com> <20200120053250.igkwofqfzvmqb3c3@vireshk-i7> <20200120055822.GB5185@cqw-OptiPlex-7050> <20200120060134.izotrbzjvzk327zx@vireshk-i7> <20200120061356.GA5605@cqw-OptiPlex-7050> <20200120062126.nmxaqhcpqcojuihr@vireshk-i7> <20200120062756.GA5802@cqw-OptiPlex-7050> <20200120063004.zzhep35vfl3urndd@vireshk-i7> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200120063004.zzhep35vfl3urndd@vireshk-i7> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 20, 2020 at 12:00:04PM +0530, Viresh Kumar wrote: > On 20-01-20, 14:27, chenqiwu wrote: > > On Mon, Jan 20, 2020 at 11:51:26AM +0530, Viresh Kumar wrote: > > > On 20-01-20, 14:13, chenqiwu wrote: > > > > On Mon, Jan 20, 2020 at 11:31:34AM +0530, Viresh Kumar wrote: > > > > > On 20-01-20, 13:58, chenqiwu wrote: > > > > > > On Mon, Jan 20, 2020 at 11:02:50AM +0530, Viresh Kumar wrote: > > > > > > > On 19-01-20, 15:09, qiwuchen55@gmail.com wrote: > > > > > > > > From: chenqiwu > > > > > > > > > > > > > > > > brcm_avs_cpufreq_get() calls cpufreq_cpu_get() to get the cpufreq policy, > > > > > > > > meanwhile, it also increments the kobject reference count to mark it busy. > > > > > > > > However, a corresponding call of cpufreq_cpu_put() is ignored to decrement > > > > > > > > the kobject reference count back, which may lead to a potential stuck risk > > > > > > > > that the cpuhp thread deadly waits for dropping of kobject refcount when > > > > > > > > cpufreq policy free. > > > > > > > > > > > > > > > > For fixing this bug, cpufreq_get_policy() is referenced to do a proper > > > > > > > > cpufreq_cpu_get()/cpufreq_cpu_put() and fill a policy copy for the user. > > > > > > > > If the policy return NULL, we just return 0 to hit the code path of > > > > > > > > cpufreq_driver->get. > > > > > > > > > > > > > > > > Signed-off-by: chenqiwu > > > > > > > > --- > > > > > > > > drivers/cpufreq/brcmstb-avs-cpufreq.c | 12 ++++++++++-- > > > > > > > > 1 file changed, 10 insertions(+), 2 deletions(-) > > > > > > > > > > > > > > > > diff --git a/drivers/cpufreq/brcmstb-avs-cpufreq.c b/drivers/cpufreq/brcmstb-avs-cpufreq.c > > > > > > > > index 77b0e5d..ee0d404 100644 > > > > > > > > --- a/drivers/cpufreq/brcmstb-avs-cpufreq.c > > > > > > > > +++ b/drivers/cpufreq/brcmstb-avs-cpufreq.c > > > > > > > > @@ -452,8 +452,16 @@ static bool brcm_avs_is_firmware_loaded(struct private_data *priv) > > > > > > > > > > > > > > > > static unsigned int brcm_avs_cpufreq_get(unsigned int cpu) > > > > > > > > { > > > > > > > > - struct cpufreq_policy *policy = cpufreq_cpu_get(cpu); > > > > > > > > > > > > > > Why can't we just add a corresponding cpufreq_cpu_put() instead of all this ? > > > > > > > > > > > > > > > > > > > cpufreq_get_policy() does a proper cpufreq_cpu_get()/cpufreq_cpu_put(), > > > > > > meanwhile fills a policy copy for the user. It equals to using > > > > > > cpufreq_cpu_get() and a corresponding cpufreq_cpu_put() around access > > > > > > to the policy pointer. I think both methods are fine here. > > > > > > What do you think? > > > > > > > > > > cpufreq_get_policy() does an extra memcpy as well, which isn't required at all > > > > > in your case. > > > > > > > > > > -- > > > > > viresh > > > > > > > > Huha..Do you worry about the race conditon with cpufreq policy free path? > > > > > > No. I just worry about an unnecessary memcpy, nothing else. > > > > > Is there any question about this extra memcpy? > > What do you mean by that? > > The whole point I am trying to make is that for your specific case, doing an > explicit cpufreq_cpu_get() and cpufreq_cpu_put() is far more efficient than > calling cpufreq_get_policy() which has a different purpose and usecase. > For efficiency, I agree your idea. So we have change as follows: diff --git a/drivers/cpufreq/brcmstb-avs-cpufreq.c b/drivers/cpufreq/brcmstb-avs-cpufreq.c index 77b0e5d..b2ddde3 100644 --- a/drivers/cpufreq/brcmstb-avs-cpufreq.c +++ b/drivers/cpufreq/brcmstb-avs-cpufreq.c @@ -455,6 +455,11 @@ static unsigned int brcm_avs_cpufreq_get(unsigned int cpu) struct cpufreq_policy *policy = cpufreq_cpu_get(cpu); struct private_data *priv = policy->driver_data; + if (!policy) + return 0; + + cpufreq_cpu_put(policy); + return brcm_avs_get_frequency(priv->base); } Qiwu