Re: [PATCH] coresight: cti: write regsiters directly in cti_enable_hw()

[Tingwei Zhang]

On Tue, Sep 01, 2020 at 04:28:19AM +0800, Mathieu Poirier wrote:
> On Fri, Aug 28, 2020 at 11:18:18AM +0800, Tingwei Zhang wrote:
> > On Fri, Aug 28, 2020 at 02:12:53AM +0800, Mathieu Poirier wrote:
> > > Hi Tingwei,
> > > 
> > > On Tue, Aug 18, 2020 at 07:10:57PM +0800, Tingwei Zhang wrote:
> > > > Deadlock as below is triggered by one CPU holds drvdata->spinlock
> > > > and calls cti_enable_hw(). Smp_call_function_single() is called
> > > > in cti_enable_hw() and tries to let another CPU write CTI registers.
> > > > That CPU is trying to get drvdata->spinlock in cti_cpu_pm_notify()
> > > > and doesn't response to IPI from smp_call_function_single().
> > > > 
> > > > [  988.335937] CPU: 6 PID: 10258 Comm: sh Tainted: G        W    L
> > > > 5.8.0-rc6-mainline-16783-gc38daa79b26b-dirty #1
> > > > [  988.346364] Hardware name: Thundercomm Dragonboard 845c (DT)
> > > > [  988.352073] pstate: 20400005 (nzCv daif +PAN -UAO BTYPE=--)
> > > > [  988.357689] pc : smp_call_function_single+0x158/0x1b8
> > > > [  988.362782] lr : smp_call_function_single+0x124/0x1b8
> > > > ...
> > > > [  988.451638] Call trace:
> > > > [  988.454119]  smp_call_function_single+0x158/0x1b8
> > > > [  988.458866]  cti_enable+0xb4/0xf8 [coresight_cti]
> > > > [  988.463618]  coresight_control_assoc_ectdev+0x6c/0x128
> [coresight]
> > > > [  988.469855]  coresight_enable+0x1f0/0x364 [coresight]
> > > > [  988.474957]  enable_source_store+0x5c/0x9c [coresight]
> > > > [  988.480140]  dev_attr_store+0x14/0x28
> > > > [  988.483839]  sysfs_kf_write+0x38/0x4c
> > > > [  988.487532]  kernfs_fop_write+0x1c0/0x2b0
> > > > [  988.491585]  vfs_write+0xfc/0x300
> > > > [  988.494931]  ksys_write+0x78/0xe0
> > > > [  988.498283]  __arm64_sys_write+0x18/0x20
> > > > [  988.502240]  el0_svc_common+0x98/0x160
> > > > [  988.506024]  do_el0_svc+0x78/0x80
> > > > [  988.509377]  el0_sync_handler+0xd4/0x270
> > > > [  988.513337]  el0_sync+0x164/0x180
> > > > 
> > > 
> > > Was this the full log or you did cut some of it?
> > > 
> > 
> > I cut some CPU registers' value since it's too long and not relevant.
> > The Call trace is full.
> > 
> > > > This change write CTI registers directly in cti_enable_hw().
> > > > Config->hw_powered has been checked to be true with spinlock holded.
> > > > CTI is powered and can be programmed until spinlock is released.
> > > > 
> > > 
> > > From your explanation above it seems that cti_enable_hw() was called
> from,
> > > say
> > > CPUy, to enable the CTI associated to CPUx.  CTIx's drvdata->spinlock
> was
> > > taken
> > > and smp_call_function_single() called right after.  That woke up CPUx
> and
> > > cti_cpu_pm_notify() was executed on CPUx in interrupt context, trying
> to
> > > take
> > > CTIx's drvdata->spinlock.  That hung CPUx and the kernel got angry.
> Is my
> > > assessment correct?
> > > 
> > 
> > Most of them is correct. The only difference is CPUx is power on when
> > cti_enable_hw() is called.  Otherwise it will goto cti_state_unchanged:
> > and won't call cti_enable_hw_smp_call(). cti_cpu_pm_notify() is called
> > when CPUx tries to suspend instead of resume.
> > 
> > > If so I don't think the fix suggested in this patch will work.  The
> same
> > > condition will happen whenever cti_enable_hw() is called on a CPU to
> > > enable a
> > > CTI that belongs to another CPU and that cti_cpu_pm_notify() is called
> on
> > > latter
> > > CPU at the same time.
> > > 
> > 
> > I'm not sure I understand this correctly.  Let me clarify it a little
> bit.
> > It's a deadlock since cti_enable_hw() holds the spinlock and calls
> > cti_enable_hw_smp_call() from CPUx to enable CTI associated to CPUy. It
> > waits for cti_enable_hw_smp_call() to return. IPI is sent to CPUy while
> > CPUy is in cti_cpu_pm_notify() and waits for spinlock. In this patch,
> > I remove cti_enable_hw_smp_call() and write CTI CPU directly on CPUx.
> > It won't wait for CPUy and release spinlock after program registers of
> > CTI. After cti_enable_hw() releases spinlock, cti_cpu_pm_notify() will
> > continue to run. Since spinlock is held and config->hw_powered is true,
> > we don't need to worry about CPUy power down when we program CTI on
> CPUx.
> > 
> > > I think a better solution is to grab the lock in cti_enable_hw() and
> check
> > > the
> > > value of ->ctidev.cpu.  If not a global CPU, i.e >= 0, then release
> the
> > > lock and
> > > call smp_call_function_single().  In cti_enable_hw_smp_call() take the
> > > lock
> > > again and move forward from there. 
> > > 
> > 
> > After cti_enable_hw() releases the lock, it's possible that CPU is
> offline
> > by user, cti_enable_hw_smp_call() will fail in this case.
> > 
> >
> 
> I took another look at your solution and you are correct, CPUy won't lock
> but
> rather spin waiting for cti_enable_hw() to finish on CPUx.  To make sure
> CPUx is
> not interrupted while executing cti_enable_hw() I suggest to use
> spin_lock_irq().
> 
> Thanks,
> Mathieu
>  
Sure. I've sent v2 according to your comments.
> > 
> > > I have applied the other two patches in this set so no need to send
> them
> > > again.
> > >
> > Thanks,
> > Tingwei 
> > > Thanks,
> > > Mathieu
> > > 
> > > > Fixes: 6a0953ce7de9 ("coresight: cti: Add CPU idle pm notifer to CTI
> > > devices")
> > > > Signed-off-by: Tingwei Zhang <tingwei@xxxxxxxxxxxxxx>
> > > > ---
> > > >  drivers/hwtracing/coresight/coresight-cti.c | 17 +----------------
> > > >  1 file changed, 1 insertion(+), 16 deletions(-)
> > > > 
> > > > diff --git a/drivers/hwtracing/coresight/coresight-cti.c
> > > b/drivers/hwtracing/coresight/coresight-cti.c
> > > > index 3ccc703dc940..869569eb8c7f 100644
> > > > --- a/drivers/hwtracing/coresight/coresight-cti.c
> > > > +++ b/drivers/hwtracing/coresight/coresight-cti.c
> > > > @@ -86,13 +86,6 @@ void cti_write_all_hw_regs(struct cti_drvdata
> > > *drvdata)
> > > >  	CS_LOCK(drvdata->base);
> > > >  }
> > > >  
> > > > -static void cti_enable_hw_smp_call(void *info)
> > > > -{
> > > > -	struct cti_drvdata *drvdata = info;
> > > > -
> > > > -	cti_write_all_hw_regs(drvdata);
> > > > -}
> > > > -
> > > >  /* write regs to hardware and enable */
> > > >  static int cti_enable_hw(struct cti_drvdata *drvdata)
> > > >  {
> > > > @@ -112,15 +105,7 @@ static int cti_enable_hw(struct cti_drvdata
> > > *drvdata)
> > > >  	if (rc)
> > > >  		goto cti_err_not_enabled;
> > > >  
> > > > -	if (drvdata->ctidev.cpu >= 0) {
> > > > -		rc = smp_call_function_single(drvdata->ctidev.cpu,
> > > > -					      cti_enable_hw_smp_call,
> > > > -					      drvdata, 1);
> > > > -		if (rc)
> > > > -			goto cti_err_not_enabled;
> > > > -	} else {
> > > > -		cti_write_all_hw_regs(drvdata);
> > > > -	}
> > > > +	cti_write_all_hw_regs(drvdata);
> > > >  
> > > >  	config->hw_enabled = true;
> > > >  	atomic_inc(&drvdata->config.enable_req_count);
> > > > -- 
> > > > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
> > > Forum,
> > > > a Linux Foundation Collaborative Project
> > > > 
> > > 
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