Re: [PATCH] coresight: cti: write regsiters directly in cti_enable_hw() | linux-arm-kernel

quoted

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 <redacted>
---
 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);
-- 
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