there is following race condition,
CPU1 CPU2
dwc3_runtime_suspend() dwc3_gadget_stop()
spin_lock(&dwc->lock)
dwc3_gadget_suspend()
dwc3_disconnect_gadget()
dwc->gadget_driver != NULL
spin_unlock(&dwc->lock)
spin_lock(&dwc->lock)
dwc->gadget_driver = NULL
spin_unlock(&dwc->lock)
dwc->gadget_driver->disconnect(dwc->gadget);
system will crash when access NULL gadget_driver.
7dc0c55e9f30 ('USB: UDC core: Add udc_async_callbacks gadget op')
suggest a common way to avoid such kind of race.
this change implment the callback in dwc3 and
change related functions which have callback to UDC core.
Signed-off-by: Linyu Yuan <[email protected]>
---
drivers/usb/dwc3/core.h | 1 +
drivers/usb/dwc3/ep0.c | 5 +++--
drivers/usb/dwc3/gadget.c | 39 +++++++++++++++++++++++----------------
3 files changed, 27 insertions(+), 18 deletions(-)
diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
index dccdf13b5f9e..5991766239ba 100644
--- a/drivers/usb/dwc3/core.h
+++ b/drivers/usb/dwc3/core.h
@@ -1279,6 +1279,7 @@ struct dwc3 {
unsigned dis_metastability_quirk:1;
unsigned dis_split_quirk:1;
+ unsigned async_callbacks:1;
u16 imod_interval;
};
diff --git a/drivers/usb/dwc3/ep0.c b/drivers/usb/dwc3/ep0.c
index 3cd294264372..26419077c7c9 100644
--- a/drivers/usb/dwc3/ep0.c
+++ b/drivers/usb/dwc3/ep0.c
@@ -597,10 +597,11 @@ static int dwc3_ep0_set_address(struct dwc3 *dwc, struct usb_ctrlrequest *ctrl)
static int dwc3_ep0_delegate_req(struct dwc3 *dwc, struct usb_ctrlrequest *ctrl)
{
- int ret;
+ int ret = 0;
spin_unlock(&dwc->lock);
- ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
+ if (dwc->async_callbacks)
+ ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
spin_lock(&dwc->lock);
return ret;
}
diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index af6d7f157989..d868f30007cc 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -2585,6 +2585,16 @@ static int dwc3_gadget_vbus_draw(struct usb_gadget *g, unsigned int mA)
return ret;
}
+static void dwc3_gadget_async_callbacks(struct usb_gadget *g, bool enable)
+{
+ struct dwc3 *dwc = gadget_to_dwc(g);
+ unsigned long flags;
+
+ spin_lock_irqsave(&dwc->lock, flags);
+ dwc->async_callbacks = enable;
+ spin_unlock_irqrestore(&dwc->lock, flags);
+}
+
static const struct usb_gadget_ops dwc3_gadget_ops = {
.get_frame = dwc3_gadget_get_frame,
.wakeup = dwc3_gadget_wakeup,
@@ -2596,6 +2606,7 @@ static const struct usb_gadget_ops dwc3_gadget_ops = {
.udc_set_ssp_rate = dwc3_gadget_set_ssp_rate,
.get_config_params = dwc3_gadget_config_params,
.vbus_draw = dwc3_gadget_vbus_draw,
+ .udc_async_callbacks = dwc3_gadget_async_callbacks,
};
/* -------------------------------------------------------------------------- */
@@ -3231,29 +3242,26 @@ static void dwc3_endpoint_interrupt(struct dwc3 *dwc,
static void dwc3_disconnect_gadget(struct dwc3 *dwc)
{
- if (dwc->gadget_driver && dwc->gadget_driver->disconnect) {
- spin_unlock(&dwc->lock);
+ spin_unlock(&dwc->lock);
+ if (dwc->async_callbacks && dwc->gadget_driver->disconnect)
dwc->gadget_driver->disconnect(dwc->gadget);
- spin_lock(&dwc->lock);
- }
+ spin_lock(&dwc->lock);
}
static void dwc3_suspend_gadget(struct dwc3 *dwc)
{
- if (dwc->gadget_driver && dwc->gadget_driver->suspend) {
- spin_unlock(&dwc->lock);
+ spin_unlock(&dwc->lock);
+ if (dwc->async_callbacks && dwc->gadget_driver->suspend)
dwc->gadget_driver->suspend(dwc->gadget);
- spin_lock(&dwc->lock);
- }
+ spin_lock(&dwc->lock);
}
static void dwc3_resume_gadget(struct dwc3 *dwc)
{
- if (dwc->gadget_driver && dwc->gadget_driver->resume) {
- spin_unlock(&dwc->lock);
+ spin_unlock(&dwc->lock);
+ if (dwc->async_callbacks && dwc->gadget_driver->resume)
dwc->gadget_driver->resume(dwc->gadget);
- spin_lock(&dwc->lock);
- }
+ spin_lock(&dwc->lock);
}
static void dwc3_reset_gadget(struct dwc3 *dwc)
@@ -3585,11 +3593,10 @@ static void dwc3_gadget_wakeup_interrupt(struct dwc3 *dwc)
* implemented.
*/
- if (dwc->gadget_driver && dwc->gadget_driver->resume) {
- spin_unlock(&dwc->lock);
+ spin_unlock(&dwc->lock);
+ if (dwc->async_callbacks && dwc->gadget_driver->resume)
dwc->gadget_driver->resume(dwc->gadget);
- spin_lock(&dwc->lock);
- }
+ spin_lock(&dwc->lock);
}
static void dwc3_gadget_linksts_change_interrupt(struct dwc3 *dwc,
--
2.25.1
On Fri, Jun 25, 2021 at 06:44:15PM +0800, Linyu Yuan wrote:
> there is following race condition,
>
> CPU1 CPU2
> dwc3_runtime_suspend() dwc3_gadget_stop()
> spin_lock(&dwc->lock)
> dwc3_gadget_suspend()
> dwc3_disconnect_gadget()
> dwc->gadget_driver != NULL
> spin_unlock(&dwc->lock)
> spin_lock(&dwc->lock)
> dwc->gadget_driver = NULL
> spin_unlock(&dwc->lock)
> dwc->gadget_driver->disconnect(dwc->gadget);
>
> system will crash when access NULL gadget_driver.
>
> 7dc0c55e9f30 ('USB: UDC core: Add udc_async_callbacks gadget op')
> suggest a common way to avoid such kind of race.
>
> this change implment the callback in dwc3 and
> change related functions which have callback to UDC core.
>
> Signed-off-by: Linyu Yuan <[email protected]>
> ---
> drivers/usb/dwc3/core.h | 1 +
> drivers/usb/dwc3/ep0.c | 5 +++--
> drivers/usb/dwc3/gadget.c | 39 +++++++++++++++++++++++----------------
> 3 files changed, 27 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
> index dccdf13b5f9e..5991766239ba 100644
> --- a/drivers/usb/dwc3/core.h
> +++ b/drivers/usb/dwc3/core.h
> @@ -1279,6 +1279,7 @@ struct dwc3 {
> unsigned dis_metastability_quirk:1;
>
> unsigned dis_split_quirk:1;
> + unsigned async_callbacks:1;
>
> u16 imod_interval;
> };
> diff --git a/drivers/usb/dwc3/ep0.c b/drivers/usb/dwc3/ep0.c
> index 3cd294264372..26419077c7c9 100644
> --- a/drivers/usb/dwc3/ep0.c
> +++ b/drivers/usb/dwc3/ep0.c
> @@ -597,10 +597,11 @@ static int dwc3_ep0_set_address(struct dwc3 *dwc, struct usb_ctrlrequest *ctrl)
>
> static int dwc3_ep0_delegate_req(struct dwc3 *dwc, struct usb_ctrlrequest *ctrl)
> {
> - int ret;
> + int ret = 0;
>
> spin_unlock(&dwc->lock);
> - ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
> + if (dwc->async_callbacks)
> + ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
> spin_lock(&dwc->lock);
Here and in the other places, you should test dwc->async_callbacks
_before_ dropping the spinlock. Otherwise there is a race (the flag
could be written at about the same time it is checked).
Alan Stern
> return ret;
> }
> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
> index af6d7f157989..d868f30007cc 100644
> --- a/drivers/usb/dwc3/gadget.c
> +++ b/drivers/usb/dwc3/gadget.c
> @@ -2585,6 +2585,16 @@ static int dwc3_gadget_vbus_draw(struct usb_gadget *g, unsigned int mA)
> return ret;
> }
>
> +static void dwc3_gadget_async_callbacks(struct usb_gadget *g, bool enable)
> +{
> + struct dwc3 *dwc = gadget_to_dwc(g);
> + unsigned long flags;
> +
> + spin_lock_irqsave(&dwc->lock, flags);
> + dwc->async_callbacks = enable;
> + spin_unlock_irqrestore(&dwc->lock, flags);
> +}
> +
> static const struct usb_gadget_ops dwc3_gadget_ops = {
> .get_frame = dwc3_gadget_get_frame,
> .wakeup = dwc3_gadget_wakeup,
> @@ -2596,6 +2606,7 @@ static const struct usb_gadget_ops dwc3_gadget_ops = {
> .udc_set_ssp_rate = dwc3_gadget_set_ssp_rate,
> .get_config_params = dwc3_gadget_config_params,
> .vbus_draw = dwc3_gadget_vbus_draw,
> + .udc_async_callbacks = dwc3_gadget_async_callbacks,
> };
>
> /* -------------------------------------------------------------------------- */
> @@ -3231,29 +3242,26 @@ static void dwc3_endpoint_interrupt(struct dwc3 *dwc,
>
> static void dwc3_disconnect_gadget(struct dwc3 *dwc)
> {
> - if (dwc->gadget_driver && dwc->gadget_driver->disconnect) {
> - spin_unlock(&dwc->lock);
> + spin_unlock(&dwc->lock);
> + if (dwc->async_callbacks && dwc->gadget_driver->disconnect)
> dwc->gadget_driver->disconnect(dwc->gadget);
> - spin_lock(&dwc->lock);
> - }
> + spin_lock(&dwc->lock);
> }
>
> static void dwc3_suspend_gadget(struct dwc3 *dwc)
> {
> - if (dwc->gadget_driver && dwc->gadget_driver->suspend) {
> - spin_unlock(&dwc->lock);
> + spin_unlock(&dwc->lock);
> + if (dwc->async_callbacks && dwc->gadget_driver->suspend)
> dwc->gadget_driver->suspend(dwc->gadget);
> - spin_lock(&dwc->lock);
> - }
> + spin_lock(&dwc->lock);
> }
>
> static void dwc3_resume_gadget(struct dwc3 *dwc)
> {
> - if (dwc->gadget_driver && dwc->gadget_driver->resume) {
> - spin_unlock(&dwc->lock);
> + spin_unlock(&dwc->lock);
> + if (dwc->async_callbacks && dwc->gadget_driver->resume)
> dwc->gadget_driver->resume(dwc->gadget);
> - spin_lock(&dwc->lock);
> - }
> + spin_lock(&dwc->lock);
> }
>
> static void dwc3_reset_gadget(struct dwc3 *dwc)
> @@ -3585,11 +3593,10 @@ static void dwc3_gadget_wakeup_interrupt(struct dwc3 *dwc)
> * implemented.
> */
>
> - if (dwc->gadget_driver && dwc->gadget_driver->resume) {
> - spin_unlock(&dwc->lock);
> + spin_unlock(&dwc->lock);
> + if (dwc->async_callbacks && dwc->gadget_driver->resume)
> dwc->gadget_driver->resume(dwc->gadget);
> - spin_lock(&dwc->lock);
> - }
> + spin_lock(&dwc->lock);
> }
>
> static void dwc3_gadget_linksts_change_interrupt(struct dwc3 *dwc,
> --
> 2.25.1
>
On 2021-06-26 00:37, Alan Stern wrote:
> On Fri, Jun 25, 2021 at 06:44:15PM +0800, Linyu Yuan wrote:
>> there is following race condition,
>>
>> CPU1 CPU2
>> dwc3_runtime_suspend() dwc3_gadget_stop()
>> spin_lock(&dwc->lock)
>> dwc3_gadget_suspend()
>> dwc3_disconnect_gadget()
>> dwc->gadget_driver != NULL
>> spin_unlock(&dwc->lock)
>> spin_lock(&dwc->lock)
>> dwc->gadget_driver = NULL
>> spin_unlock(&dwc->lock)
>> dwc->gadget_driver->disconnect(dwc->gadget);
>>
>> system will crash when access NULL gadget_driver.
>>
>> 7dc0c55e9f30 ('USB: UDC core: Add udc_async_callbacks gadget op')
>> suggest a common way to avoid such kind of race.
>>
>> this change implment the callback in dwc3 and
>> change related functions which have callback to UDC core.
>>
>> Signed-off-by: Linyu Yuan <[email protected]>
>> ---
>> drivers/usb/dwc3/core.h | 1 +
>> drivers/usb/dwc3/ep0.c | 5 +++--
>> drivers/usb/dwc3/gadget.c | 39
>> +++++++++++++++++++++++----------------
>> 3 files changed, 27 insertions(+), 18 deletions(-)
>>
>> diff --git a/drivers/usb/dwc3/core.h b/drivers/usb/dwc3/core.h
>> index dccdf13b5f9e..5991766239ba 100644
>> --- a/drivers/usb/dwc3/core.h
>> +++ b/drivers/usb/dwc3/core.h
>> @@ -1279,6 +1279,7 @@ struct dwc3 {
>> unsigned dis_metastability_quirk:1;
>>
>> unsigned dis_split_quirk:1;
>> + unsigned async_callbacks:1;
>>
>> u16 imod_interval;
>> };
>> diff --git a/drivers/usb/dwc3/ep0.c b/drivers/usb/dwc3/ep0.c
>> index 3cd294264372..26419077c7c9 100644
>> --- a/drivers/usb/dwc3/ep0.c
>> +++ b/drivers/usb/dwc3/ep0.c
>> @@ -597,10 +597,11 @@ static int dwc3_ep0_set_address(struct dwc3
>> *dwc, struct usb_ctrlrequest *ctrl)
>>
>> static int dwc3_ep0_delegate_req(struct dwc3 *dwc, struct
>> usb_ctrlrequest *ctrl)
>> {
>> - int ret;
>> + int ret = 0;
>>
>> spin_unlock(&dwc->lock);
>> - ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
>> + if (dwc->async_callbacks)
>> + ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
>> spin_lock(&dwc->lock);
>
> Here and in the other places, you should test dwc->async_callbacks
> _before_ dropping the spinlock. Otherwise there is a race (the flag
> could be written at about the same time it is checked).
thanks for your comments,
if you think there is race here, how to make sure gadget_driver pointer
is safe,
this is closest place where we can confirm it is non-NULL by checking
async_callbacks ?
>
> Alan Stern
>
>> return ret;
>> }
>> diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
>> index af6d7f157989..d868f30007cc 100644
>> --- a/drivers/usb/dwc3/gadget.c
>> +++ b/drivers/usb/dwc3/gadget.c
>> @@ -2585,6 +2585,16 @@ static int dwc3_gadget_vbus_draw(struct
>> usb_gadget *g, unsigned int mA)
>> return ret;
>> }
>>
>> +static void dwc3_gadget_async_callbacks(struct usb_gadget *g, bool
>> enable)
>> +{
>> + struct dwc3 *dwc = gadget_to_dwc(g);
>> + unsigned long flags;
>> +
>> + spin_lock_irqsave(&dwc->lock, flags);
>> + dwc->async_callbacks = enable;
>> + spin_unlock_irqrestore(&dwc->lock, flags);
>> +}
>> +
>> static const struct usb_gadget_ops dwc3_gadget_ops = {
>> .get_frame = dwc3_gadget_get_frame,
>> .wakeup = dwc3_gadget_wakeup,
>> @@ -2596,6 +2606,7 @@ static const struct usb_gadget_ops
>> dwc3_gadget_ops = {
>> .udc_set_ssp_rate = dwc3_gadget_set_ssp_rate,
>> .get_config_params = dwc3_gadget_config_params,
>> .vbus_draw = dwc3_gadget_vbus_draw,
>> + .udc_async_callbacks = dwc3_gadget_async_callbacks,
>> };
>>
>> /*
>> --------------------------------------------------------------------------
>> */
>> @@ -3231,29 +3242,26 @@ static void dwc3_endpoint_interrupt(struct
>> dwc3 *dwc,
>>
>> static void dwc3_disconnect_gadget(struct dwc3 *dwc)
>> {
>> - if (dwc->gadget_driver && dwc->gadget_driver->disconnect) {
>> - spin_unlock(&dwc->lock);
>> + spin_unlock(&dwc->lock);
>> + if (dwc->async_callbacks && dwc->gadget_driver->disconnect)
>> dwc->gadget_driver->disconnect(dwc->gadget);
>> - spin_lock(&dwc->lock);
>> - }
>> + spin_lock(&dwc->lock);
>> }
>>
>> static void dwc3_suspend_gadget(struct dwc3 *dwc)
>> {
>> - if (dwc->gadget_driver && dwc->gadget_driver->suspend) {
>> - spin_unlock(&dwc->lock);
>> + spin_unlock(&dwc->lock);
>> + if (dwc->async_callbacks && dwc->gadget_driver->suspend)
>> dwc->gadget_driver->suspend(dwc->gadget);
>> - spin_lock(&dwc->lock);
>> - }
>> + spin_lock(&dwc->lock);
>> }
>>
>> static void dwc3_resume_gadget(struct dwc3 *dwc)
>> {
>> - if (dwc->gadget_driver && dwc->gadget_driver->resume) {
>> - spin_unlock(&dwc->lock);
>> + spin_unlock(&dwc->lock);
>> + if (dwc->async_callbacks && dwc->gadget_driver->resume)
>> dwc->gadget_driver->resume(dwc->gadget);
>> - spin_lock(&dwc->lock);
>> - }
>> + spin_lock(&dwc->lock);
>> }
>>
>> static void dwc3_reset_gadget(struct dwc3 *dwc)
>> @@ -3585,11 +3593,10 @@ static void
>> dwc3_gadget_wakeup_interrupt(struct dwc3 *dwc)
>> * implemented.
>> */
>>
>> - if (dwc->gadget_driver && dwc->gadget_driver->resume) {
>> - spin_unlock(&dwc->lock);
>> + spin_unlock(&dwc->lock);
>> + if (dwc->async_callbacks && dwc->gadget_driver->resume)
>> dwc->gadget_driver->resume(dwc->gadget);
>> - spin_lock(&dwc->lock);
>> - }
>> + spin_lock(&dwc->lock);
>> }
>>
>> static void dwc3_gadget_linksts_change_interrupt(struct dwc3 *dwc,
>> --
>> 2.25.1
>>
On Sat, Jun 26, 2021 at 09:16:25AM +0800, [email protected] wrote:
> On 2021-06-26 00:37, Alan Stern wrote:
> > On Fri, Jun 25, 2021 at 06:44:15PM +0800, Linyu Yuan wrote:
> > > --- a/drivers/usb/dwc3/ep0.c
> > > +++ b/drivers/usb/dwc3/ep0.c
> > > @@ -597,10 +597,11 @@ static int dwc3_ep0_set_address(struct dwc3
> > > *dwc, struct usb_ctrlrequest *ctrl)
> > >
> > > static int dwc3_ep0_delegate_req(struct dwc3 *dwc, struct
> > > usb_ctrlrequest *ctrl)
> > > {
> > > - int ret;
> > > + int ret = 0;
> > >
> > > spin_unlock(&dwc->lock);
> > > - ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
> > > + if (dwc->async_callbacks)
> > > + ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
> > > spin_lock(&dwc->lock);
> >
> > Here and in the other places, you should test dwc->async_callbacks
> > _before_ dropping the spinlock. Otherwise there is a race (the flag
> > could be written at about the same time it is checked).
> thanks for your comments,
>
> if you think there is race here, how to make sure gadget_driver pointer is
> safe,
> this is closest place where we can confirm it is non-NULL by checking
> async_callbacks ?
I explained this twice already: We know that gadget_driver is not
NULL because usb_gadget_remove_driver calls synchronize_irq before
doing usb_gadget_udc_stop.
Look at this timing diagram:
CPU0 CPU1
---- ----
IRQ happens for setup packet
Handler sees async_callbacks
is enabled
Handler unlocks dwc->lock
usb_gadget_remove_driver runs
Disables async callbacks
Calls synchronize_irq
Handler calls dwc-> . waits for IRQ handler to
gadget_driver->setup . return
Handler locks dwc-lock .
... .
Handler returns .
. synchronize_irq returns
Calls usb_gadget_udc_stop
dwc->gadget_driver is
set to NULL
As you can see, dwc->gadget_driver is non-NULL when CPU0 uses it,
even though async_callbacks gets cleared during the time when the
lock is released.
Alan Stern
On 2021-06-26 23:03, Alan Stern wrote:
> On Sat, Jun 26, 2021 at 09:16:25AM +0800, [email protected]
> wrote:
>> On 2021-06-26 00:37, Alan Stern wrote:
>> > On Fri, Jun 25, 2021 at 06:44:15PM +0800, Linyu Yuan wrote:
>
>> > > --- a/drivers/usb/dwc3/ep0.c
>> > > +++ b/drivers/usb/dwc3/ep0.c
>> > > @@ -597,10 +597,11 @@ static int dwc3_ep0_set_address(struct dwc3
>> > > *dwc, struct usb_ctrlrequest *ctrl)
>> > >
>> > > static int dwc3_ep0_delegate_req(struct dwc3 *dwc, struct
>> > > usb_ctrlrequest *ctrl)
>> > > {
>> > > - int ret;
>> > > + int ret = 0;
>> > >
>> > > spin_unlock(&dwc->lock);
>> > > - ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
>> > > + if (dwc->async_callbacks)
>> > > + ret = dwc->gadget_driver->setup(dwc->gadget, ctrl);
>> > > spin_lock(&dwc->lock);
>> >
>> > Here and in the other places, you should test dwc->async_callbacks
>> > _before_ dropping the spinlock. Otherwise there is a race (the flag
>> > could be written at about the same time it is checked).
>> thanks for your comments,
>>
>> if you think there is race here, how to make sure gadget_driver
>> pointer is
>> safe,
>> this is closest place where we can confirm it is non-NULL by checking
>> async_callbacks ?
>
> I explained this twice already: We know that gadget_driver is not
> NULL because usb_gadget_remove_driver calls synchronize_irq before
> doing usb_gadget_udc_stop.
>
> Look at this timing diagram:
>
> CPU0 CPU1
> ---- ----
> IRQ happens for setup packet
> Handler sees async_callbacks
> is enabled
> Handler unlocks dwc->lock
> usb_gadget_remove_driver runs
> Disables async callbacks
> Calls synchronize_irq
> Handler calls dwc-> . waits for IRQ handler to
> gadget_driver->setup . return
> Handler locks dwc-lock .
> ... .
> Handler returns .
> . synchronize_irq returns
> Calls usb_gadget_udc_stop
> dwc->gadget_driver is
> set to NULL
>
> As you can see, dwc->gadget_driver is non-NULL when CPU0 uses it,
> even though async_callbacks gets cleared during the time when the
> lock is released.
thanks for your patient explanation,
but from this part, seem it is synchronize_irq() help to avoid NULL
pointer crash.
can you also explain how async_callbacks flag help here ?
>
> Alan Stern
On Sun, Jun 27, 2021 at 10:48:56AM +0800, [email protected] wrote:
> On 2021-06-26 23:03, Alan Stern wrote:
> > On Sat, Jun 26, 2021 at 09:16:25AM +0800, [email protected] wrote:
> > > On 2021-06-26 00:37, Alan Stern wrote:
> > > > Here and in the other places, you should test dwc->async_callbacks
> > > > _before_ dropping the spinlock. Otherwise there is a race (the flag
> > > > could be written at about the same time it is checked).
> > > thanks for your comments,
> > >
> > > if you think there is race here, how to make sure gadget_driver
> > > pointer is
> > > safe,
> > > this is closest place where we can confirm it is non-NULL by checking
> > > async_callbacks ?
> >
> > I explained this twice already: We know that gadget_driver is not
> > NULL because usb_gadget_remove_driver calls synchronize_irq before
> > doing usb_gadget_udc_stop.
> >
> > Look at this timing diagram:
> >
> > CPU0 CPU1
> > ---- ----
> > IRQ happens for setup packet
> > Handler sees async_callbacks
> > is enabled
> > Handler unlocks dwc->lock
> > usb_gadget_remove_driver runs
> > Disables async callbacks
> > Calls synchronize_irq
> > Handler calls dwc-> . waits for IRQ handler to
> > gadget_driver->setup . return
> > Handler locks dwc-lock .
> > ... .
> > Handler returns .
> > . synchronize_irq returns
> > Calls usb_gadget_udc_stop
> > dwc->gadget_driver is
> > set to NULL
> >
> > As you can see, dwc->gadget_driver is non-NULL when CPU0 uses it,
> > even though async_callbacks gets cleared during the time when the
> > lock is released.
> thanks for your patient explanation,
> but from this part, seem it is synchronize_irq() help to avoid NULL pointer
> crash.
That's right.
> can you also explain how async_callbacks flag help here ?
It doesn't help in the situation shown above, but it does help in other
situations. Consider this timing diagram:
CPU0 CPU1
---- ----
usb_gadget_remove_driver runs
Disables async callbacks
Calls synchronize_irq
synchronize_irq returns
Calls udc_driver_unbind
IRQ happens for disconnect
Handler sees async_callbacks
is disabled
Handler returns
Calls usb_gadget_udc_stop
dwc->gadget_driver is
set to NULL
With the async_callbacks check, everything works okay. But now look at
what would happen without the async_callbacks mechanism:
CPU0 CPU1
---- ----
usb_gadget_remove_driver runs
Calls synchronize_irq
synchronize_irq returns
Calls udc_driver_unbind
IRQ happens for disconnect
Handler unlocks dwc->lock
Calls dwc->gadget_driver->disconnect
Gadget driver has already been unbound
and is not prepared to handle a
callback, so it crashes
Calls usb_gadget_udc_stop
dwc->gadget_driver is
set to NULL
Without the async_callbacks mechanism, the gadget driver can get a
callback at the wrong time (after it has been unbound), which might
cause it to crash.
Alan Stern
On 2021-06-27 22:09, Alan Stern wrote:
> On Sun, Jun 27, 2021 at 10:48:56AM +0800, [email protected]
> wrote:
>> On 2021-06-26 23:03, Alan Stern wrote:
>> > On Sat, Jun 26, 2021 at 09:16:25AM +0800, [email protected] wrote:
>> > > On 2021-06-26 00:37, Alan Stern wrote:
>
>> > > > Here and in the other places, you should test dwc->async_callbacks
>> > > > _before_ dropping the spinlock. Otherwise there is a race (the flag
>> > > > could be written at about the same time it is checked).
>> > > thanks for your comments,
>> > >
>> > > if you think there is race here, how to make sure gadget_driver
>> > > pointer is
>> > > safe,
>> > > this is closest place where we can confirm it is non-NULL by checking
>> > > async_callbacks ?
>> >
>> > I explained this twice already: We know that gadget_driver is not
>> > NULL because usb_gadget_remove_driver calls synchronize_irq before
>> > doing usb_gadget_udc_stop.
>> >
>> > Look at this timing diagram:
>> >
>> > CPU0 CPU1
>> > ---- ----
>> > IRQ happens for setup packet
>> > Handler sees async_callbacks
>> > is enabled
>> > Handler unlocks dwc->lock
>> > usb_gadget_remove_driver runs
>> > Disables async callbacks
>> > Calls synchronize_irq
>> > Handler calls dwc-> . waits for IRQ handler to
>> > gadget_driver->setup . return
>> > Handler locks dwc-lock .
>> > ... .
>> > Handler returns .
>> > . synchronize_irq returns
>> > Calls usb_gadget_udc_stop
>> > dwc->gadget_driver is
>> > set to NULL
>> >
>> > As you can see, dwc->gadget_driver is non-NULL when CPU0 uses it,
>> > even though async_callbacks gets cleared during the time when the
>> > lock is released.
>> thanks for your patient explanation,
>> but from this part, seem it is synchronize_irq() help to avoid NULL
>> pointer
>> crash.
>
> That's right.
>
>> can you also explain how async_callbacks flag help here ?
>
> It doesn't help in the situation shown above, but it does help in other
> situations. Consider this timing diagram:
>
> CPU0 CPU1
> ---- ----
> usb_gadget_remove_driver runs
> Disables async callbacks
> Calls synchronize_irq
> synchronize_irq returns
> Calls udc_driver_unbind
> IRQ happens for disconnect
> Handler sees async_callbacks
> is disabled
> Handler returns
> Calls usb_gadget_udc_stop
> dwc->gadget_driver is
> set to NULL
>
> With the async_callbacks check, everything works okay. But now look at
> what would happen without the async_callbacks mechanism:
>
> CPU0 CPU1
> ---- ----
> usb_gadget_remove_driver runs
> Calls synchronize_irq
> synchronize_irq returns
> Calls udc_driver_unbind
> IRQ happens for disconnect
> Handler unlocks dwc->lock
> Calls dwc->gadget_driver->disconnect
> Gadget driver has already been unbound
> and is not prepared to handle a
> callback, so it crashes
> Calls usb_gadget_udc_stop
> dwc->gadget_driver is
> set to NULL
>
> Without the async_callbacks mechanism, the gadget driver can get a
> callback at the wrong time (after it has been unbound), which might
> cause it to crash.
1. do you think we need to back to my original patch,
https://lore.kernel.org/linux-usb/[email protected]/T/#t
i think we can add the spin lock or mutex lock to protect this kind of
race from UDC layer, it will be easy understanding.
2. if you insist this kind of change, how to change following code in
dwc3 ?
if (dwc->gadget_driver && dwc->gadget_driver->disconnect) {
2.1 if (dwc->async_callbacks && dwc->gadget_driver->disconnect) {
or
2.2 if (dwc->async_callbacks && vdwc->gadget_driver &&
dwc->gadget_driver->disconnect) {
>
> Alan Stern
On Mon, Jun 28, 2021 at 05:36:22PM +0800, [email protected] wrote:
> On 2021-06-27 22:09, Alan Stern wrote:
> >
> > CPU0 CPU1
> > ---- ----
> > usb_gadget_remove_driver runs
> > Calls synchronize_irq
> > synchronize_irq returns
> > Calls udc_driver_unbind
> > IRQ happens for disconnect
> > Handler unlocks dwc->lock
> > Calls dwc->gadget_driver->disconnect
> > Gadget driver has already been unbound
> > and is not prepared to handle a
> > callback, so it crashes
> > Calls usb_gadget_udc_stop
> > dwc->gadget_driver is
> > set to NULL
> >
> > Without the async_callbacks mechanism, the gadget driver can get a
> > callback at the wrong time (after it has been unbound), which might
> > cause it to crash.
> 1. do you think we need to back to my original patch,
> https://lore.kernel.org/linux-usb/[email protected]/T/#t
No, I think the async_callbacks approach is slightly better.
> i think we can add the spin lock or mutex lock to protect this kind of race
> from UDC layer, it will be easy understanding.
We don't actually need a lock or mutex to fix this problem. We just
need to make the remove_driver sequence issue two calls to the UDC
driver rather than just one: async_callbacks and udc_stop.
> 2. if you insist this kind of change, how to change following code in dwc3 ?
> if (dwc->gadget_driver && dwc->gadget_driver->disconnect) {
>
> 2.1 if (dwc->async_callbacks && dwc->gadget_driver->disconnect) {
> or
> 2.2 if (dwc->async_callbacks && vdwc->gadget_driver &&
> dwc->gadget_driver->disconnect) {
Either one would be okay. If async_callbacks is enabled then
dwc->gadget_driver should never be NULL, but it won't hurt to check.
After all, disconnect does not occur often and it doesn't need to run
extremely quickly.
Alan Stern