[v4] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

[PATCH v4] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

Posted by Alexandra Diupina 2 years, 5 months ago

Process the result of hdlc_open() and call uhdlc_close()
in case of an error. It is necessary to pass the error
code up the control flow, similar to a possible
error in request_irq().
Also add a hdlc_close() call to the uhdlc_close()
because the comment to hdlc_close() says it must be called
by the hardware driver when the HDLC device is being closed

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: c19b6d246a35 ("drivers/net: support hdlc function for QE-UCC")
Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
---
v4: undo all the things done prior to hdlc_open() as 
Jakub Kicinski <kuba@kernel.org> suggested, 
add hdlc_close() call to the uhdlc_close() to match the function comment, 
add uhdlc_close() declaration to the top of the file not to put the 
uhdlc_close() function definition before uhdlc_open()
v3: Fix the commits tree
v2: Remove the 'rc' variable (stores the return value of the 
hdlc_open()) as Christophe Leroy <christophe.leroy@csgroup.eu> suggested
 drivers/net/wan/fsl_ucc_hdlc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wan/fsl_ucc_hdlc.c b/drivers/net/wan/fsl_ucc_hdlc.c
index 47c2ad7a3e42..fd999dabdd39 100644
--- a/drivers/net/wan/fsl_ucc_hdlc.c
+++ b/drivers/net/wan/fsl_ucc_hdlc.c
@@ -34,6 +34,8 @@
 #define TDM_PPPOHT_SLIC_MAXIN
 #define RX_BD_ERRORS (R_CD_S | R_OV_S | R_CR_S | R_AB_S | R_NO_S | R_LG_S)
 
+static int uhdlc_close(struct net_device *dev);
+
 static struct ucc_tdm_info utdm_primary_info = {
 	.uf_info = {
 		.tsa = 0,
@@ -731,7 +733,9 @@ static int uhdlc_open(struct net_device *dev)
 		napi_enable(&priv->napi);
 		netdev_reset_queue(dev);
 		netif_start_queue(dev);
-		hdlc_open(dev);
+
+		int rc = hdlc_open(dev);
+		return rc == 0 ? 0 : (uhdlc_close(dev), rc);
 	}
 
 	return 0;
@@ -824,6 +828,8 @@ static int uhdlc_close(struct net_device *dev)
 	netdev_reset_queue(dev);
 	priv->hdlc_busy = 0;
 
+	hdlc_close(dev);
+
 	return 0;
 }
 
-- 
2.30.2

Re: [PATCH v4] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

Posted by Christophe Leroy 2 years, 5 months ago


Le 04/09/2023 à 14:31, Alexandra Diupina a écrit :
> Process the result of hdlc_open() and call uhdlc_close()
> in case of an error. It is necessary to pass the error
> code up the control flow, similar to a possible
> error in request_irq().
> Also add a hdlc_close() call to the uhdlc_close()
> because the comment to hdlc_close() says it must be called
> by the hardware driver when the HDLC device is being closed
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: c19b6d246a35 ("drivers/net: support hdlc function for QE-UCC")
> Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
> ---
> v4: undo all the things done prior to hdlc_open() as
> Jakub Kicinski <kuba@kernel.org> suggested,
> add hdlc_close() call to the uhdlc_close() to match the function comment,
> add uhdlc_close() declaration to the top of the file not to put the
> uhdlc_close() function definition before uhdlc_open()
> v3: Fix the commits tree
> v2: Remove the 'rc' variable (stores the return value of the
> hdlc_open()) as Christophe Leroy <christophe.leroy@csgroup.eu> suggested
>   drivers/net/wan/fsl_ucc_hdlc.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/wan/fsl_ucc_hdlc.c b/drivers/net/wan/fsl_ucc_hdlc.c
> index 47c2ad7a3e42..fd999dabdd39 100644
> --- a/drivers/net/wan/fsl_ucc_hdlc.c
> +++ b/drivers/net/wan/fsl_ucc_hdlc.c
> @@ -34,6 +34,8 @@
>   #define TDM_PPPOHT_SLIC_MAXIN
>   #define RX_BD_ERRORS (R_CD_S | R_OV_S | R_CR_S | R_AB_S | R_NO_S | R_LG_S)
>   
> +static int uhdlc_close(struct net_device *dev);
> +
>   static struct ucc_tdm_info utdm_primary_info = {
>   	.uf_info = {
>   		.tsa = 0,
> @@ -731,7 +733,9 @@ static int uhdlc_open(struct net_device *dev)
>   		napi_enable(&priv->napi);
>   		netdev_reset_queue(dev);
>   		netif_start_queue(dev);
> -		hdlc_open(dev);
> +
> +		int rc = hdlc_open(dev);

Do not mix declarations and code. Please put all declaration at the top 
of the block.

> +		return rc == 0 ? 0 : (uhdlc_close(dev), rc);
>   	}

That's not easy to read.

I know that's more changes, but I'd prefer something like:

static int uhdlc_open(struct net_device *dev)
{
	u32 cecr_subblock;
	hdlc_device *hdlc = dev_to_hdlc(dev);
	struct ucc_hdlc_private *priv = hdlc->priv;
	struct ucc_tdm *utdm = priv->utdm;
	int rc;

	if (priv->hdlc_busy != 1)
		return 0;

	if (request_irq(priv->ut_info->uf_info.irq,
			ucc_hdlc_irq_handler, 0, "hdlc", priv))
		return -ENODEV;

	cecr_subblock = ucc_fast_get_qe_cr_subblock(
				priv->ut_info->uf_info.ucc_num);

	qe_issue_cmd(QE_INIT_TX_RX, cecr_subblock,
		     QE_CR_PROTOCOL_UNSPECIFIED, 0);

	ucc_fast_enable(priv->uccf, COMM_DIR_RX | COMM_DIR_TX);

	/* Enable the TDM port */
	if (priv->tsa)
		qe_setbits_8(&utdm->si_regs->siglmr1_h, 0x1 << utdm->tdm_port);

	priv->hdlc_busy = 1;
	netif_device_attach(priv->ndev);
	napi_enable(&priv->napi);
	netdev_reset_queue(dev);
	netif_start_queue(dev);

	rc = hdlc_open(dev);
	if (rc)
		uhdlc_close(dev);

	return rc;
}



>   
>   	return 0;
> @@ -824,6 +828,8 @@ static int uhdlc_close(struct net_device *dev)
>   	netdev_reset_queue(dev);
>   	priv->hdlc_busy = 0;
>   
> +	hdlc_close(dev);
> +
>   	return 0;
>   }
>   


And while you are looking at the correctness of this code, is it sure 
that uhdlc_open() cannot be called twice in parallele ?
If it can be called in parallèle I think the "if (priv->hdlc_busy != 1)" 
should be replaced by something using cmpxchg()

Re: [PATCH v4] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

Posted by Paolo Abeni 2 years, 5 months ago

On Mon, 2023-09-04 at 17:03 +0000, Christophe Leroy wrote:
> 
> Le 04/09/2023 à 14:31, Alexandra Diupina a écrit :
> > diff --git a/drivers/net/wan/fsl_ucc_hdlc.c b/drivers/net/wan/fsl_ucc_hdlc.c
> > index 47c2ad7a3e42..fd999dabdd39 100644
> > --- a/drivers/net/wan/fsl_ucc_hdlc.c
> > +++ b/drivers/net/wan/fsl_ucc_hdlc.c
> > @@ -34,6 +34,8 @@
> >   #define TDM_PPPOHT_SLIC_MAXIN
> >   #define RX_BD_ERRORS (R_CD_S | R_OV_S | R_CR_S | R_AB_S | R_NO_S | R_LG_S)
> >   
> > +static int uhdlc_close(struct net_device *dev);
> > +
> >   static struct ucc_tdm_info utdm_primary_info = {
> >   	.uf_info = {
> >   		.tsa = 0,
> > @@ -731,7 +733,9 @@ static int uhdlc_open(struct net_device *dev)
> >   		napi_enable(&priv->napi);
> >   		netdev_reset_queue(dev);
> >   		netif_start_queue(dev);
> > -		hdlc_open(dev);
> > +
> > +		int rc = hdlc_open(dev);
> 
> Do not mix declarations and code. Please put all declaration at the top 
> of the block.
> 
> > +		return rc == 0 ? 0 : (uhdlc_close(dev), rc);
> >   	}
> 
> That's not easy to read.
> 
> I know that's more changes, but I'd prefer something like:
> 
> static int uhdlc_open(struct net_device *dev)
> {
> 	u32 cecr_subblock;
> 	hdlc_device *hdlc = dev_to_hdlc(dev);
> 	struct ucc_hdlc_private *priv = hdlc->priv;
> 	struct ucc_tdm *utdm = priv->utdm;
> 	int rc;
> 
> 	if (priv->hdlc_busy != 1)
> 		return 0;
> 
> 	if (request_irq(priv->ut_info->uf_info.irq,
> 			ucc_hdlc_irq_handler, 0, "hdlc", priv))
> 		return -ENODEV;
> 
> 	cecr_subblock = ucc_fast_get_qe_cr_subblock(
> 				priv->ut_info->uf_info.ucc_num);
> 
> 	qe_issue_cmd(QE_INIT_TX_RX, cecr_subblock,
> 		     QE_CR_PROTOCOL_UNSPECIFIED, 0);
> 
> 	ucc_fast_enable(priv->uccf, COMM_DIR_RX | COMM_DIR_TX);
> 
> 	/* Enable the TDM port */
> 	if (priv->tsa)
> 		qe_setbits_8(&utdm->si_regs->siglmr1_h, 0x1 << utdm->tdm_port);
> 
> 	priv->hdlc_busy = 1;
> 	netif_device_attach(priv->ndev);
> 	napi_enable(&priv->napi);
> 	netdev_reset_queue(dev);
> 	netif_start_queue(dev);
> 
> 	rc = hdlc_open(dev);
> 	if (rc)
> 		uhdlc_close(dev);
> 
> 	return rc;
> }

I agree the above is more readable, but I don't think the whole
refactor is not worthy for a -net fix. I think simply rewriting the
final statements as:

		rc = hdlc_open(dev);
		if (rc)
			uhdlc_close(dev);

		return rc;	

would be good for -net.
 
> >   	return 0;
> > @@ -824,6 +828,8 @@ static int uhdlc_close(struct net_device *dev)
> >   	netdev_reset_queue(dev);
> >   	priv->hdlc_busy = 0;
> >   
> > +	hdlc_close(dev);
> > +
> >   return 0;
> >     
> 
> And while you are looking at the correctness of this code, is it sure 
> that uhdlc_open() cannot be called twice in parallele ?
> If it can be called in parallèle I think the "if (priv->hdlc_busy != 1)" 
> should be replaced by something using cmpxchg()

That part is safe, ndo_open() is invoked under the rtnl lock.

The other comments are IMHO relevant, @Alexandra: please address them.

Thanks!

Paolo

[PATCH v5] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

Posted by Alexandra Diupina 2 years, 4 months ago

Process the result of hdlc_open() and call uhdlc_close()
in case of an error. It is necessary to pass the error
code up the control flow, similar to a possible
error in request_irq().
Also add a hdlc_close() call to the uhdlc_close()
because the comment to hdlc_close() says it must be called
by the hardware driver when the HDLC device is being closed

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: c19b6d246a35 ("drivers/net: support hdlc function for QE-UCC")
Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
---
v5: do some refactoring as Paolo Abeni <pabeni@redhat.com> and 
Christophe Leroy <christophe.leroy@csgroup.eu> suggested
v4: undo all the things done prior to hdlc_open() as 
Jakub Kicinski <kuba@kernel.org> suggested, 
add hdlc_close() call to the uhdlc_close() to match the function comment, 
add uhdlc_close() declaration to the top of the file not to put the 
uhdlc_close() function definition before uhdlc_open()
v3: Fix the commits tree
v2: Remove the 'rc' variable (stores the return value of the 
hdlc_open()) as Christophe Leroy <christophe.leroy@csgroup.eu> suggested
 drivers/net/wan/fsl_ucc_hdlc.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wan/fsl_ucc_hdlc.c b/drivers/net/wan/fsl_ucc_hdlc.c
index 47c2ad7a3e42..fd50bb313b92 100644
--- a/drivers/net/wan/fsl_ucc_hdlc.c
+++ b/drivers/net/wan/fsl_ucc_hdlc.c
@@ -34,6 +34,8 @@
 #define TDM_PPPOHT_SLIC_MAXIN
 #define RX_BD_ERRORS (R_CD_S | R_OV_S | R_CR_S | R_AB_S | R_NO_S | R_LG_S)
 
+static int uhdlc_close(struct net_device *dev);
+
 static struct ucc_tdm_info utdm_primary_info = {
 	.uf_info = {
 		.tsa = 0,
@@ -708,6 +710,7 @@ static int uhdlc_open(struct net_device *dev)
 	hdlc_device *hdlc = dev_to_hdlc(dev);
 	struct ucc_hdlc_private *priv = hdlc->priv;
 	struct ucc_tdm *utdm = priv->utdm;
+	int rc = 0;
 
 	if (priv->hdlc_busy != 1) {
 		if (request_irq(priv->ut_info->uf_info.irq,
@@ -731,10 +734,13 @@ static int uhdlc_open(struct net_device *dev)
 		napi_enable(&priv->napi);
 		netdev_reset_queue(dev);
 		netif_start_queue(dev);
-		hdlc_open(dev);
+
+		rc = hdlc_open(dev);
+		if (rc)
+			uhdlc_close(dev);
 	}
 
-	return 0;
+	return rc;
 }
 
 static void uhdlc_memclean(struct ucc_hdlc_private *priv)
@@ -824,6 +830,8 @@ static int uhdlc_close(struct net_device *dev)
 	netdev_reset_queue(dev);
 	priv->hdlc_busy = 0;
 
+	hdlc_close(dev);
+
 	return 0;
 }
 
-- 
2.30.2

Re: [PATCH v5] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

Posted by patchwork-bot+netdevbpf@kernel.org 2 years, 4 months ago

Hello:

This patch was applied to netdev/net.git (main)
by David S. Miller <davem@davemloft.net>:

On Tue, 19 Sep 2023 17:25:02 +0300 you wrote:
> Process the result of hdlc_open() and call uhdlc_close()
> in case of an error. It is necessary to pass the error
> code up the control flow, similar to a possible
> error in request_irq().
> Also add a hdlc_close() call to the uhdlc_close()
> because the comment to hdlc_close() says it must be called
> by the hardware driver when the HDLC device is being closed
> 
> [...]

Here is the summary with links:
  - [v5] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()
    https://git.kernel.org/netdev/net/c/a59addacf899

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

Re: [PATCH v5] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

Posted by Christophe Leroy 2 years, 4 months ago


Le 19/09/2023 à 16:25, Alexandra Diupina a écrit :
> Process the result of hdlc_open() and call uhdlc_close()
> in case of an error. It is necessary to pass the error
> code up the control flow, similar to a possible
> error in request_irq().
> Also add a hdlc_close() call to the uhdlc_close()
> because the comment to hdlc_close() says it must be called
> by the hardware driver when the HDLC device is being closed
> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: c19b6d246a35 ("drivers/net: support hdlc function for QE-UCC")
> Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> ---
> v5: do some refactoring as Paolo Abeni <pabeni@redhat.com> and
> Christophe Leroy <christophe.leroy@csgroup.eu> suggested
> v4: undo all the things done prior to hdlc_open() as
> Jakub Kicinski <kuba@kernel.org> suggested,
> add hdlc_close() call to the uhdlc_close() to match the function comment,
> add uhdlc_close() declaration to the top of the file not to put the
> uhdlc_close() function definition before uhdlc_open()
> v3: Fix the commits tree
> v2: Remove the 'rc' variable (stores the return value of the
> hdlc_open()) as Christophe Leroy <christophe.leroy@csgroup.eu> suggested
>   drivers/net/wan/fsl_ucc_hdlc.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/wan/fsl_ucc_hdlc.c b/drivers/net/wan/fsl_ucc_hdlc.c
> index 47c2ad7a3e42..fd50bb313b92 100644
> --- a/drivers/net/wan/fsl_ucc_hdlc.c
> +++ b/drivers/net/wan/fsl_ucc_hdlc.c
> @@ -34,6 +34,8 @@
>   #define TDM_PPPOHT_SLIC_MAXIN
>   #define RX_BD_ERRORS (R_CD_S | R_OV_S | R_CR_S | R_AB_S | R_NO_S | R_LG_S)
>   
> +static int uhdlc_close(struct net_device *dev);
> +
>   static struct ucc_tdm_info utdm_primary_info = {
>   	.uf_info = {
>   		.tsa = 0,
> @@ -708,6 +710,7 @@ static int uhdlc_open(struct net_device *dev)
>   	hdlc_device *hdlc = dev_to_hdlc(dev);
>   	struct ucc_hdlc_private *priv = hdlc->priv;
>   	struct ucc_tdm *utdm = priv->utdm;
> +	int rc = 0;
>   
>   	if (priv->hdlc_busy != 1) {
>   		if (request_irq(priv->ut_info->uf_info.irq,
> @@ -731,10 +734,13 @@ static int uhdlc_open(struct net_device *dev)
>   		napi_enable(&priv->napi);
>   		netdev_reset_queue(dev);
>   		netif_start_queue(dev);
> -		hdlc_open(dev);
> +
> +		rc = hdlc_open(dev);
> +		if (rc)
> +			uhdlc_close(dev);
>   	}
>   
> -	return 0;
> +	return rc;
>   }
>   
>   static void uhdlc_memclean(struct ucc_hdlc_private *priv)
> @@ -824,6 +830,8 @@ static int uhdlc_close(struct net_device *dev)
>   	netdev_reset_queue(dev);
>   	priv->hdlc_busy = 0;
>   
> +	hdlc_close(dev);
> +
>   	return 0;
>   }
>

Re: [PATCH v4] drivers/net: process the result of hdlc_open() and add call of hdlc_close() in uhdlc_close()

Posted by Christophe Leroy 2 years, 5 months ago


Le 04/09/2023 à 19:03, Christophe Leroy a écrit :
> 
> 
> Le 04/09/2023 à 14:31, Alexandra Diupina a écrit :
>> Process the result of hdlc_open() and call uhdlc_close()
>> in case of an error. It is necessary to pass the error
>> code up the control flow, similar to a possible
>> error in request_irq().
>> Also add a hdlc_close() call to the uhdlc_close()
>> because the comment to hdlc_close() says it must be called
>> by the hardware driver when the HDLC device is being closed
>>
>> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>>
>> Fixes: c19b6d246a35 ("drivers/net: support hdlc function for QE-UCC")
>> Signed-off-by: Alexandra Diupina <adiupina@astralinux.ru>
>> ---
>> v4: undo all the things done prior to hdlc_open() as
>> Jakub Kicinski <kuba@kernel.org> suggested,
>> add hdlc_close() call to the uhdlc_close() to match the function comment,
>> add uhdlc_close() declaration to the top of the file not to put the
>> uhdlc_close() function definition before uhdlc_open()
>> v3: Fix the commits tree
>> v2: Remove the 'rc' variable (stores the return value of the
>> hdlc_open()) as Christophe Leroy <christophe.leroy@csgroup.eu> suggested
>>   drivers/net/wan/fsl_ucc_hdlc.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/wan/fsl_ucc_hdlc.c 
>> b/drivers/net/wan/fsl_ucc_hdlc.c
>> index 47c2ad7a3e42..fd999dabdd39 100644
>> --- a/drivers/net/wan/fsl_ucc_hdlc.c
>> +++ b/drivers/net/wan/fsl_ucc_hdlc.c
>> @@ -34,6 +34,8 @@
>>   #define TDM_PPPOHT_SLIC_MAXIN
>>   #define RX_BD_ERRORS (R_CD_S | R_OV_S | R_CR_S | R_AB_S | R_NO_S | 
>> R_LG_S)
>> +static int uhdlc_close(struct net_device *dev);
>> +
>>   static struct ucc_tdm_info utdm_primary_info = {
>>       .uf_info = {
>>           .tsa = 0,
>> @@ -731,7 +733,9 @@ static int uhdlc_open(struct net_device *dev)
>>           napi_enable(&priv->napi);
>>           netdev_reset_queue(dev);
>>           netif_start_queue(dev);
>> -        hdlc_open(dev);
>> +
>> +        int rc = hdlc_open(dev);
> 
> Do not mix declarations and code. Please put all declaration at the top 
> of the block.
> 
>> +        return rc == 0 ? 0 : (uhdlc_close(dev), rc);
>>       }
> 
> That's not easy to read.
> 
> I know that's more changes, but I'd prefer something like:
> 
> static int uhdlc_open(struct net_device *dev)
> {
>      u32 cecr_subblock;
>      hdlc_device *hdlc = dev_to_hdlc(dev);
>      struct ucc_hdlc_private *priv = hdlc->priv;
>      struct ucc_tdm *utdm = priv->utdm;
>      int rc;
> 
>      if (priv->hdlc_busy != 1)

Of course should be:

	if (priv->hdlc_busy == 1)

>          return 0;
> 
>      if (request_irq(priv->ut_info->uf_info.irq,
>              ucc_hdlc_irq_handler, 0, "hdlc", priv))
>          return -ENODEV;
> 
>      cecr_subblock = ucc_fast_get_qe_cr_subblock(
>                  priv->ut_info->uf_info.ucc_num);
> 
>      qe_issue_cmd(QE_INIT_TX_RX, cecr_subblock,
>               QE_CR_PROTOCOL_UNSPECIFIED, 0);
> 
>      ucc_fast_enable(priv->uccf, COMM_DIR_RX | COMM_DIR_TX);
> 
>      /* Enable the TDM port */
>      if (priv->tsa)
>          qe_setbits_8(&utdm->si_regs->siglmr1_h, 0x1 << utdm->tdm_port);
> 
>      priv->hdlc_busy = 1;
>      netif_device_attach(priv->ndev);
>      napi_enable(&priv->napi);
>      netdev_reset_queue(dev);
>      netif_start_queue(dev);
> 
>      rc = hdlc_open(dev);
>      if (rc)
>          uhdlc_close(dev);
> 
>      return rc;
> }
> 
> 
> 
>>       return 0;
>> @@ -824,6 +828,8 @@ static int uhdlc_close(struct net_device *dev)
>>       netdev_reset_queue(dev);
>>       priv->hdlc_busy = 0;
>> +    hdlc_close(dev);
>> +
>>       return 0;
>>   }
> 
> 
> And while you are looking at the correctness of this code, is it sure 
> that uhdlc_open() cannot be called twice in parallele ?
> If it can be called in parallèle I think the "if (priv->hdlc_busy != 1)" 
> should be replaced by something using cmpxchg()