[v2] xen/events: bug fixes and some diagnostic aids

[PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices

Posted by Juergen Gross 4 years, 12 months ago

For avoiding read- and write-tearing by the compiler use READ_ONCE()
and WRITE_ONCE() for accessing the ring indices in evtchn.c.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- modify all accesses (Julien Grall)
---
 drivers/xen/evtchn.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
index 421382c73d88..620008f89dbe 100644
--- a/drivers/xen/evtchn.c
+++ b/drivers/xen/evtchn.c
@@ -162,6 +162,7 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
 {
 	struct user_evtchn *evtchn = data;
 	struct per_user_data *u = evtchn->user;
+	unsigned int prod, cons;
 
 	WARN(!evtchn->enabled,
 	     "Interrupt for port %u, but apparently not enabled; per-user %p\n",
@@ -171,10 +172,14 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
 
 	spin_lock(&u->ring_prod_lock);
 
-	if ((u->ring_prod - u->ring_cons) < u->ring_size) {
-		*evtchn_ring_entry(u, u->ring_prod) = evtchn->port;
+	prod = READ_ONCE(u->ring_prod);
+	cons = READ_ONCE(u->ring_cons);
+
+	if ((prod - cons) < u->ring_size) {
+		*evtchn_ring_entry(u, prod) = evtchn->port;
 		smp_wmb(); /* Ensure ring contents visible */
-		if (u->ring_cons == u->ring_prod++) {
+		if (cons == prod++) {
+			WRITE_ONCE(u->ring_prod, prod);
 			wake_up_interruptible(&u->evtchn_wait);
 			kill_fasync(&u->evtchn_async_queue,
 				    SIGIO, POLL_IN);
@@ -210,8 +215,8 @@ static ssize_t evtchn_read(struct file *file, char __user *buf,
 		if (u->ring_overflow)
 			goto unlock_out;
 
-		c = u->ring_cons;
-		p = u->ring_prod;
+		c = READ_ONCE(u->ring_cons);
+		p = READ_ONCE(u->ring_prod);
 		if (c != p)
 			break;
 
@@ -221,7 +226,7 @@ static ssize_t evtchn_read(struct file *file, char __user *buf,
 			return -EAGAIN;
 
 		rc = wait_event_interruptible(u->evtchn_wait,
-					      u->ring_cons != u->ring_prod);
+			READ_ONCE(u->ring_cons) != READ_ONCE(u->ring_prod));
 		if (rc)
 			return rc;
 	}
@@ -251,7 +256,7 @@ static ssize_t evtchn_read(struct file *file, char __user *buf,
 	     copy_to_user(&buf[bytes1], &u->ring[0], bytes2)))
 		goto unlock_out;
 
-	u->ring_cons += (bytes1 + bytes2) / sizeof(evtchn_port_t);
+	WRITE_ONCE(u->ring_cons, c + (bytes1 + bytes2) / sizeof(evtchn_port_t));
 	rc = bytes1 + bytes2;
 
  unlock_out:
@@ -552,7 +557,9 @@ static long evtchn_ioctl(struct file *file,
 		/* Initialise the ring to empty. Clear errors. */
 		mutex_lock(&u->ring_cons_mutex);
 		spin_lock_irq(&u->ring_prod_lock);
-		u->ring_cons = u->ring_prod = u->ring_overflow = 0;
+		WRITE_ONCE(u->ring_cons, 0);
+		WRITE_ONCE(u->ring_prod, 0);
+		u->ring_overflow = 0;
 		spin_unlock_irq(&u->ring_prod_lock);
 		mutex_unlock(&u->ring_cons_mutex);
 		rc = 0;
@@ -595,7 +602,7 @@ static __poll_t evtchn_poll(struct file *file, poll_table *wait)
 	struct per_user_data *u = file->private_data;
 
 	poll_wait(file, &u->evtchn_wait, wait);
-	if (u->ring_cons != u->ring_prod)
+	if (READ_ONCE(u->ring_cons) != READ_ONCE(u->ring_prod))
 		mask |= EPOLLIN | EPOLLRDNORM;
 	if (u->ring_overflow)
 		mask = EPOLLERR;
-- 
2.26.2

Re: [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices

Posted by Ross Lagerwall 4 years, 11 months ago

On 2021-02-11 10:16, Juergen Gross wrote:
> For avoiding read- and write-tearing by the compiler use READ_ONCE()
> and WRITE_ONCE() for accessing the ring indices in evtchn.c.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - modify all accesses (Julien Grall)
> ---
>  drivers/xen/evtchn.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
> index 421382c73d88..620008f89dbe 100644
> --- a/drivers/xen/evtchn.c
> +++ b/drivers/xen/evtchn.c
> @@ -162,6 +162,7 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
>  {
>  	struct user_evtchn *evtchn = data;
>  	struct per_user_data *u = evtchn->user;
> +	unsigned int prod, cons;
>  
>  	WARN(!evtchn->enabled,
>  	     "Interrupt for port %u, but apparently not enabled; per-user %p\n",
> @@ -171,10 +172,14 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
>  
>  	spin_lock(&u->ring_prod_lock);
>  
> -	if ((u->ring_prod - u->ring_cons) < u->ring_size) {
> -		*evtchn_ring_entry(u, u->ring_prod) = evtchn->port;
> +	prod = READ_ONCE(u->ring_prod);
> +	cons = READ_ONCE(u->ring_cons);
> +
> +	if ((prod - cons) < u->ring_size) {
> +		*evtchn_ring_entry(u, prod) = evtchn->port;
>  		smp_wmb(); /* Ensure ring contents visible */
> -		if (u->ring_cons == u->ring_prod++) {
> +		if (cons == prod++) {
> +			WRITE_ONCE(u->ring_prod, prod);
>  			wake_up_interruptible(&u->evtchn_wait);
>  			kill_fasync(&u->evtchn_async_queue,
>  				    SIGIO, POLL_IN);

This doesn't work correctly since now u->ring_prod is only updated if cons == prod++.

Ross

Re: [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices

Posted by Jürgen Groß 4 years, 11 months ago

On 17.02.21 14:29, Ross Lagerwall wrote:
> On 2021-02-11 10:16, Juergen Gross wrote:
>> For avoiding read- and write-tearing by the compiler use READ_ONCE()
>> and WRITE_ONCE() for accessing the ring indices in evtchn.c.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> V2:
>> - modify all accesses (Julien Grall)
>> ---
>>   drivers/xen/evtchn.c | 25 ++++++++++++++++---------
>>   1 file changed, 16 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
>> index 421382c73d88..620008f89dbe 100644
>> --- a/drivers/xen/evtchn.c
>> +++ b/drivers/xen/evtchn.c
>> @@ -162,6 +162,7 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
>>   {
>>   	struct user_evtchn *evtchn = data;
>>   	struct per_user_data *u = evtchn->user;
>> +	unsigned int prod, cons;
>>   
>>   	WARN(!evtchn->enabled,
>>   	     "Interrupt for port %u, but apparently not enabled; per-user %p\n",
>> @@ -171,10 +172,14 @@ static irqreturn_t evtchn_interrupt(int irq, void *data)
>>   
>>   	spin_lock(&u->ring_prod_lock);
>>   
>> -	if ((u->ring_prod - u->ring_cons) < u->ring_size) {
>> -		*evtchn_ring_entry(u, u->ring_prod) = evtchn->port;
>> +	prod = READ_ONCE(u->ring_prod);
>> +	cons = READ_ONCE(u->ring_cons);
>> +
>> +	if ((prod - cons) < u->ring_size) {
>> +		*evtchn_ring_entry(u, prod) = evtchn->port;
>>   		smp_wmb(); /* Ensure ring contents visible */
>> -		if (u->ring_cons == u->ring_prod++) {
>> +		if (cons == prod++) {
>> +			WRITE_ONCE(u->ring_prod, prod);
>>   			wake_up_interruptible(&u->evtchn_wait);
>>   			kill_fasync(&u->evtchn_async_queue,
>>   				    SIGIO, POLL_IN);
> 
> This doesn't work correctly since now u->ring_prod is only updated if cons == prod++.

Right. Thanks for noticing.


Juergen