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

["Jürgen Groß" <jgross@suse.com>]

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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


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