Re: [PATCH 7/7] xen/evtchn: read producer index only once

From: "Jürgen Groß" <jgross@suse.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org
Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once
Date: Mon, 8 Feb 2021 11:41:08 +0100	[thread overview]
Message-ID: <626f500a-494a-0141-7bf3-94fb86b47ed4@suse.com> (raw)
In-Reply-To: <72334160-cffe-2d8a-23b7-2ea9ab1d803a@suse.com>

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On 08.02.21 10:48, Jan Beulich wrote:
> On 06.02.2021 11:49, Juergen Gross wrote:
>> In evtchn_read() use READ_ONCE() for reading the producer index in
>> order to avoid the compiler generating multiple accesses.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>   drivers/xen/evtchn.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
>> index 421382c73d88..f6b199b597bf 100644
>> --- a/drivers/xen/evtchn.c
>> +++ b/drivers/xen/evtchn.c
>> @@ -211,7 +211,7 @@ static ssize_t evtchn_read(struct file *file, char __user *buf,
>>   			goto unlock_out;
>>   
>>   		c = u->ring_cons;
>> -		p = u->ring_prod;
>> +		p = READ_ONCE(u->ring_prod);
>>   		if (c != p)
>>   			break;
> 
> Why only here and not also in
> 
> 		rc = wait_event_interruptible(u->evtchn_wait,
> 					      u->ring_cons != u->ring_prod);
> 
> or in evtchn_poll()? I understand it's not needed when
> ring_prod_lock is held, but that's not the case in the two
> afaics named places. Plus isn't the same then true for
> ring_cons and ring_cons_mutex, i.e. aren't the two named
> places plus evtchn_interrupt() also in need of READ_ONCE()
> for ring_cons?

The problem solved here is the further processing using "p" multiple
times. p must not be silently replaced with u->ring_prod by the
compiler, so I probably should reword the commit message to say:

... in order to not allow the compiler to refetch p.

Juergen

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