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Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once
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
References: <20210206104932.29064-1-jgross@suse.com>
 <20210206104932.29064-8-jgross@suse.com>
 <72334160-cffe-2d8a-23b7-2ea9ab1d803a@suse.com>
 <626f500a-494a-0141-7bf3-94fb86b47ed4@suse.com>
 <e88526ac-6972-fe08-c58f-ea872cbdcc14@suse.com>
 <d0ca217c-ecc9-55f7-abb1-30a687a46b31@suse.com>
 <a30db278-087b-554c-d5bf-1317e14e8508@suse.com>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <9d725a1b-ec8e-c078-5ec6-9c4899d4c7aa@suse.com>
Date:   Mon, 8 Feb 2021 13:15:29 +0100
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From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <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
Message-ID: <9d725a1b-ec8e-c078-5ec6-9c4899d4c7aa@suse.com>
Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once
References: <20210206104932.29064-1-jgross@suse.com>
 <20210206104932.29064-8-jgross@suse.com>
 <72334160-cffe-2d8a-23b7-2ea9ab1d803a@suse.com>
 <626f500a-494a-0141-7bf3-94fb86b47ed4@suse.com>
 <e88526ac-6972-fe08-c58f-ea872cbdcc14@suse.com>
 <d0ca217c-ecc9-55f7-abb1-30a687a46b31@suse.com>
 <a30db278-087b-554c-d5bf-1317e14e8508@suse.com>
In-Reply-To: <a30db278-087b-554c-d5bf-1317e14e8508@suse.com>

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On 08.02.21 12:54, Jan Beulich wrote:
> On 08.02.2021 11:59, J=C3=BCrgen Gro=C3=9F wrote:
>> On 08.02.21 11:51, Jan Beulich wrote:
>>> On 08.02.2021 11:41, J=C3=BCrgen Gro=C3=9F wrote:
>>>> 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;
>>>>>>    =20
>>>>>>     		c =3D u->ring_cons;
>>>>>> -		p =3D u->ring_prod;
>>>>>> +		p =3D READ_ONCE(u->ring_prod);
>>>>>>     		if (c !=3D p)
>>>>>>     			break;
>>>>>
>>>>> Why only here and not also in
>>>>>
>>>>> 		rc =3D wait_event_interruptible(u->evtchn_wait,
>>>>> 					      u->ring_cons !=3D 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.
>>>
>>> I still wouldn't understand the change (and the lack of
>>> further changes) then: The first further use of p is
>>> outside the loop, alongside one of c. IOW why would c
>>> then not need treating the same as p?
>>
>> Its value wouldn't change, as ring_cons is being modified only at
>> the bottom of this function, and nowhere else (apart from the reset
>> case, but this can't run concurrently due to ring_cons_mutex).
>>
>>> I also still don't see the difference between latching a
>>> value into a local variable vs a "freestanding" access -
>>> neither are guaranteed to result in exactly one memory
>>> access afaict.
>>
>> READ_ONCE() is using a pointer to volatile, so any refetching by
>> the compiler would be a bug.
>=20
> Of course, but this wasn't my point. I was contrasting
>=20
> 		c =3D u->ring_cons;
> 		p =3D u->ring_prod;
>=20
> which you change with
>=20
> 		rc =3D wait_event_interruptible(u->evtchn_wait,
> 					      u->ring_cons !=3D u->ring_prod);
>=20
> which you leave alone.

Can you point out which problem might arise from that?


Juergen

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