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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>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once
Message-ID: <d0ca217c-ecc9-55f7-abb1-30a687a46b31@suse.com>
Date:   Mon, 8 Feb 2021 11:59:27 +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: <d0ca217c-ecc9-55f7-abb1-30a687a46b31@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>
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 <e88526ac-6972-fe08-c58f-ea872cbdcc14@suse.com>
In-Reply-To: <e88526ac-6972-fe08-c58f-ea872cbdcc14@suse.com>

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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, ch=
ar __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.
>=20
> 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.

> And of course there's also our beloved topic of access
> tearing here: READ_ONCE() also excludes that (at least as
> per its intentions aiui).

Yes, but I don't see an urgent need to fix that, as there would
be thousands of accesses in the kernel needing a fix. A compiler
tearing a naturally aligned access into multiple memory accesses
would be rejected as buggy from the kernel community IMO.


Juergen

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