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a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1613648766; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=IBzfMKiSeUEFSAHrVku/Sd9rNaUrGBe91U4xWJTdn5k=; b=pbyeaglKkYHe3qoageLH/xq1rK520BEBJihrV1YqsS0Ux9Azs2fSTUkHwz5+hKLzahIEOe MJX2pyQ5wcGYkqsAlGBDIfJRjyxPLBqk0wBhx54ZaOWpgOblErKLNJhNjioCfTv3p+KXIJ yR6HKu3KiaT452SdbFdDpNtnlCubZfQ= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 3B2AEACD4; Thu, 18 Feb 2021 11:46:06 +0000 (UTC) Subject: Re: [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices To: Ross Lagerwall , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org Cc: Boris Ostrovsky , Stefano Stabellini References: <20210211101616.13788-1-jgross@suse.com> <20210211101616.13788-9-jgross@suse.com> <6818fcde-abab-1250-119c-d0ccb8c80488@citrix.com> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Message-ID: Date: Thu, 18 Feb 2021 12:46:05 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <6818fcde-abab-1250-119c-d0ccb8c80488@citrix.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="wPTjx05UixzXWOdncPbHQbYBNlaLnhaEg" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --wPTjx05UixzXWOdncPbHQbYBNlaLnhaEg Content-Type: multipart/mixed; boundary="gBsCtQn6MpSbLUGhbnq3HWLUqY6yu4TxQ"; protected-headers="v1" From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= To: Ross Lagerwall , xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org Cc: Boris Ostrovsky , Stefano Stabellini Message-ID: Subject: Re: [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices References: <20210211101616.13788-1-jgross@suse.com> <20210211101616.13788-9-jgross@suse.com> <6818fcde-abab-1250-119c-d0ccb8c80488@citrix.com> In-Reply-To: <6818fcde-abab-1250-119c-d0ccb8c80488@citrix.com> --gBsCtQn6MpSbLUGhbnq3HWLUqY6yu4TxQ Content-Type: multipart/mixed; boundary="------------C2FAD25BBA0FE1C6ACA23C63" Content-Language: en-US This is a multi-part message in MIME format. --------------C2FAD25BBA0FE1C6ACA23C63 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable 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 >> --- >> 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 =3D data; >> struct per_user_data *u =3D evtchn->user; >> + unsigned int prod, cons; >> =20 >> 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, voi= d *data) >> =20 >> spin_lock(&u->ring_prod_lock); >> =20 >> - if ((u->ring_prod - u->ring_cons) < u->ring_size) { >> - *evtchn_ring_entry(u, u->ring_prod) =3D evtchn->port; >> + prod =3D READ_ONCE(u->ring_prod); >> + cons =3D READ_ONCE(u->ring_cons); >> + >> + if ((prod - cons) < u->ring_size) { >> + *evtchn_ring_entry(u, prod) =3D evtchn->port; >> smp_wmb(); /* Ensure ring contents visible */ >> - if (u->ring_cons =3D=3D u->ring_prod++) { >> + if (cons =3D=3D prod++) { >> + WRITE_ONCE(u->ring_prod, prod); >> wake_up_interruptible(&u->evtchn_wait); >> kill_fasync(&u->evtchn_async_queue, >> SIGIO, POLL_IN); >=20 > This doesn't work correctly since now u->ring_prod is only updated if c= ons =3D=3D prod++. Right. Thanks for noticing. 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