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d="scan'208";a="37785213" Subject: Re: [PATCH v2 8/8] xen/evtchn: use READ/WRITE_ONCE() for accessing ring indices To: Juergen Gross , , CC: Boris Ostrovsky , Stefano Stabellini References: <20210211101616.13788-1-jgross@suse.com> <20210211101616.13788-9-jgross@suse.com> From: Ross Lagerwall Message-ID: <6818fcde-abab-1250-119c-d0ccb8c80488@citrix.com> Date: Wed, 17 Feb 2021 13:29:19 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <20210211101616.13788-9-jgross@suse.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit 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 = 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++. Ross