Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot()

[Chuck Lever <chuck.lever@oracle.com>]

> On Jan 11, 2019, at 4:52 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
> 
> 
> 
>> On Jan 11, 2019, at 4:12 PM, bfields@fieldses.org wrote:
>> 
>> On Wed, Jan 09, 2019 at 05:41:36PM +0000, Trond Myklebust wrote:
>>> The above is stating that
>>> 
>>> smp_rmb();
>>> smp_read_barrier_depends();
>>> if (xprt->xpt_flags & ....)
>>> 
>>> is redundant and can be replaced with just
>>> 
>>> smp_rmb();
>>> if (xprt->xpt_flags & ....)
>>> 
>>> However that's not the case for smp_rmb() followed by READ_ONCE(). That
>>> would expand to
>>> 
>>> smp_rmb();
>>> if (xprt->xpt_flags & ...) {
>>>   smp_read_barrier_depends();
>>> } else
>>>   smp_read_barrier_depends();
>>> 
>>> which is not redundant. It is ensuring (on alpha only) that the read of
>>> xprt->xpt_flags is also not re-ordered w.r.t. other data reads that
>>> follow.
>>> 
>>> See, for instance, kernel/events/core.c which has several examples, or
>>> kernel/exit.c.
>> 
>> You're right, I was confused.
>> 
>> So, I think we need your patch plus something like this.
>> 
>> Chuck, maybe you could help me with the "XXX: Chuck:" parts?
> 
> I haven't been following. Why do you think those are necessary?
> We've had set_bit and atomic_{inc,dec} in this code for ages,
> and I've never noticed a problem.
> 
> Rather than adding another CPU pipeline bubble in the RDMA code,
> though, could you simply move the set_bit() call site inside the
> critical sections?

er, inside the preceding critical section. Just reverse the order
of the spin_unlock and the set_bit.


> 
> 
>> (This applies on top of your patch plus another that just renames the
>> stupidly long svc_xprt_has_something_to_do() to svc_xprt_read().)
>> 
>> --b.
>> 
>> commit d7356c3250d4
>> Author: J. Bruce Fields <bfields@redhat.com>
>> Date:   Fri Jan 11 15:36:40 2019 -0500
>> 
>>   svcrpc: fix unlikely races preventing queueing of sockets
>> 
>>   In the rpc server, When something happens that might be reason to wake
>>   up a thread to do something, what we do is
>> 
>>           - modify xpt_flags, sk_sock->flags, xpt_reserved, or
>>             xpt_nr_rqsts to indicate the new situation
>>           - call svc_xprt_enqueue() to decide whether to wake up a thread.
>> 
>>   svc_xprt_enqueue may require multiple conditions to be true before
>>   queueing up a thread to handle the xprt.  In the SMP case, one of the
>>   other CPU's may have set another required condition, and in that case,
>>   although both CPUs run svc_xprt_enqueue(), it's possible that neither
>>   call sees the writes done by the other CPU in time, and neither one
>>   recognizes that all the required conditions have been set.  A socket
>>   could therefore be ignored indefinitely.
>> 
>>   Add memory barries to ensure that any svc_xprt_enqueue() call will
>>   always see the conditions changed by other CPUs before deciding to
>>   ignore a socket.
>> 
>>   I've never seen this race reported.  In the unlikely event it happens,
>>   another event will usually come along and the problem will fix itself.
>>   So I don't think this is worth backporting to stable.
>> 
>>   Signed-off-by: J. Bruce Fields <bfields@redhat.com>
>> 
>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>> index d410ae512b02..2af21b84b3b6 100644
>> --- a/net/sunrpc/svc_xprt.c
>> +++ b/net/sunrpc/svc_xprt.c
>> @@ -357,6 +357,7 @@ static void svc_xprt_release_slot(struct svc_rqst *rqstp)
>> 	struct svc_xprt	*xprt = rqstp->rq_xprt;
>> 	if (test_and_clear_bit(RQ_DATA, &rqstp->rq_flags)) {
>> 		atomic_dec(&xprt->xpt_nr_rqsts);
>> +		smp_wmb(); /* See smp_rmb() in svc_xprt_ready() */
>> 		svc_xprt_enqueue(xprt);
>> 	}
>> }
>> @@ -365,6 +366,15 @@ static bool svc_xprt_ready(struct svc_xprt *xprt)
>> {
>> 	unsigned long xpt_flags;
>> 
>> +	/*
>> +	 * If another cpu has recently updated xpt_flags,
>> +	 * sk_sock->flags, xpt_reserved, or xpt_nr_rqsts, we need to
>> +	 * know about it; otherwise it's possible that both that cpu and
>> +	 * this one could call svc_xprt_enqueue() without either
>> +	 * svc_xprt_enqueue() recognizing that the conditions below
>> +	 * are satisfied, and we could stall indefinitely:
>> +	 */
>> +	smp_rmb();
>> 	READ_ONCE(xprt->xpt_flags);
>> 
>> 	if (xpt_flags & (BIT(XPT_CONN) | BIT(XPT_CLOSE)))
>> @@ -479,7 +489,7 @@ void svc_reserve(struct svc_rqst *rqstp, int space)
>> 	if (xprt && space < rqstp->rq_reserved) {
>> 		atomic_sub((rqstp->rq_reserved - space), &xprt->xpt_reserved);
>> 		rqstp->rq_reserved = space;
>> -
>> +		smp_wmb(); /* See smp_rmb() in svc_xprt_ready() */
>> 		svc_xprt_enqueue(xprt);
>> 	}
>> }
>> diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
>> index 828b149eaaef..377244992ae8 100644
>> --- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
>> +++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
>> @@ -316,6 +316,7 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
>> 	list_add_tail(&ctxt->rc_list, &rdma->sc_rq_dto_q);
>> 	spin_unlock(&rdma->sc_rq_dto_lock);
>> 	set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
>> +	/* XXX: Chuck: do we need an smp_mb__after_atomic() here? */
>> 	if (!test_bit(RDMAXPRT_CONN_PENDING, &rdma->sc_flags))
>> 		svc_xprt_enqueue(&rdma->sc_xprt);
>> 	goto out;
>> diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c b/net/sunrpc/xprtrdma/svc_rdma_rw.c
>> index dc1951759a8e..e1a790487d69 100644
>> --- a/net/sunrpc/xprtrdma/svc_rdma_rw.c
>> +++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c
>> @@ -290,6 +290,7 @@ static void svc_rdma_wc_read_done(struct ib_cq *cq, struct ib_wc *wc)
>> 		spin_unlock(&rdma->sc_rq_dto_lock);
>> 
>> 		set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
>> +		/* XXX: Chuck: do we need a smp_mb__after_atomic() here? */
>> 		svc_xprt_enqueue(&rdma->sc_xprt);
>> 	}
>> 
> 
> --
> Chuck Lever

--
Chuck Lever