> On Jan 11, 2019, at 4:12 PM, bfields@xxxxxxxxxxxx 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? > (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@xxxxxxxxxx> > 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@xxxxxxxxxx> > > 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