> On Jan 11, 2019, at 4:52 PM, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > > > >> 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? 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@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 -- Chuck Lever