On Thu, Feb 16, 2023 at 02:39:05PM +0800, D. Wythe wrote: > From: "D. Wythe" <alibuda@xxxxxxxxxxxxxxxxx> > > There is a certain probability that following > exceptions will occur in the wrk benchmark test: > > Running 10s test @ http://11.213.45.6:80 > 8 threads and 64 connections > Thread Stats Avg Stdev Max +/- Stdev > Latency 3.72ms 13.94ms 245.33ms 94.17% > Req/Sec 1.96k 713.67 5.41k 75.16% > 155262 requests in 10.10s, 23.10MB read > Non-2xx or 3xx responses: 3 > > We will find that the error is HTTP 400 error, which is a serious > exception in our test, which means the application data was > corrupted. > > Consider the following scenarios: > > CPU0 CPU1 > > buf_desc->used = 0; > cmpxchg(buf_desc->used, 0, 1) > deal_with(buf_desc) > > memset(buf_desc->cpu_addr,0); > > This will cause the data received by a victim connection to be cleared, > thus triggering an HTTP 400 error in the server. > > This patch exchange the order between clear used and memset, add > barrier to ensure memory consistency. > > Fixes: 1c5526968e27 ("net/smc: Clear memory when release and reuse buffer") > Signed-off-by: D. Wythe <alibuda@xxxxxxxxxxxxxxxxx> > --- > v2: rebase it with latest net tree. > > net/smc/smc_core.c | 17 ++++++++--------- > 1 file changed, 8 insertions(+), 9 deletions(-) > > diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c > index c305d8d..c19d4b7 100644 > --- a/net/smc/smc_core.c > +++ b/net/smc/smc_core.c > @@ -1120,8 +1120,9 @@ static void smcr_buf_unuse(struct smc_buf_desc *buf_desc, bool is_rmb, > > smc_buf_free(lgr, is_rmb, buf_desc); > } else { > - buf_desc->used = 0; > - memset(buf_desc->cpu_addr, 0, buf_desc->len); > + /* memzero_explicit provides potential memory barrier semantics */ > + memzero_explicit(buf_desc->cpu_addr, buf_desc->len); > + WRITE_ONCE(buf_desc->used, 0); This looks odd to me. memzero_explicit() is only sort of a compiler barrier, since it is a function call, but not a real memory barrier. You may want to check Documentation/memory-barriers.txt and Documentation/atomic_t.txt. To me the proper solution looks like buf_desc->used should be converted to an atomic_t, and then you could do: memset(buf_desc->cpu_addr, 0, buf_desc->len); smp_mb__before_atomic(); atomic_set(&buf_desc->used, 0); and in a similar way use atomic_cmpxchg() instead of the now used cmpxchg() for the part that sets buf_desc->used to 1. Adding experts to cc, since s390 has such strong memory ordering semantics that you can basically do whatever you want without breaking anything. So I don't consider myself an expert here at all. :) But given that this is common code, let's make sure this is really correct.
