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Message-ID: <3D834ADB-09E7-4E28-B62F-CB6281987E41@redhat.com>
Date: Wed, 14 Oct 2020 12:44:23 +0200
From: "Eelco Chaudron" <echaudro@...hat.com>
To: "Sebastian Andrzej Siewior" <bigeasy@...utronix.de>
Cc: netdev@...r.kernel.org, davem@...emloft.net, dev@...nvswitch.org,
kuba@...nel.org, pabeni@...hat.com, pshelar@....org,
jlelli@...hat.com
Subject: Re: [PATCH net-next] net: openvswitch: fix to make sure flow_lookup()
is not preempted
On 13 Oct 2020, at 14:53, Sebastian Andrzej Siewior wrote:
> On 2020-10-13 14:44:19 [+0200], Eelco Chaudron wrote:
>> The flow_lookup() function uses per CPU variables, which must not be
>> preempted. However, this is fine in the general napi use case where
>> the local BH is disabled. But, it's also called in the netlink
>> context, which is preemptible. The below patch makes sure that even
>> in the netlink path, preemption is disabled.
>>
>> Fixes: eac87c413bf9 ("net: openvswitch: reorder masks array based on
>> usage")
>> Reported-by: Juri Lelli <jlelli@...hat.com>
>> Signed-off-by: Eelco Chaudron <echaudro@...hat.com>
>> ---
>> net/openvswitch/flow_table.c | 10 +++++++++-
>> 1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/openvswitch/flow_table.c
>> b/net/openvswitch/flow_table.c
>> index 87c286ad660e..16289386632b 100644
>> --- a/net/openvswitch/flow_table.c
>> +++ b/net/openvswitch/flow_table.c
>> @@ -850,9 +850,17 @@ struct sw_flow *ovs_flow_tbl_lookup(struct
>> flow_table *tbl,
>> struct mask_array *ma = rcu_dereference_ovsl(tbl->mask_array);
>> u32 __always_unused n_mask_hit;
>> u32 __always_unused n_cache_hit;
>> + struct sw_flow *flow;
>> u32 index = 0;
>>
>> - return flow_lookup(tbl, ti, ma, key, &n_mask_hit, &n_cache_hit,
>> &index);
>> + /* This function gets called trough the netlink interface and
>> therefore
>> + * is preemptible. However, flow_lookup() function needs to be
>> called
>> + * with preemption disabled due to CPU specific variables.
>> + */
>
> Once again. u64_stats_update_begin(). What protects you against
> concurrent access.
Thanks Sebastian for repeating this, as I thought I went over the
seqcount code and thought it should be fine for my use case. However
based on this comment I went over it again, and found the logic part I
was constantly missing :)
My idea is to send a v2 patch and in addition to the preempt_disable()
also make the seqcount part per CPU. I noticed other parts of the
networking stack doing it the same way. So the patch would look
something like:
@@ -731,7 +732,7 @@ static struct sw_flow *flow_lookup(struct flow_table
*tbl,
u32 *n_cache_hit,
u32 *index)
{
- u64 *usage_counters = this_cpu_ptr(ma->masks_usage_cntr);
+ struct mask_array_stats *stats =
this_cpu_ptr(ma->masks_usage_stats);
struct sw_flow *flow;
struct sw_flow_mask *mask;
int i;
@@ -741,9 +742,9 @@ static struct sw_flow *flow_lookup(struct flow_table
*tbl,
if (mask) {
flow = masked_flow_lookup(ti, key, mask,
n_mask_hit);
if (flow) {
- u64_stats_update_begin(&ma->syncp);
- usage_counters[*index]++;
- u64_stats_update_end(&ma->syncp);
+ u64_stats_update_begin(&stats->syncp);
+ stats->usage_cntr[*index]++;
+ u64_stats_update_end(&stats->syncp);
(*n_cache_hit)++;
return flow;
}
Let me know your thoughts.
Thanks,
Eelco
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