xt_replace_table relies on table replacement counter retrieval (which
uses xt_recseq to synchronize pcpu counters).
This is fine, however with large rule set get_counters() can take
a very long time -- it needs to synchronize all counters because
it has to assume concurrent modifications can occur.
Make xt_replace_table synchronize by itself by waiting until all cpus
had an even seqcount.
This allows a followup patch to copy the counters of the old ruleset
without any synchonization after xt_replace_table has completed.
Cc: Dan Williams <dcbw@xxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
---
v4: add smb_wmb to make sure ->private is visible
before checking xt_recseq
v3: also continue if seq has changed in any way
net/netfilter/x_tables.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/net/netfilter/x_tables.c b/net/netfilter/x_tables.c
index c83a3b5e1c6c..a164e5123d59 100644
--- a/net/netfilter/x_tables.c
+++ b/net/netfilter/x_tables.c
@@ -1153,6 +1153,7 @@ xt_replace_table(struct xt_table *table,
int *error)
{
struct xt_table_info *private;
+ unsigned int cpu;
int ret;
ret = xt_jumpstack_alloc(newinfo);
@@ -1182,14 +1183,28 @@ xt_replace_table(struct xt_table *table,
smp_wmb();
table->private = newinfo;
+ /* make sure all cpus see new ->private value */
+ smp_wmb();
+
/*
* Even though table entries have now been swapped, other CPU's
- * may still be using the old entries. This is okay, because
- * resynchronization happens because of the locking done
- * during the get_counters() routine.
+ * may still be using the old entries...
*/
local_bh_enable();
+ /* ... so wait for even xt_recseq on all cpus */
+ for_each_possible_cpu(cpu) {
+ seqcount_t *s = &per_cpu(xt_recseq, cpu);
+ u32 seq = raw_read_seqcount(s);
+
+ if (seq & 1) {
+ do {
+ cond_resched();
+ cpu_relax();
+ } while (seq == raw_read_seqcount(s));
+ }
+ }
+
#ifdef CONFIG_AUDIT
if (audit_enabled) {
audit_log(current->audit_context, GFP_KERNEL,
--
2.13.6
--
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