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PG-223: Remove relations and num_relations from pgssSharedState.

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These variables can't be in shared state, as the following problem was
taking place:
1. Process1 call pgss_ExecutorCheckPerms(), acquire lock, update
   relations and num_relations, release lock.
2. Process 2 call pgss_ExecutorCheckPerms(), acquire lock, update
   num_relations = 0;
3. Process 1 read num_relations = 0 in pgss_update_entry, this value is
   wrong as it was updated by Process 2.

Even if we acquire the lock in pgss_update_entry to read num_relations
and relations variable, Process 1 may end up acquiring the lock after
Process 2 has ovewritten the variable values, leading to Process 1
reading of wrong data.

By defining relations and num_relations to be static and global in
pg_stat_monitor.c we take advantage that each individual PostgreSQL
backend will have its own copy of this data, which allows us to remove
the locking in pgss_ExecutorCheckPerms to update these variables,
improving pg_stat_monitor overall performance.
This commit is contained in:
Diego Fronza
2021-08-27 15:53:11 -04:00
parent f3962509fb
commit a847ed95de
2 changed files with 9 additions and 15 deletions
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22
pg_stat_monitor.c
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@@ -66,7 +66,8 @@ static int plan_nested_level = 0;
/* The array to store outer layer query id*/
uint64 *nested_queryids;
FILE *qfile;
static char relations[REL_LST][REL_LEN];
static int num_relations; /* Number of relation in the query */
static bool system_init = false;
static struct rusage rusage_start;
static struct rusage rusage_end;
@@ -645,7 +646,6 @@ static void
pgss_ExecutorEnd(QueryDesc *queryDesc)
{
uint64 queryId = queryDesc->plannedstmt->queryId;
pgssSharedState *pgss = pgsm_get_ss();
SysInfo sys_info;
PlanInfo plan_info;
@@ -693,7 +693,7 @@ pgss_ExecutorEnd(QueryDesc *queryDesc)
prev_ExecutorEnd(queryDesc);
else
standard_ExecutorEnd(queryDesc);
pgss->num_relations = 0;
num_relations = 0;
}
#ifdef BENCHMARK
@@ -713,13 +713,11 @@ static bool
pgss_ExecutorCheckPerms(List *rt, bool abort)
{
ListCell *lr = NULL;
pgssSharedState *pgss = pgsm_get_ss();
int i = 0;
int j = 0;
Oid list_oid[20];
LWLockAcquire(pgss->lock, LW_EXCLUSIVE);
pgss->num_relations = 0;
num_relations = 0;
foreach(lr, rt)
{
@@ -744,14 +742,13 @@ pgss_ExecutorCheckPerms(List *rt, bool abort)
namespace_name = get_namespace_name(get_rel_namespace(rte->relid));
relation_name = get_rel_name(rte->relid);
if (rte->relkind == 'v')
snprintf(pgss->relations[i++], REL_LEN, "%s.%s*", namespace_name, relation_name);
snprintf(relations[i++], REL_LEN, "%s.%s*", namespace_name, relation_name);
else
snprintf(pgss->relations[i++], REL_LEN, "%s.%s", namespace_name, relation_name);
snprintf(relations[i++], REL_LEN, "%s.%s", namespace_name, relation_name);
}
}
}
pgss->num_relations = i;
LWLockRelease(pgss->lock);
num_relations = i;
if (prev_ExecutorCheckPerms_hook)
return prev_ExecutorCheckPerms_hook(rt, abort);
@@ -1184,7 +1181,6 @@ pgss_update_entry(pgssEntry *entry,
int index;
char application_name[APPLICATIONNAME_LEN];
int application_name_len = pg_get_application_name(application_name);
pgssSharedState *pgss = pgsm_get_ss();
double old_mean;
int message_len = error_info ? strlen (error_info->message) : 0;
int comments_len = comments ? strlen (comments) : 0;
@@ -1255,8 +1251,8 @@ pgss_update_entry(pgssEntry *entry,
_snprintf(e->counters.planinfo.plan_text, plan_info->plan_text, plan_text_len, PLAN_TEXT_LEN);
_snprintf(e->counters.info.application_name, application_name, application_name_len, APPLICATIONNAME_LEN);
e->counters.info.num_relations = pgss->num_relations;
_snprintf2(e->counters.info.relations, pgss->relations, pgss->num_relations, REL_LEN);
e->counters.info.num_relations = num_relations;
_snprintf2(e->counters.info.relations, relations, num_relations, REL_LEN);
e->counters.info.cmd_type = cmd_type;