At Mon, 8 Aug 2022 11:53:19 -0400, Greg Stark <[email protected]> wrote in
> I'm trying to wrap my head around the shared memory stats collector
> infrastructure from
> <[email protected]> committed in
> 5891c7a8ed8f2d3d577e7eea34dacff12d7b6bbd.
>
> I have one question about locking -- afaics there's nothing protecting
> reading the shared memory stats. There is an lwlock protecting
> concurrent updates of the shared memory stats, but that lock isn't
> taken when we read the stats. Are we ok relying on atomic 64-bit reads
> or is there something else going on that I'm missing?
>
> In particular I'm looking at pgstat.c:847 in pgstat_fetch_entry()
> which does this:
>
> memcpy(stats_data,
> pgstat_get_entry_data(kind, entry_ref->shared_stats),
> kind_info->shared_data_len);
>
> stats_data is the returned copy of the stats entry with all the
> statistics in it. But it's copied from the shared memory location
> directly using memcpy and there's no locking or change counter or
> anything protecting this memcpy that I can see.
We take LW_SHARED while creating a snapshot of fixed-numbered
stats. On the other hand we don't for variable-numbered stats. I
agree to you, that we need that also for variable-numbered stats.
If I'm not missing something, it's strange that pgstat_lock_entry()
only takes LW_EXCLUSIVE. The atached changes the interface of
pgstat_lock_entry() but there's only one user since another read of
shared stats entry is not using reference. Thus the interface change
might be too much. If I just add bare LWLockAcquire/Release() to
pgstat_fetch_entry,the amount of the patch will be reduced.
regards.
--
Kyotaro Horiguchi
NTT Open Source Software Center
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 88e5dd1b2b..7c4e5f0238 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -844,9 +844,11 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, Oid objoid)
else
stats_data = MemoryContextAlloc(pgStatLocal.snapshot.context,
kind_info->shared_data_len);
+ pgstat_lock_entry(entry_ref, LW_SHARED, false);
memcpy(stats_data,
pgstat_get_entry_data(kind, entry_ref->shared_stats),
kind_info->shared_data_len);
+ pgstat_unlock_entry(entry_ref);
if (pgstat_fetch_consistency > PGSTAT_FETCH_CONSISTENCY_NONE)
{
@@ -983,9 +985,15 @@ pgstat_build_snapshot(void)
entry->data = MemoryContextAlloc(pgStatLocal.snapshot.context,
kind_info->shared_size);
+ /*
+ * We're directly accesing the shared stats entry not using
+ * reference. pg_stat_lock_entry() cannot be used here.
+ */
+ LWLockAcquire(&stats_data->lock, LW_SHARED);
memcpy(entry->data,
pgstat_get_entry_data(kind, stats_data),
kind_info->shared_size);
+ LWLockRelease(&stats_data->lock);
}
dshash_seq_term(&hstat);
diff --git a/src/backend/utils/activity/pgstat_database.c b/src/backend/utils/activity/pgstat_database.c
index d9275611f0..fdf4d022c1 100644
--- a/src/backend/utils/activity/pgstat_database.c
+++ b/src/backend/utils/activity/pgstat_database.c
@@ -364,7 +364,7 @@ pgstat_database_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
pendingent = (PgStat_StatDBEntry *) entry_ref->pending;
sharedent = (PgStatShared_Database *) entry_ref->shared_stats;
- if (!pgstat_lock_entry(entry_ref, nowait))
+ if (!pgstat_lock_entry(entry_ref, LW_EXCLUSIVE, nowait))
return false;
#define PGSTAT_ACCUM_DBCOUNT(item) \
diff --git a/src/backend/utils/activity/pgstat_function.c b/src/backend/utils/activity/pgstat_function.c
index 427d8c47fc..318db0b3c8 100644
--- a/src/backend/utils/activity/pgstat_function.c
+++ b/src/backend/utils/activity/pgstat_function.c
@@ -200,7 +200,7 @@ pgstat_function_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
/* localent always has non-zero content */
- if (!pgstat_lock_entry(entry_ref, nowait))
+ if (!pgstat_lock_entry(entry_ref, LW_EXCLUSIVE, nowait))
return false;
shfuncent->stats.f_numcalls += localent->f_counts.f_numcalls;
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index a846d9ffb6..98dda726db 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -782,7 +782,7 @@ pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
return true;
}
- if (!pgstat_lock_entry(entry_ref, nowait))
+ if (!pgstat_lock_entry(entry_ref, LW_EXCLUSIVE, nowait))
return false;
/* add the values to the shared entry. */
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index 89060ef29a..fdd20d80a1 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -568,14 +568,14 @@ pgstat_release_entry_ref(PgStat_HashKey key, PgStat_EntryRef *entry_ref,
}
bool
-pgstat_lock_entry(PgStat_EntryRef *entry_ref, bool nowait)
+pgstat_lock_entry(PgStat_EntryRef *entry_ref, LWLockMode mode, bool nowait)
{
LWLock *lock = &entry_ref->shared_stats->lock;
if (nowait)
- return LWLockConditionalAcquire(lock, LW_EXCLUSIVE);
+ return LWLockConditionalAcquire(lock, mode);
- LWLockAcquire(lock, LW_EXCLUSIVE);
+ LWLockAcquire(lock, mode);
return true;
}
@@ -598,7 +598,7 @@ pgstat_get_entry_ref_locked(PgStat_Kind kind, Oid dboid, Oid objoid,
entry_ref = pgstat_get_entry_ref(kind, dboid, objoid, true, NULL);
/* lock the shared entry to protect the content, skip if failed */
- if (!pgstat_lock_entry(entry_ref, nowait))
+ if (!pgstat_lock_entry(entry_ref, LW_EXCLUSIVE, nowait))
return NULL;
return entry_ref;
@@ -920,7 +920,7 @@ pgstat_reset_entry(PgStat_Kind kind, Oid dboid, Oid objoid, TimestampTz ts)
if (!entry_ref || entry_ref->shared_entry->dropped)
return;
- (void) pgstat_lock_entry(entry_ref, false);
+ (void) pgstat_lock_entry(entry_ref, LW_EXCLUSIVE, false);
shared_stat_reset_contents(kind, entry_ref->shared_stats, ts);
pgstat_unlock_entry(entry_ref);
}
diff --git a/src/backend/utils/activity/pgstat_subscription.c b/src/backend/utils/activity/pgstat_subscription.c
index e1072bd5ba..eeb2e3370f 100644
--- a/src/backend/utils/activity/pgstat_subscription.c
+++ b/src/backend/utils/activity/pgstat_subscription.c
@@ -91,7 +91,7 @@ pgstat_subscription_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
/* localent always has non-zero content */
- if (!pgstat_lock_entry(entry_ref, nowait))
+ if (!pgstat_lock_entry(entry_ref, LW_EXCLUSIVE, nowait))
return false;
#define SUB_ACC(fld) shsubent->stats.fld += localent->fld
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index 9303d05427..fe1ac0f274 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -580,7 +580,7 @@ extern void pgstat_detach_shmem(void);
extern PgStat_EntryRef *pgstat_get_entry_ref(PgStat_Kind kind, Oid dboid, Oid objoid,
bool create, bool *found);
-extern bool pgstat_lock_entry(PgStat_EntryRef *entry_ref, bool nowait);
+extern bool pgstat_lock_entry(PgStat_EntryRef *entry_ref, LWLockMode mode, bool nowait);
extern void pgstat_unlock_entry(PgStat_EntryRef *entry_ref);
extern bool pgstat_drop_entry(PgStat_Kind kind, Oid dboid, Oid objoid);
extern void pgstat_drop_all_entries(void);
