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Commit045ec34

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pgstat: Acquire lock when reading variable-numbered stats
Somewhere during the development of the patch acquiring a lock during readaccess to variable-numbered stats got lost. The missing lock acquisition won'tcause corruption, but can lead to reading torn values when accessingstats. Add the missing lock acquisitions.Reported-by: Greg Stark <stark@mit.edu>Reviewed-by: "Drouvot, Bertrand" <bdrouvot@amazon.com>Reviewed-by: Andres Freund <andres@anarazel.de>Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>Discussion:https://postgr.es/m/CAM-w4HMYkM_DkYhWtUGV+qE_rrBxKOzOF0+5faozxO3vXrc9wA@mail.gmail.comBackpatch: 15-
1 parenta2caf18 commit045ec34

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3 files changed

+26
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lines changed

3 files changed

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‎src/backend/utils/activity/pgstat.c

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -844,9 +844,12 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, Oid objoid)
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else
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stats_data=MemoryContextAlloc(pgStatLocal.snapshot.context,
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kind_info->shared_data_len);
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pgstat_lock_entry_shared(entry_ref, false);
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memcpy(stats_data,
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pgstat_get_entry_data(kind,entry_ref->shared_stats),
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kind_info->shared_data_len);
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pgstat_unlock_entry(entry_ref);
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if (pgstat_fetch_consistency>PGSTAT_FETCH_CONSISTENCY_NONE)
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{
@@ -983,9 +986,15 @@ pgstat_build_snapshot(void)
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entry->data=MemoryContextAlloc(pgStatLocal.snapshot.context,
985988
kind_info->shared_size);
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/*
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* Acquire the LWLock directly instead of using
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* pg_stat_lock_entry_shared() which requires a reference.
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*/
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LWLockAcquire(&stats_data->lock,LW_SHARED);
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memcpy(entry->data,
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pgstat_get_entry_data(kind,stats_data),
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kind_info->shared_size);
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LWLockRelease(&stats_data->lock);
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}
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dshash_seq_term(&hstat);
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‎src/backend/utils/activity/pgstat_shmem.c

Lines changed: 16 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -579,6 +579,22 @@ pgstat_lock_entry(PgStat_EntryRef *entry_ref, bool nowait)
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return true;
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}
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/*
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* Separate from pgstat_lock_entry() as most callers will need to lock
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* exclusively.
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*/
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bool
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pgstat_lock_entry_shared(PgStat_EntryRef*entry_ref,boolnowait)
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{
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LWLock*lock=&entry_ref->shared_stats->lock;
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if (nowait)
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returnLWLockConditionalAcquire(lock,LW_SHARED);
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LWLockAcquire(lock,LW_SHARED);
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return true;
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}
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void
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pgstat_unlock_entry(PgStat_EntryRef*entry_ref)
584600
{

‎src/include/utils/pgstat_internal.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -581,6 +581,7 @@ extern void pgstat_detach_shmem(void);
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externPgStat_EntryRef*pgstat_get_entry_ref(PgStat_Kindkind,Oiddboid,Oidobjoid,
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boolcreate,bool*found);
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externboolpgstat_lock_entry(PgStat_EntryRef*entry_ref,boolnowait);
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externboolpgstat_lock_entry_shared(PgStat_EntryRef*entry_ref,boolnowait);
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externvoidpgstat_unlock_entry(PgStat_EntryRef*entry_ref);
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externboolpgstat_drop_entry(PgStat_Kindkind,Oiddboid,Oidobjoid);
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externvoidpgstat_drop_all_entries(void);

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