external/pg_stat_monitor
1
0
Fork 0
mirror of https://github.com/percona/pg_stat_monitor.git synced 2026-02-04 05:56:21 +00:00
Code Issues Packages Projects Releases Wiki Activity

PG-244: Fix race condition in get_next_wbucket().

Browse Source 
The if condition bellow in geta_next_wbucket() was subject to a race
condition:
if ((current_usec - pgss->prev_bucket_usec) > (PGSM_BUCKET_TIME * 1000 *
1000))

Two or more threads/processes could easily evaluate this condition to
true, thus executing more than once the block that would calculate a
new bucket id, clear/move old entries in the pgss_query_hash and
pgss_hash hash tables.

To avoid this problem, we define prev_bucket_usec and current_wbucket
variables as atomic and execute a loop to check if another thread has
updated prev_bucket_usec before the current one.
This commit is contained in:
Diego Fronza
2021-09-30 17:13:27 -03:00
parent f269af3da2
commit 89743e9243
3 changed files with 70 additions and 50 deletions
Download Patch File Download Diff File Expand all files Collapse all files

24
pg_stat_monitor.h
View File

@@ -301,16 +301,16 @@ typedef struct pgssEntry
*/
typedef struct pgssSharedState
{
LWLock *lock; /* protects hashtable search/modification */
double cur_median_usage; /* current median usage in hashtable */
slock_t mutex; /* protects following fields only: */
Size extent; /* current extent of query file */
int64 n_writers; /* number of active writers to query file */
uint64 current_wbucket;
uint64 prev_bucket_usec;
uint64 bucket_entry[MAX_BUCKETS];
int64 query_buf_size_bucket;
char bucket_start_time[MAX_BUCKETS][60]; /* start time of the bucket */
LWLock *lock; /* protects hashtable search/modification */
double cur_median_usage; /* current median usage in hashtable */
slock_t mutex; /* protects following fields only: */
Size extent; /* current extent of query file */
int64 n_writers; /* number of active writers to query file */
pg_atomic_uint64 current_wbucket;
pg_atomic_uint64 prev_bucket_usec;
uint64 bucket_entry[MAX_BUCKETS];
int64 query_buf_size_bucket;
char bucket_start_time[MAX_BUCKETS][60]; /* start time of the bucket */
} pgssSharedState;
#define ResetSharedState(x) \
@@ -318,8 +318,8 @@ do { \
x->cur_median_usage = ASSUMED_MEDIAN_INIT; \
x->cur_median_usage = ASSUMED_MEDIAN_INIT; \
x->n_writers = 0; \
x->current_wbucket = 0; \
x->prev_bucket_usec = 0; \
pg_atomic_init_u64(&x->current_wbucket, 0); \
pg_atomic_init_u64(&x->prev_bucket_usec, 0); \
memset(&x->bucket_entry, 0, MAX_BUCKETS * sizeof(uint64)); \
} while(0)