Hello,
At Wed, 20 Apr 2016 15:14:16 +0200, Petr Jelinek <p...@2ndquadrant.com> wrote
in <571780a8.4070...@2ndquadrant.com>
> I noticed sporadic segfaults when selecting from pg_stat_activity on
> current HEAD.
>
> The culprit is the 53be0b1add7064ca5db3cd884302dfc3268d884e commit
> which added more wait info into the pg_stat_get_activity(). More
> specifically, the following code is broken:
>
> + proc = BackendPidGetProc(beentry->st_procpid);
> + wait_event_type = pgstat_get_wait_event_type(proc->wait_event_info);
>
> This needs to check if proc is NULL. When reading the code I noticed
> that the new functions pg_stat_get_backend_wait_event_type() and
> pg_stat_get_backend_wait_event() suffer from the same problem.
Good catch.
> Here is PoC patch which fixes the problem. I am wondering if we should
> raise warning in the pg_stat_get_backend_wait_event_type() and
> pg_stat_get_backend_wait_event() like the pg_signal_backend() does
> when proc is NULL instead of just returning NULL which is what this
> patch does though.
It still makes the two relevant columns in pg_stat_activity
inconsistent each other since it reads the procarray entry twice
without a lock on procarray.
The attached patch adds pgstat_get_wait_event_info to read
wait_event_info in more appropriate way. Then change
pg_stat_get_wait_event(_type) to take the wait_event_info.
Does this work for you?
We still may have an inconsistency between weit_event and query,
or beentry itself but preventing it would need to have local
copies of wait_event_info of all corresponding entries in
procarray, which will be overdone.
regards,
--
Kyotaro Horiguchi
NTT Open Source Software Center
diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 41f4374..999b7e7 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -55,6 +55,7 @@
#include "storage/latch.h"
#include "storage/lmgr.h"
#include "storage/pg_shmem.h"
+#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "storage/sinvaladt.h"
#include "utils/ascii.h"
@@ -3118,6 +3119,27 @@ pgstat_read_current_status(void)
}
/* ----------
+ * pgstat_get_wait_event_info() -
+ *
+ * Return the wait_event_info for a procid. 0 if the proc is no longer
+ * living or has no waiting event.
+ */
+uint32
+pgstat_get_wait_event_info(int procpid)
+{
+ uint32 ret = 0;
+ PGPROC *proc;
+
+ LWLockAcquire(ProcArrayLock, LW_SHARED);
+ proc = BackendPidGetProcWithLock(procpid);
+ if (proc)
+ ret = proc->wait_event_info;
+ LWLockRelease(ProcArrayLock);
+
+ return ret;
+}
+
+/* ----------
* pgstat_get_wait_event_type() -
*
* Return a string representing the current wait event type, backend is
diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 64c4cc4..72776ab 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -679,7 +679,7 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
bool nulls[PG_STAT_GET_ACTIVITY_COLS];
LocalPgBackendStatus *local_beentry;
PgBackendStatus *beentry;
- PGPROC *proc;
+ uint32 wait_event_info;
const char *wait_event_type;
const char *wait_event;
@@ -716,6 +716,14 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
continue;
}
+ /*
+ * Read wait event. This may be inconsistent with the beentry when the
+ * corresponding procarray entry has been removed or modified after
+ * the beentry was copied, but we don't need such strict consistency
+ * for this information.
+ */
+ wait_event_info = pgstat_get_wait_event_info(beentry->st_procpid);
+
/* Values available to all callers */
values[0] = ObjectIdGetDatum(beentry->st_databaseid);
values[1] = Int32GetDatum(beentry->st_procpid);
@@ -782,14 +790,13 @@ pg_stat_get_activity(PG_FUNCTION_ARGS)
values[5] = CStringGetTextDatum(beentry->st_activity);
- proc = BackendPidGetProc(beentry->st_procpid);
- wait_event_type = pgstat_get_wait_event_type(proc->wait_event_info);
+ wait_event_type = pgstat_get_wait_event_type(wait_event_info);
if (wait_event_type)
values[6] = CStringGetTextDatum(wait_event_type);
else
nulls[6] = true;
- wait_event = pgstat_get_wait_event(proc->wait_event_info);
+ wait_event = pgstat_get_wait_event(wait_event_info);
if (wait_event)
values[7] = CStringGetTextDatum(wait_event);
else
@@ -983,7 +990,6 @@ pg_stat_get_backend_wait_event_type(PG_FUNCTION_ARGS)
{
int32 beid = PG_GETARG_INT32(0);
PgBackendStatus *beentry;
- PGPROC *proc;
const char *wait_event_type;
if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
@@ -992,8 +998,8 @@ pg_stat_get_backend_wait_event_type(PG_FUNCTION_ARGS)
wait_event_type = "<insufficient privilege>";
else
{
- proc = BackendPidGetProc(beentry->st_procpid);
- wait_event_type = pgstat_get_wait_event_type(proc->wait_event_info);
+ wait_event_type = pgstat_get_wait_event_type(
+ pgstat_get_wait_event_info(beentry->st_procpid));
}
if (!wait_event_type)
@@ -1007,7 +1013,6 @@ pg_stat_get_backend_wait_event(PG_FUNCTION_ARGS)
{
int32 beid = PG_GETARG_INT32(0);
PgBackendStatus *beentry;
- PGPROC *proc;
const char *wait_event;
if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
@@ -1016,8 +1021,8 @@ pg_stat_get_backend_wait_event(PG_FUNCTION_ARGS)
wait_event = "<insufficient privilege>";
else
{
- proc = BackendPidGetProc(beentry->st_procpid);
- wait_event = pgstat_get_wait_event(proc->wait_event_info);
+ wait_event = pgstat_get_wait_event(
+ pgstat_get_wait_event_info(beentry->st_procpid));
}
if (!wait_event)
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index 4be09fe..228e865 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -969,6 +969,7 @@ extern void pgstat_report_activity(BackendState state, const char *cmd_str);
extern void pgstat_report_tempfile(size_t filesize);
extern void pgstat_report_appname(const char *appname);
extern void pgstat_report_xact_timestamp(TimestampTz tstamp);
+extern uint32 pgstat_get_wait_event_info(int procpid);
extern const char *pgstat_get_wait_event(uint32 wait_event_info);
extern const char *pgstat_get_wait_event_type(uint32 wait_event_info);
extern const char *pgstat_get_backend_current_activity(int pid, bool checkUser);
--
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