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Commit38628db

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Add memory barriers for PgBackendStatus.st_changecount protocol.
st_changecount protocol needs the memory barriers to ensure thatthe apparent order of execution is as it desires. Otherwise,for example, the CPU might rearrange the code so that st_changecountis incremented twice before the modification on a machine withweak memory ordering. This surprising result can lead to bugs.This commit introduces the macros to load and store st_changecountwith the memory barriers. These are called before and afterPgBackendStatus entries are modified or copied into private memory,in order to prevent CPU from reordering PgBackendStatus access.Per discussion on pgsql-hackers, we decided not to back-patch thisto 9.4 or before until we get an actual bug report about this.Patch by me. Review by Robert Haas.
1 parent19e065c commit38628db

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2 files changed

+71
-24
lines changed

2 files changed

+71
-24
lines changed

‎src/backend/postmaster/pgstat.c

Lines changed: 27 additions & 24 deletions
Original file line numberDiff line numberDiff line change
@@ -2563,7 +2563,7 @@ pgstat_bestart(void)
25632563
beentry=MyBEEntry;
25642564
do
25652565
{
2566-
beentry->st_changecount++;
2566+
pgstat_increment_changecount_before(beentry);
25672567
}while ((beentry->st_changecount&1)==0);
25682568

25692569
beentry->st_procpid=MyProcPid;
@@ -2588,8 +2588,7 @@ pgstat_bestart(void)
25882588
beentry->st_appname[NAMEDATALEN-1]='\0';
25892589
beentry->st_activity[pgstat_track_activity_query_size-1]='\0';
25902590

2591-
beentry->st_changecount++;
2592-
Assert((beentry->st_changecount&1)==0);
2591+
pgstat_increment_changecount_after(beentry);
25932592

25942593
/* Update app name to current GUC setting */
25952594
if (application_name)
@@ -2624,12 +2623,11 @@ pgstat_beshutdown_hook(int code, Datum arg)
26242623
* before and after. We use a volatile pointer here to ensure the
26252624
* compiler doesn't try to get cute.
26262625
*/
2627-
beentry->st_changecount++;
2626+
pgstat_increment_changecount_before(beentry);
26282627

26292628
beentry->st_procpid=0;/* mark invalid */
26302629

2631-
beentry->st_changecount++;
2632-
Assert((beentry->st_changecount&1)==0);
2630+
pgstat_increment_changecount_after(beentry);
26332631
}
26342632

26352633

@@ -2666,16 +2664,15 @@ pgstat_report_activity(BackendState state, const char *cmd_str)
26662664
* non-disabled state. As our final update, change the state and
26672665
* clear fields we will not be updating anymore.
26682666
*/
2669-
beentry->st_changecount++;
2667+
pgstat_increment_changecount_before(beentry);
26702668
beentry->st_state=STATE_DISABLED;
26712669
beentry->st_state_start_timestamp=0;
26722670
beentry->st_activity[0]='\0';
26732671
beentry->st_activity_start_timestamp=0;
26742672
/* st_xact_start_timestamp and st_waiting are also disabled */
26752673
beentry->st_xact_start_timestamp=0;
26762674
beentry->st_waiting= false;
2677-
beentry->st_changecount++;
2678-
Assert((beentry->st_changecount&1)==0);
2675+
pgstat_increment_changecount_after(beentry);
26792676
}
26802677
return;
26812678
}
@@ -2695,7 +2692,7 @@ pgstat_report_activity(BackendState state, const char *cmd_str)
26952692
/*
26962693
* Now update the status entry
26972694
*/
2698-
beentry->st_changecount++;
2695+
pgstat_increment_changecount_before(beentry);
26992696

27002697
beentry->st_state=state;
27012698
beentry->st_state_start_timestamp=current_timestamp;
@@ -2707,8 +2704,7 @@ pgstat_report_activity(BackendState state, const char *cmd_str)
27072704
beentry->st_activity_start_timestamp=start_timestamp;
27082705
}
27092706

2710-
beentry->st_changecount++;
2711-
Assert((beentry->st_changecount&1)==0);
2707+
pgstat_increment_changecount_after(beentry);
27122708
}
27132709

27142710
/* ----------
@@ -2734,13 +2730,12 @@ pgstat_report_appname(const char *appname)
27342730
* st_changecount before and after. We use a volatile pointer here to
27352731
* ensure the compiler doesn't try to get cute.
27362732
*/
2737-
beentry->st_changecount++;
2733+
pgstat_increment_changecount_before(beentry);
27382734

27392735
memcpy((char*)beentry->st_appname,appname,len);
27402736
beentry->st_appname[len]='\0';
27412737

2742-
beentry->st_changecount++;
2743-
Assert((beentry->st_changecount&1)==0);
2738+
pgstat_increment_changecount_after(beentry);
27442739
}
27452740

27462741
/*
@@ -2760,10 +2755,9 @@ pgstat_report_xact_timestamp(TimestampTz tstamp)
27602755
* st_changecount before and after. We use a volatile pointer here to
27612756
* ensure the compiler doesn't try to get cute.
27622757
*/
2763-
beentry->st_changecount++;
2758+
pgstat_increment_changecount_before(beentry);
27642759
beentry->st_xact_start_timestamp=tstamp;
2765-
beentry->st_changecount++;
2766-
Assert((beentry->st_changecount&1)==0);
2760+
pgstat_increment_changecount_after(beentry);
27672761
}
27682762

27692763
/* ----------
@@ -2839,7 +2833,10 @@ pgstat_read_current_status(void)
28392833
*/
28402834
for (;;)
28412835
{
2842-
intsave_changecount=beentry->st_changecount;
2836+
intbefore_changecount;
2837+
intafter_changecount;
2838+
2839+
pgstat_save_changecount_before(beentry,before_changecount);
28432840

28442841
localentry->backendStatus.st_procpid=beentry->st_procpid;
28452842
if (localentry->backendStatus.st_procpid>0)
@@ -2856,8 +2853,9 @@ pgstat_read_current_status(void)
28562853
localentry->backendStatus.st_activity=localactivity;
28572854
}
28582855

2859-
if (save_changecount==beentry->st_changecount&&
2860-
(save_changecount&1)==0)
2856+
pgstat_save_changecount_after(beentry,after_changecount);
2857+
if (before_changecount==after_changecount&&
2858+
(before_changecount&1)==0)
28612859
break;
28622860

28632861
/* Make sure we can break out of loop if stuck... */
@@ -2927,12 +2925,17 @@ pgstat_get_backend_current_activity(int pid, bool checkUser)
29272925

29282926
for (;;)
29292927
{
2930-
intsave_changecount=vbeentry->st_changecount;
2928+
intbefore_changecount;
2929+
intafter_changecount;
2930+
2931+
pgstat_save_changecount_before(vbeentry,before_changecount);
29312932

29322933
found= (vbeentry->st_procpid==pid);
29332934

2934-
if (save_changecount==vbeentry->st_changecount&&
2935-
(save_changecount&1)==0)
2935+
pgstat_save_changecount_after(vbeentry,after_changecount);
2936+
2937+
if (before_changecount==after_changecount&&
2938+
(before_changecount&1)==0)
29362939
break;
29372940

29382941
/* Make sure we can break out of loop if stuck... */

‎src/include/pgstat.h

Lines changed: 44 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -16,6 +16,7 @@
1616
#include"libpq/pqcomm.h"
1717
#include"portability/instr_time.h"
1818
#include"postmaster/pgarch.h"
19+
#include"storage/barrier.h"
1920
#include"utils/hsearch.h"
2021
#include"utils/relcache.h"
2122

@@ -714,6 +715,12 @@ typedef struct PgBackendStatus
714715
* st_changecount again. If the value hasn't changed, and if it's even,
715716
* the copy is valid; otherwise start over. This makes updates cheap
716717
* while reads are potentially expensive, but that's the tradeoff we want.
718+
*
719+
* The above protocol needs the memory barriers to ensure that
720+
* the apparent order of execution is as it desires. Otherwise,
721+
* for example, the CPU might rearrange the code so that st_changecount
722+
* is incremented twice before the modification on a machine with
723+
* weak memory ordering. This surprising result can lead to bugs.
717724
*/
718725
intst_changecount;
719726

@@ -745,6 +752,43 @@ typedef struct PgBackendStatus
745752
char*st_activity;
746753
}PgBackendStatus;
747754

755+
/*
756+
* Macros to load and store st_changecount with the memory barriers.
757+
*
758+
* pgstat_increment_changecount_before() and
759+
* pgstat_increment_changecount_after() need to be called before and after
760+
* PgBackendStatus entries are modified, respectively. This makes sure that
761+
* st_changecount is incremented around the modification.
762+
*
763+
* Also pgstat_save_changecount_before() and pgstat_save_changecount_after()
764+
* need to be called before and after PgBackendStatus entries are copied into
765+
* private memory, respectively.
766+
*/
767+
#definepgstat_increment_changecount_before(beentry)\
768+
do {\
769+
beentry->st_changecount++;\
770+
pg_write_barrier();\
771+
} while (0)
772+
773+
#definepgstat_increment_changecount_after(beentry)\
774+
do {\
775+
pg_write_barrier();\
776+
beentry->st_changecount++;\
777+
Assert((beentry->st_changecount & 1) == 0);\
778+
} while (0)
779+
780+
#definepgstat_save_changecount_before(beentry,save_changecount)\
781+
do {\
782+
save_changecount = beentry->st_changecount;\
783+
pg_read_barrier();\
784+
} while (0)
785+
786+
#definepgstat_save_changecount_after(beentry,save_changecount)\
787+
do {\
788+
pg_read_barrier();\
789+
save_changecount = beentry->st_changecount;\
790+
} while (0)
791+
748792
/* ----------
749793
* LocalPgBackendStatus
750794
*

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