*** a/src/backend/storage/lmgr/proc.c
--- b/src/backend/storage/lmgr/proc.c
***************
*** 1086,1097 **** ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable)
  				!(autovac_pgxact->vacuumFlags & PROC_VACUUM_FOR_WRAPAROUND))
  			{
  				int			pid = autovac->pid;
  
! 				elog(DEBUG2, "sending cancel to blocking autovacuum PID %d",
! 					 pid);
  
! 				/* don't hold the lock across the kill() syscall */
! 				LWLockRelease(ProcArrayLock);
  
  				/* send the autovacuum worker Back to Old Kent Road */
  				if (kill(pid, SIGINT) < 0)
--- 1086,1114 ----
  				!(autovac_pgxact->vacuumFlags & PROC_VACUUM_FOR_WRAPAROUND))
  			{
  				int			pid = autovac->pid;
+ 				StringInfoData locktagbuf;
+ 				StringInfoData logbuf;		/* errdetail for server log */
+ 
+ 				initStringInfo(&locktagbuf);
+ 				initStringInfo(&logbuf);
+ 				DescribeLockTag(&locktagbuf, &lock->tag);
+ 				appendStringInfo(&logbuf,
+ 					  _("Process %d waits for %s on %s"),
+ 						 MyProcPid,
+ 						 GetLockmodeName(lock->tag.locktag_lockmethodid,
+ 										 lockmode),
+ 						 locktagbuf.data);
+ 
+ 				/* release lock as quickly as possible */
+ 				LWLockRelease(ProcArrayLock);
  
! 				ereport(LOG,
! 						(errmsg("sending cancel to blocking autovacuum PID %d",
! 							pid),
! 						 errdetail_log("%s", logbuf.data)));
  
! 				pfree(logbuf.data);
! 				pfree(locktagbuf.data);
  
  				/* send the autovacuum worker Back to Old Kent Road */
  				if (kill(pid, SIGINT) < 0)