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Conference vaxaxp::vmsnotes

Title:	VAX and Alpha VMS
Notice:	This is a new VMSnotes, please read note 2.1
Moderator:	VAXAXP::BERNARDO

Created:	Wed Jan 22 1997
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	703
Total number of notes:	3722

204.0. "Process hangs in LEF state " by NIXFIX::LOBSANG (A good bug is hard to find) Mon Feb 17 1997 15:53

I have a process that hangs in LEF state. On going through ther archived
VMSNotes files I came across Note 3150.* in VMSNotes_V10 which deals with
Processes that hang when $UPDSECW is called without specifying an event
flag and iosb. However, it is not clear from these Notes if this pertains
to both $UPDSEC and $UPDSECW. On reading the System Services documentation
regarding $UPDSEC, it seems that this would be the case for both $UPDSEC
and $UPDSECW since in $UPDSECW, if I am not mistaken, a $SYNCH is 
implicitly called with event flag 0 if no event flag is specified. 

The Notes in VMSNotes_V10 described how it was possible for a process
to hang upon calling $UPDSECW if no event flag is specified. It seems
that it is possible for another event to clear event flag 0 before
$UPDSECW reads it thus resulting in the Process hanging in a LEF state.

Upon reading the code for the hung process, I see that a call to $UPDSECW
is made with no event flag and iosb being specified. Could this be the
reason that the process hangs in LEF state. This happens only once in a
rare while - twice within the past six months and only when the application
is very, very busy handling lots of transactions. In the code it looks like
the $UPDSECW is the culprit because using SDA I could ascertain that the
process had taken out a lock on a resource that preceeds the call to
$UPDSECW and the actions following $UPDSECW do not appear to have been
executed. Would the use of $UPDSECW in this manner be the the reason
why the process remains hung in the LEF state ?


Lobsang

T.R	Title	User	Personal Name	Date	Lines
204.1	Use SDA or SHOW PROCESS, Locate The Hang...	XDELTA::HOFFMAN	Steve, OpenVMS Engineering	`Mon Feb 17 1997 16:30`	15
	LEF is often a normal state. Flakey application behaviour is usually due to incorrect use of the event flags, use of event flag zero, incorrect declaration or failure to use an IOSB whenever one is permitted, failure to synchronize the completion of asycnhronous calls via sys$synch or similar, incorrect or unintended sharing of event flags and/or IOSBs, or failure to check all system service or run-time return status codes... When the process is wedged, use SDA or SHOW PROCESS to try to determine what was going on in the context of the process -- you may be able to locate the PC where the process is wedged, and back-track to the user (or system) code that is involved in the hang.
204.2		AUSS::GARSON	DECcharity Program Office	`Mon Feb 17 1997 21:36`	27
	re .0 >Processes that hang when $UPDSECW is called without specifying an event >flag and iosb. That can happen. In fact it can happen when any of the $*W services is called and the IOSB is not specified and the EFN is not specified and there is inopportune asynchronous activity that uses event flag 0 (explicitly or implicitly). Later versions of VMS allow you to avoid event flag usage completely. Traditionally though you are always using an event flag whether you like it or not. > However, it is not clear from these Notes if this pertains >to both $UPDSEC and $UPDSECW. It can't happen with $UPDSEC because that service does not wait. It could happen if you then subsequently explicitly issued a call to $WAITFR or to $SYNCH. It really doesn't matter whether these discussions explain your observed behaviour. Put in an IOSB. But make sure that the lifetime of the IOSB variable exceeds the lifetime of its usage e.g. static if you have to. Having done that ensure that each concurrent asynchronous activity has a unique non-zero event flag.
204.3		UTRTSC::utoras-198-48-93.uto.dec.com::JurVanDerBurg	Change mode to Panic!	`Tue Feb 18 1997 01:27`	5
	If you don't specify an iosb for the call to $updsecw, then how do you know that the call completed without any errors? Right, you don't. Jur.
204.4	Need help with SDA	NIXFIX::LOBSANG	A good bug is hard to find	`Tue Feb 18 1997 13:40`	112
	The process in question hung today in the LEF state. I was able to use SDA to get some information but being not very proficient in SDA was unable to pinpoint the place where the process was hung. I used some commands that I saw in some SDA examples to exame some of the instructions and it seems that SYS$SYNCH is in action but I could not find out if SYS$SYNCH was called by SYS$UPDSECW. I have attached below the output from the SDA commands that I used. The <CR>s are a result of cut and paste operations that I used to save the output from a terminal emulator. Can someone verify if the process is hannging in $SYNCH ? If so, how do I find out in SDA which event flag $SYNCH is waitng for and if $SYNCH has been called by $UPDSECW and if the evnt flag is set or cleared ??? Lobsang SDA> sho stack/user Process stacks -------------- Current operating stack (USER): 7FEC515C 00000004 7FEC5160 0014EB78 7FEC5164 0FFC0000 7FEC5168 7FEC523C 7FEC516C 7FEC51B4 7FEC5170 8AC1635F EVENT_FLAGS_AND_ASTS+0055F 7FEC5174 00233BFF 7FEC5178 7FFEFEE0 CTL$GL_IPAGEFL SP => 7FEC517C 00000000 7FEC5180 00000000 7FEC5184 207C0000 7FEC5188 7FEC523C 7FEC518C 7FEC51B4 7FEC5190 80000343 EXE$WAIT_FORM+00023 7FEC5194 00000001 7FEC5198 7FFEFEE0 CTL$GL_IPAGEFL 7FEC519C 8CAEA140 PCB 7FEC51A0 7FF4FA00 7FEC51A4 FFFFFFFF PR$_XSID_N8NNN 7FEC51A8 00000002 7FEC51AC 00000000 7FEC51B0 00000000 7FEC51B4 00000000 7FEC51B8 01FC0020 7FEC51BC 7FEC52B0 7FEC51C0 7FEC5274 7FEC51C4 00223D11 7FEC51C8 00000001 7FEC51CC 7FEC5B01 7FEC51D0 00000001 7FEC51D4 00000000 7FEC51D8 00000001 7FEC51DC 00000400 IRP$M_MBXIO 7FEC51E0 7FEC51E4 7FEC51E4 00000005 7FEC51E8 00000001 7FEC51EC 00000001 7FEC51F0 00000000 7FEC51F4 7FFEE450 SYS$SYNCH+00010 7FEC51F8 03C00004 7FEC51FC 00546081 7FEC5200 00000000 7FEC5204 0FFC0020 7FEC5208 7FEC52B4 7FEC520C 7FEC5288 7FEC5210 0022289B 7FEC5214 00000001 7FEC5218 7FEC5B01 7FEC521C 00000000 7FEC5220 00000000 7FEC5224 7FEC5330 7FEC5228 00000400 IRP$M_MBXIO 7FEC522C 0000016C 7FEC5230 7FF28680 ............ ............ SDA>sho call Call Frame Information ---------------------- Call Frame Generated by CALLS Instruction Condition Handler 7FEC5180 00000000 SP Align Bits = 00 7FEC5184 207C0000 Saved AP 7FEC5188 7FEC523C Saved FP 7FEC518C 7FEC51B4 Return PC 7FEC5190 80000343 EXE$WAIT_FORM+00023 R2 7FEC5194 00000001 R3 7FEC5198 7FFEFEE0 CTL$GL_IPAGEFL R4 7FEC519C 8CAEA140 PCB R5 7FEC51A0 7FF4FA00 R6 7FEC51A4 FFFFFFFF PR$_XSID_N8NNN Align Stack by 0 Bytes => Argument List 7FEC51A8 00000002 7FEC51AC 00000000 7FEC51B0 00000000 SDA> e/i 80000343-20;20 EXE$WAIT_FORM+00003: HALT EXE$WAIT_FORM+00004: BLBC R0,EXE$WAIT_FORM+00029 EXE$WAIT_FORM+00007: MOVL R0,R2 EXE$WAIT_FORM+0000A: MOVQ 14(AP),-(SP) EXE$WAIT_FORM+0000E: BRB EXE$WAIT_FORM+0001C EXE$WAIT_FORM+00010: BLBC R0,EXE$WAIT_FORM+00029 EXE$WAIT_FORM+00013: MOVL R0,R2 EXE$WAIT_FORM+00016: PUSHL 14(AP) EXE$WAIT_FORM+00019: PUSHL 04(AP) EXE$WAIT_FORM+0001C: CALLS #02,@#EXE$SYNCH EXE$WAIT_FORM+00023: BLBC R0,EXE$WAIT_FORM+00029
204.5	fix the bug, always use an IOSB	GIDDAY::GILLINGS	a crucible of informative mistakes	`Tue Feb 18 1997 17:41`	14
	Lobsang, >Align Stack by 0 Bytes => >Argument List 7FEC51A8 00000002 > 7FEC51AC 00000000 > 7FEC51B0 00000000 If this is the argument to SYS$SYNCH, then you're using event flag 0 with no IOSB. This is a LEF hang looking for a place to happen. I would strongly recommend you stop attempting to repeat the diagnosis of this problem performed many times before, and simply correct your program. I don't care what the manual says, the IOSB argument is NOT optional. John Gillings, Sydney CSC
204.6	Code already fixed	ATZIS1::GYALPO		`Wed Feb 19 1997 03:01`	9
	John, Thanks for your viewpoint. I have already fixed the problem in the code by using an iosb and an event flag in the call to $UPDSECW. I just wanted to be sure that this was the cause of this particular hang. I know that the error in the code can/will cause a process to hang. Lobsang