Experimental Physics and
Industrial Control System

Dirk Zimoch <[email protected]> · Tue, 16 Oct 2018 15:56:25 +0200

Hi folks,

Sometimes I get a strange segmentation fault in the cbLow thread in

seqRecord function processCallback(). My EPICS base version is 3.14.12.7.

#0  processCallback (arg=<optimized out>) at ../seqRecord.c:373

The line is:

  dbGetLink(&(pcb->plinks[pcb->index]->dol), DBR_DOUBLE,
	&(pcb->plinks[pcb->index]->dov),0,0);

(gdb) print *pcb

$2 = {callback = {callback = 0xff97a24 <processCallback>, priority = 0,

user = 0xaffbdbe0, timer = 0x0}, pseqRecord = 0xb121a958, plinks =

{0xb121aae8, 0xb121ab38,

 0xb121ab88, 0xb121abd8, 0xb121ac28, 0xb121ac78, 0xb121acc8,

0xb121ad18, 0x0, 0x0, 0x0}, index = 8}

As you can see, index = 8, but plinks has only 8 valid entries. (This is

consistent with the record having 8 links.) Thus pcb->plinks[pcb->index]

== NULL, causing the crash.

I cannot see how this could possibly happen.

processCallback() is triggered by processNextLink(), but only if

pcb->plinks[pcb->index] != NULL.

It looks like something changes pcb->index after callbackRequest() but

before the callback runs. But the only code that modifies pcb->index is

the callback function processCallback() itself and the process()

function. Both while holding the dbScanLock semaphore. Also

processNextLink is only called while holding the dbScanLock semaphore.

The record looks like this:
record(seq, "$(DEV)$(SYS)$(CH)-CALC-AVG-RMS") {
   field(LNK1, "$(DEV)$(SYS)$(CH)-MIN PP")
   field(LNK2, "$(DEV)$(SYS)$(CH)-MAX PP")
   field(LNK3, "$(DEV)$(SYS)$(CH)-PEAK-PEAK PP")
   field(LNK4, "$(DEV)$(SYS)$(CH)-AVG PP")
   field(LNK5, "$(DEV)$(SYS)$(CH)-RMS PP")
   field(LNK6, "$(DEV)$(SYS)$(CH)-RMS-AVG PP")
   field(LNK7, "$(DEV)$(SYS)$(CH)-AVG2 PP")
   field(LNK8, "$(DEV)$(SYS)$(CH)-AVG-RMS PP")
}

It is FLNK'ed from a record which is scanned "I/O Intr" and which may

receive very quick "double triggers". So I suspect a race condition but

I cannot find it.

Any ideas?

Dirk

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System