Experimental Physics and Industrial Control System
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
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