"Qingqing Zhou" <zhouqq@cs.toronto.edu> writes:
> "Tom Lane" <tgl@sss.pgh.pa.us> wrote
>> "Qingqing Zhou" <zhouqq@cs.toronto.edu> writes:
>>> After this, the proc->sem will be bumped to 1 unexpectedly ... Since
>>> this problem is rare, a possible fix is to put a critical section
>>> around line 1 to 7 and remove UnlockBuffers() accordingly.
>>
>> No, that would make any attempt to control-C a VACUUM have a significant
>> probability for panicking the whole database.
> Why panicking by control-C?
Because the entire point of a critical section is that any error (eg
"Query cancelled") is turned into a panic. So a query-cancel attempt
while a vacuum is blocked here would either do nothing (bad) or panic
the database (worse).
regards, tom lane