Data race in interfaces/libpq/fe-exec.c
This line
https://github.com/postgres/postgres/blob/30012a04a6c8127397a8ab71e160d9c7e7fbe874/src/interfaces/libpq/fe-exec.c#L1073
triggers data race errors when run under ThreadSanitizer (*)
As far as I can tell, the static variable in question is a hack to allow a
couple of deprecated functions that are already unsafe to use
(PQescapeString and PQescapeBytea) to be fractionally less unsafe to use.
Would there be any interest in a patch changing the type of
static_client_coding
and static_std_strings
<https://github.com/postgres/postgres/blob/30012a04a6c8127397a8ab71e160d9c7e7fbe874/src/interfaces/libpq/fe-exec.c#L49>
to
some atomic equivalent, so the data race goes away?
I know that as long as clients aren't daft enough to call PQescapeString or
PQescapeBytea, then the static variables are write-only - but wiser heads
than mine assure me that even if nothing ever reads from the variables,
unguarded simultaneous writes can cause issues when built with particularly
aggressive compilers... On the plus side: "As a note for changing the
variable to be atomic - if it uses acquire-release semantics, it'll be free
on x86 (barring any potential compiler optimisations that we *want* to
stop). And relaxed semantics should be free everywhere."
And it would be nice to be able to keep running my postgres-using tests
under ThreadSanitizer...
thanks
Mark
(*) specifically in a test that kicks off two simultaneous threads whose
first action is to open a postgres connection.
Mark Charsley <mcharsley@google.com> writes:
This line
https://github.com/postgres/postgres/blob/30012a04a6c8127397a8ab71e160d9c7e7fbe874/src/interfaces/libpq/fe-exec.c#L1073
triggers data race errors when run under ThreadSanitizer (*)
As far as I can tell, the static variable in question is a hack to allow a
couple of deprecated functions that are already unsafe to use
(PQescapeString and PQescapeBytea) to be fractionally less unsafe to use.
Yup.
Would there be any interest in a patch changing the type of
static_client_coding
and static_std_strings
<https://github.com/postgres/postgres/blob/30012a04a6c8127397a8ab71e160d9c7e7fbe874/src/interfaces/libpq/fe-exec.c#L49>
to
some atomic equivalent, so the data race goes away?
I don't see that making those be some other datatype would improve anything
usefully. (1) On just about every platform known to man, int and bool are
going to be atomic anyway. (2) The *actual* hazards here, as opposed to
theoretical ones, are that you're using more than one connection with
different settings for these values, whereupon it's not clear whether
those deprecated functions will see the appropriate settings when they're
used. A different data type won't help that.
In short: this warning you're getting from ThreadSanitizer is entirely
off-point, so contorting the code to suppress it seems useless.
regards, tom lane
According to folks significantly cleverer than me, this can be a problem:
See section 2.4 in
https://www.usenix.org/legacy/events/hotpar11/tech/final_files/Boehm.pdf
tl;dr in a self-fulfilling prophecy kind of way, there are no benign
data-races. So the compiler can assume no-one would write a data race.
Therefore it can make aggressive optimisations that render what would
otherwise have been a benign race actively dangerous.
Granted the danger here is mainly theoretical, and the main problem for me
is that turning off ThreadSanitizer because of this issue means that other
more dangerous issues in my code (rather than the postgres client code)
won't be found. But the above is the reason why ThreadSanitizer folks don't
want to put in any "you can ignore this race, it's benign" functionality,
and told me that the right thing to do was to contact you folks and get a
fix in upstream...
Mark
On Thu, Jan 30, 2020 at 4:46 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Show quoted text
Mark Charsley <mcharsley@google.com> writes:
This line
triggers data race errors when run under ThreadSanitizer (*)
As far as I can tell, the static variable in question is a hack to allow
a
couple of deprecated functions that are already unsafe to use
(PQescapeString and PQescapeBytea) to be fractionally less unsafe to use.Yup.
Would there be any interest in a patch changing the type of
static_client_coding
and static_std_strings
<to
some atomic equivalent, so the data race goes away?I don't see that making those be some other datatype would improve anything
usefully. (1) On just about every platform known to man, int and bool are
going to be atomic anyway. (2) The *actual* hazards here, as opposed to
theoretical ones, are that you're using more than one connection with
different settings for these values, whereupon it's not clear whether
those deprecated functions will see the appropriate settings when they're
used. A different data type won't help that.In short: this warning you're getting from ThreadSanitizer is entirely
off-point, so contorting the code to suppress it seems useless.regards, tom lane