On 9/4/22 13:18, Philippe Waroquiers wrote:
On Sun, 2022-09-04 at 00:14 +0200, Tomas Vondra wrote:
Clearly, this is not an issue valgrind is meant to detect (like invalid
memory access, etc.) but an application issue. I've tried reproducing it
without valgrind, but it only ever happens with valgrind - my theory is
it's some sort of race condition, and valgrind changes the timing in a
way that makes it much more likely to hit. I need to analyze the core to
inspect the state more closely, etc.
Any ideas what I might be doing wrong? Or how do I load the core file?
Rather than have the core dump and analyse it, you might interactively debug
your program under valgrind.
E.g. you might put a breakpoint on the assert or at some interesting points
before the assert.
See
https://valgrind.org/docs/manual/manual-core-adv.html#manual-core-adv.gdbserver
for more info.
I know, and I've used vgdbserver before. But sometimes that's not very
practical for a number of reasons:
1) Our tests run mostly unattended, possibly even on CI machines that we
don't have access to. And we don't want the machine to just sit there
and wait for someone to debug it interactively, it's better to report
the failure. Being able to inspect the core later would be helpful, though.
2) The error may be quite rare and/or hard to trigger - we regularly see
race conditions that happen 1 in 1000 runs. True, I could automate that
using a gdb script.
3) I'd bet it's not so simple in multi-process system that forks various
processes that can trigger the issue. I'd have to do attach a gdb to
each of those.
4) It's already pretty slow under valgrind, I'd bet it'll be even worse
with gdb, but maybe it's not that bad. rpi4 is very constrained, though.
5) Race conditions are often very sensitive to change in timing. For
example I've never seen this particular issue without valgrind. I can
easily imagine gdb changing the timing just enough for the race
condition to not happen.
regards
Tomas
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