Re: Fix race condition in SSI when reading PredXact->SxactGlobalXmin
| От | Josh Curtis |
|---|---|
| Тема | Re: Fix race condition in SSI when reading PredXact->SxactGlobalXmin |
| Дата | |
| Msg-id | CAEkjoh2BTMPF_TgR7TRtxZAsWdN7qRvvCoM9E7RuqUfYf28v7g@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Fix race condition in SSI when reading PredXact->SxactGlobalXmin (Mihail Nikalayeu <mihailnikalayeu@gmail.com>) |
| Список | pgsql-hackers |
Thanks for taking a look.
I tried adding a reproduction with injection points, but had some trouble with them. I'll give it another go this weekend.
I guess one of the commitfest entries should be deleted? I had already made one already. I don't see a way to do that though.
https://commitfest.postgresql.org/patch/6037/
Josh
I tried adding a reproduction with injection points, but had some trouble with them. I'll give it another go this weekend.
I guess one of the commitfest entries should be deleted? I had already made one already. I don't see a way to do that though.
https://commitfest.postgresql.org/patch/6037/
Josh
On Mon, Oct 20, 2025 at 5:55 PM Mihail Nikalayeu <mihailnikalayeu@gmail.com> wrote:
Hello!
Josh Curtis <jcurtis825@gmail.com>:
> This is definitely a bit more complex. It requires that SetNewSxactGlobalXmin is never called when SxactGlobalXmin is invalid to prevent readers from seeing an invalid transaction ID when they should see a valid one -- I think this is the case now since before SetNewSxactGlobalXmin is called postgres checks that PredXact->SxactGlobalXminCount > 0. I assume this entails that SxactGlobalXmin is valid, but I have not checked every place the two variables are modified.
Such logic feels fragile to me. Maybe add a special flag like
'PredXact->SxactGlobalSkipAllowed' which will be updated to 'true' by
SetNewSxactGlobalXmin, and dropped to 'false' by functions affecting
PredXact->activeList?
But I prefer the first solution anyway.
Best regards,
Mikhail.
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