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libc/locale: Fix races between localeconv(3) and setlocale(3)

Authored by markj on Sep 10 2021, 2:32 PM.



Each locale embeds a lazily initialized lconv which is populated by
localeconv(3) and localeconv_l(3). When setlocale(3) updates the global
locale, the lconv needs to be (lazily) reinitialized. To signal this,
we set flag variables in the locale structure. There are two problems:

- The flags are set before the locale is fully updated, so a concurrent
  localeconv() call can observe partially initialized locale data.
- No barriers ensure that localeconv() observes a fully initialized
  locale if a flag is set.

So, move the flag update appropriately, and use acq/rel barriers to
provide some synchronization.

Thanks to Henry Hu <> for providing a test case
demonstrating the race.

PR: 258360
MFC after: 2 weeks

Diff Detail

R10 FreeBSD src repository
Automatic diff as part of commit; lint not applicable.
Automatic diff as part of commit; unit tests not applicable.

Event Timeline

markj requested review of this revision.Sep 10 2021, 2:32 PM
This revision was not accepted when it landed; it landed in state Needs Review.Fri, Sep 17, 3:00 PM
This revision was automatically updated to reflect the committed changes.