[Bug nptl/27304] New: pthread_cond_destroy does not pass private flag to futex system calls

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27304

            Bug ID: 27304
           Summary: pthread_cond_destroy does not pass private flag to
                    futex system calls
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Commit dc6cfdc934db9997c33728082d63552b9eee4563 ("nptl: Move
pthread_cond_destroy implementation into libc") dropped use of the
process-shared flag because of this optimization in
sysdeps/nptl/lowlevellock-futex.h:

# if IS_IN (libc) || IS_IN (rtld)
/* In libc.so or ld.so all futexes are private.  */
#  define __lll_private_flag(fl, private)                       \
  ({                                                            \
    /* Prevent warnings in callers of this macro.  */           \
    int __lll_private_flag_priv __attribute__ ((unused));       \
    __lll_private_flag_priv = (private);                        \
    ((fl) | FUTEX_PRIVATE_FLAG);                                \
  })
# else
#  define __lll_private_flag(fl, private) \
  (((fl) | FUTEX_PRIVATE_FLAG) ^ (private))
# endif

I think the fix is to remove the optimization because it is unused. The
internal locks are always private, so the optimization does not matter to them.

On x86-64, pthread_cond_destroy is the only place where the alternative macro
definition results in changed machine code, but we do not actually want this
change there (because it probably breaks process-shared condition variables).

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