With the current set of fences, the version update at the start
of the TM write operation is redundant, and the version update
at the end does not need to use an atomic read-modify-write
operation.
Also use relaxed MO stores during the dlclose update, and skip any
version changes there.
Suggested-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
As explained in Hans Boehm, Can Seqlocks Get Along with Programming
Language Memory Models?, an acquire fence is needed in
_dlfo_read_success. The lack of a fence resulted in an observable
bug on powerpc64le compile-time load reordering.
The fence in _dlfo_mappings_begin_update has been reordered, turning
the fence/store sequence into a release MO store equivalent.
Relaxed MO loads are used on the reader side, and relaxed MO stores
on the writer side for the shared data, to avoid formal data races.
This is just to be conservative; it should not actually be necessary
given how the data is used.
This commit also fixes the test run time. The intent was to run it
for 3 seconds, but 0.3 seconds was enough to uncover the bug very
occasionally (while 3 seconds did not reliably show the bug on every
test run).
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
I used these shell commands:
../glibc/scripts/update-copyrights $PWD/../gnulib/build-aux/update-copyright
(cd ../glibc && git commit -am"[this commit message]")
and then ignored the output, which consisted lines saying "FOO: warning:
copyright statement not found" for each of 7061 files FOO.
I then removed trailing white space from math/tgmath.h,
support/tst-support-open-dev-null-range.c, and
sysdeps/x86_64/multiarch/strlen-vec.S, to work around the following
obscure pre-commit check failure diagnostics from Savannah. I don't
know why I run into these diagnostics whereas others evidently do not.
remote: *** 912-#endif
remote: *** 913:
remote: *** 914-
remote: *** error: lines with trailing whitespace found
...
remote: *** error: sysdeps/unix/sysv/linux/statx_cp.c: trailing lines
It can be used to speed up the libgcc unwinder, and the internal
_dl_find_dso_for_object function (which is used for caller
identification in dlopen and related functions, and in dladdr).
_dl_find_object is in the internal namespace due to bug 28503.
If libgcc switches to _dl_find_object, this namespace issue will
be fixed. It is located in libc for two reasons: it is necessary
to forward the call to the static libc after static dlopen, and
there is a link ordering issue with -static-libgcc and libgcc_eh.a
because libc.so is not a linker script that includes ld.so in the
glibc build tree (so that GCC's internal -lc after libgcc_eh.a does
not pick up ld.so).
It is necessary to do the i386 customization in the
sysdeps/x86/bits/dl_find_object.h header shared with x86-64 because
otherwise, multilib installations are broken.
The implementation uses software transactional memory, as suggested
by Torvald Riegel. Two copies of the supporting data structures are
used, also achieving full async-signal-safety.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
