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Always do TLS descriptor initialization at load time during relocation processing to avoid barriers at every TLS access. In non-dlopened shared libraries the overhead of tls access vs static global access is > 3x bigger when lazy initialization is used (_dl_tlsdesc_return_lazy) compared to bind-now (_dl_tlsdesc_return) so the barriers dominate tls access performance. TLSDESC relocs are in DT_JMPREL which are processed at load time using elf_machine_lazy_rel which is only supposed to do lightweight initialization using the DT_TLSDESC_PLT trampoline (the trampoline code jumps to the entry point in DT_TLSDESC_GOT which does the lazy tlsdesc initialization at runtime). This patch changes elf_machine_lazy_rel in aarch64 to do the symbol binding and initialization as if DF_BIND_NOW was set, so the non-lazy code path of elf/do-rel.h was replicated. The static linker could be changed to emit TLSDESC relocs in DT_REL*, which are processed non-lazily, but the goal of this patch is to always guarantee bind-now semantics, even if the binary was produced with an old linker, so the barriers can be dropped in tls descriptor functions. After this change the synchronizing ldar instructions can be dropped as well as the lazy initialization machinery including the DT_TLSDESC_GOT setup. I believe this should be done on all targets, including ones where no barrier is needed for lazy initialization. There is very little gain in optimizing for large number of symbolic tlsdesc relocations which is an extremely uncommon case. And currently the tlsdesc entries are only readonly protected with -z now and some hardennings against writable JUMPSLOT relocs don't work for TLSDESC so they are a security hazard. (But to fix that the static linker has to be changed.) * sysdeps/aarch64/dl-machine.h (elf_machine_lazy_rel): Do symbol binding and initialization non-lazily for R_AARCH64_TLSDESC. |
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This directory contains the sources of the GNU C Library. See the file "version.h" for what release version you have. The GNU C Library is the standard system C library for all GNU systems, and is an important part of what makes up a GNU system. It provides the system API for all programs written in C and C-compatible languages such as C++ and Objective C; the runtime facilities of other programming languages use the C library to access the underlying operating system. In GNU/Linux systems, the C library works with the Linux kernel to implement the operating system behavior seen by user applications. In GNU/Hurd systems, it works with a microkernel and Hurd servers. The GNU C Library implements much of the POSIX.1 functionality in the GNU/Hurd system, using configurations i[4567]86-*-gnu. The current GNU/Hurd support requires out-of-tree patches that will eventually be incorporated into an official GNU C Library release. When working with Linux kernels, this version of the GNU C Library requires Linux kernel version 3.2 or later. Also note that the shared version of the libgcc_s library must be installed for the pthread library to work correctly. The GNU C Library supports these configurations for using Linux kernels: aarch64*-*-linux-gnu alpha*-*-linux-gnu arm-*-linux-gnueabi hppa-*-linux-gnu Not currently functional without patches. i[4567]86-*-linux-gnu x86_64-*-linux-gnu Can build either x86_64 or x32 ia64-*-linux-gnu m68k-*-linux-gnu microblaze*-*-linux-gnu mips-*-linux-gnu mips64-*-linux-gnu powerpc-*-linux-gnu Hardware or software floating point, BE only. powerpc64*-*-linux-gnu Big-endian and little-endian. s390-*-linux-gnu s390x-*-linux-gnu sh[34]-*-linux-gnu sparc*-*-linux-gnu sparc64*-*-linux-gnu tilegx-*-linux-gnu tilepro-*-linux-gnu If you are interested in doing a port, please contact the glibc maintainers; see http://www.gnu.org/software/libc/ for more information. See the file INSTALL to find out how to configure, build, and install the GNU C Library. You might also consider reading the WWW pages for the C library at http://www.gnu.org/software/libc/. The GNU C Library is (almost) completely documented by the Texinfo manual found in the `manual/' subdirectory. The manual is still being updated and contains some known errors and omissions; we regret that we do not have the resources to work on the manual as much as we would like. For corrections to the manual, please file a bug in the `manual' component, following the bug-reporting instructions below. Please be sure to check the manual in the current development sources to see if your problem has already been corrected. Please see http://www.gnu.org/software/libc/bugs.html for bug reporting information. We are now using the Bugzilla system to track all bug reports. This web page gives detailed information on how to report bugs properly. The GNU C Library is free software. See the file COPYING.LIB for copying conditions, and LICENSES for notices about a few contributions that require these additional notices to be distributed. License copyright years may be listed using range notation, e.g., 1996-2015, indicating that every year in the range, inclusive, is a copyrightable year that would otherwise be listed individually.