src/libthread/README.Linux - plan9 - Git at Google

 Thread support on Linux is confused by the recent thread local storage (TLS)
 support that has been put into the ELF tool chain.  The TLS libraries are
 installed in /lib/tls on most Linux systems.

 We provide two different implementations of the os-dependent parts
 of libthread for Linux.  The first is intended for use on Linux 2.4 and earlier
 kernels, which do not support TLS.  It is in Linux.c and Linuxasm.c and
 does not use the pthread interface.  The second is intended for Linux 2.6
 and later kernels, which do support TLS.  It is in pthread.c and uses the
 standard pthread interface.  It expects to be linked against the TLS-aware
 thread library aka NPTL.

 If you use Linux.c and Linuxasm.c with TLS libraries, they do not
 set up the TLS properly so you will get incorrect programs.
 For example, there will only be one errno among all the procs
 in your program instead of one per proc.  The pthread NPTL
 implementation is needed to use the TLS libraries properly.

 If you use pthread.c without TLS libraries (i.e., with the old Linux
 pthread library known as LinuxThreads), then you will also get
 incorrect programs, although more obviously so.  The LinuxThreads
 library assumes it can look at the stack pointer to distinguish between
 threads, but libthread does its own stack management, breaking this
 assumption.  If you run a pthread-compiled program with the
 LinuxThreads library, LinuxThreads itself will cause a segmentation
 fault in __pthread_getspecific() the first time it is called from a
 non-standard stack.

 So, it is important that you compile binaries that match your
 system's choice of TLS vs. not-TLS libraries.  The hard part is figuring
 out which your system has chosen.  Plan9port looks at the kernel
 version you are running and assumes that on kernels that support
 TLS (2.6+) you will be using TLS.

 Apparently Gentoo and maybe other distributions do not follow this rule.
 They use non-TLS libraries even on kernels that can support TLS.
 To accomodate them, you can add a line SYSVERSION=2.4 to $PLAN9/config
 to force the build to think you are running an old kernel.
 The INSTALL script sets up this file automatically on Linux systems.
	Thread support on Linux is confused by the recent thread local storage (TLS)
	support that has been put into the ELF tool chain. The TLS libraries are
	installed in /lib/tls on most Linux systems.

	We provide two different implementations of the os-dependent parts
	of libthread for Linux. The first is intended for use on Linux 2.4 and earlier
	kernels, which do not support TLS. It is in Linux.c and Linuxasm.c and
	does not use the pthread interface. The second is intended for Linux 2.6
	and later kernels, which do support TLS. It is in pthread.c and uses the
	standard pthread interface. It expects to be linked against the TLS-aware
	thread library aka NPTL.

	If you use Linux.c and Linuxasm.c with TLS libraries, they do not
	set up the TLS properly so you will get incorrect programs.
	For example, there will only be one errno among all the procs
	in your program instead of one per proc. The pthread NPTL
	implementation is needed to use the TLS libraries properly.

	If you use pthread.c without TLS libraries (i.e., with the old Linux
	pthread library known as LinuxThreads), then you will also get
	incorrect programs, although more obviously so. The LinuxThreads
	library assumes it can look at the stack pointer to distinguish between
	threads, but libthread does its own stack management, breaking this
	assumption. If you run a pthread-compiled program with the
	LinuxThreads library, LinuxThreads itself will cause a segmentation
	fault in __pthread_getspecific() the first time it is called from a
	non-standard stack.

	So, it is important that you compile binaries that match your
	system's choice of TLS vs. not-TLS libraries. The hard part is figuring
	out which your system has chosen. Plan9port looks at the kernel
	version you are running and assumes that on kernels that support
	TLS (2.6+) you will be using TLS.

	Apparently Gentoo and maybe other distributions do not follow this rule.
	They use non-TLS libraries even on kernels that can support TLS.
	To accomodate them, you can add a line SYSVERSION=2.4 to $PLAN9/config
	to force the build to think you are running an old kernel.
	The INSTALL script sets up this file automatically on Linux systems.