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On UP systems, only a single task can be 'current' at the same time, which means we can use a global variable to track it. This means we can also enable THREAD_INFO_IN_TASK for those systems, as in that case, thread_info is accessed via current rather than the other way around, removing the need to store thread_info at the base of the task stack. This, in turn, permits us to enable IRQ stacks and vmap'ed stacks on UP systems as well. To partially mitigate the performance overhead of this arrangement, use a ADD/ADD/LDR sequence with the appropriate PC-relative group relocations to load the value of current when needed. This means that accessing current will still only require a single load as before, avoiding the need for a literal to carry the address of the global variable in each function. However, accessing thread_info will now require this load as well. Acked-by: Linus Walleij <linus.walleij@linaro.org> Acked-by: Nicolas Pitre <nico@fluxnic.net> Signed-off-by: Ard Biesheuvel <ardb@kernel.org> Tested-by: Marc Zyngier <maz@kernel.org> Tested-by: Vladimir Murzin <vladimir.murzin@arm.com> # ARMv7M
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.