[13] On 25 August 1991, Torvalds posted the following to comp.os.minix, a newsgroup on Usenet:[14] I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones.

It is NOT protable [sic] (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(.On 17 September 1991, Torvalds prepared version 0.01 of Linux and put on the "ftp.funet.fi" – FTP server of the Finnish University and Research Network (FUNET).

With the support of the POSIX APIs, through the libC that, whether needed, acts as an entry point to the kernel address space, Linux could run software and applications that had been developed for Unix.

[21] The make *config family of commands of kbuild enable and configure options for building ad hoc kernel executables (vmlinux) and loadable modules.

[31][32] Version 2.2, released on 20 January 1999,[33] improved locking granularity and SMP management, added m68k, PowerPC, Sparc64, Alpha, and other 64-bit platforms support.

Linux 2.4 added support for the Pentium 4 and Itanium (the latter introduced the ia64 ISA that was jointly developed by Intel and Hewlett-Packard to supersede the older PA-RISC), and for the newer 64-bit MIPS processor.

Version 3.11, released on 2 September 2013,[55] added many new features such as new O_TMPFILE flag for open(2) to reduce temporary file vulnerabilities, experimental AMD Radeon dynamic power management, low-latency network polling, and zswap (compressed swap cache).

[73] Again, Garcia-Garcia and Alonso de Magdaleno from University of Oviedo (Spain) estimate that the value annually added to kernel was about €100M between 2005 and 2007 and €225M in 2008, it would cost also more than €1bn (about $1.4bn as of February 2010) to develop in the European Union.

[96] In July 2013, the maintainer of the USB 3.0 driver Sage Sharp asked Torvalds to address the abusive commentary in the kernel development community.

The patches must conform to a set of rules and to a formal language that, among other things, describes which lines of code are to be deleted and what others are to be added to the specified files.

Common challenges relate to userspace vs. kernel space access, misuse of synchronization primitives, and incorrect hardware management.

[11]: 371 The kernel provides for debugging by printing via printk() which stores messages in a circular buffer (overwriting older entries with newer).

I don't know if you may remember Yggdrasil was known for having quite extreme changes to the kernel and even today all of the major vendors have their own tweaks because they have some portion of the market they're interested in and quite frankly that's how it should be.

Unlike standard monolithic kernels, device drivers are easily configured as modules, and loaded or unloaded while the system is running and can also be pre-empted under certain conditions in order to handle hardware interrupts correctly and to better support symmetric multiprocessing.

[5][10] The Linux application binary interface (ABI) between the kernel and the user space has four degrees of stability (stable, testing, obsolete, removed);[198] The system calls are expected to never change in order to preserve compatibility for userspace programs that rely on them.

[200] The set of the Linux kernel API that regards the interfaces exposed to user applications is fundamentally composed of UNIX and Linux-specific system calls.

This issue is mainly due to distribution-specific configurations and a set of patches applied to the code of the Linux kernel, differences in system libraries, services (daemons), filesystem hierarchies, and environment variables.

In other POSIX kernels, a similar policy known as SCHED_OTHER allocates CPU timeslices (i.e, it assigns absolute slices of the processor time depending on either predetermined or dynamically computed priority of each process).

[11]: 46–50 With user preemption, the kernel scheduler can replace the current process with the execution of a context switch to a different one that therefore acquires the computing resources for running (CPU, memory, and more).

Real-time PREEMPT_RT patches, included into the mainline Linux since version 2.6, provide a deterministic scheduler, the removal of preemption and interrupt disabling (where possible), PI Mutexes (i.e., locking primitives that avoid priority inversion),[256][257] support for High Precision Event Timers (HPET), preemptive read-copy-update (RCU), (forced) IRQ threads, and other minor features.

They consist of atomic types (which can only be manipulated by a set of specific operators), spinlocks, semaphores, mutexes,[265][11]: 176–198 [266] and lockless algorithms (e.g., RCUs).

[270][271][272][273] PREEMPT_RT code included in mainline Linux provide RT-mutexes, a special kind of Mutex which do not disable preemption and have support for priority inheritance.

[284] Small chunks of memory can be dynamically allocated in kernel space via the family of kmalloc() APIs and freed with the appropriate variant of kfree().

Those foundations, known as livepatch and based primarily on the kernel's ftrace functionality, form a common core capable of supporting hot patching by both kGraft and kpatch, by providing an application programming interface (API) for kernel modules that contain hot patches and an application binary interface (ABI) for the userspace management utilities.

However, the common core included into Linux kernel 4.0 supports only the x86 architecture and does not provide any mechanisms for ensuring function-level consistency while the hot patches are applied.

Many offer long-term support releases that receive security updates for a certain Linux kernel version for an extended period of time.

"[315] This group of high-profile kernel developers, including Torvalds, Greg Kroah-Hartman and Andrew Morton, commented on mass media about their objections to the GPLv3.

The official kernel, that is the Linus git branch at the kernel.org repository, contains binary blobs released under the terms of the GNU GPLv2 license.

[5][10] Linux can also search filesystems to locate binary blobs, proprietary firmware, drivers, or other executable modules, then it can load and link them into kernel space.

[333] James Bottomley, a kernel developer, issued an apology for the handling of the situation and clarified that the action was a consequence of U.S. sanctions against Russia.