Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. Quick Start ----------- * Report a bug: See Documentation/admin-guide/reporting-issues.rst * Get the latest kernel: https://kernel.org * Build the kernel: See Documentation/admin-guide/quickly-build-trimmed-linux.rst * Join the community: https://lore.kernel.org/ Essential Documentation ----------------------- All users should be familiar with: * Building requirements: Documentation/process/changes.rst * Code of Conduct: Documentation/process/code-of-conduct.rst * License: See COPYING Documentation can be built with make htmldocs or viewed online at: https://www.kernel.org/doc/html/latest/ Who Are You? ============ Find your role below: * New Kernel Developer - Getting started with kernel development * Academic Researcher - Studying kernel internals and architecture * Security Expert - Hardening and vulnerability analysis * Backport/Maintenance Engineer - Maintaining stable kernels * System Administrator - Configuring and troubleshooting * Maintainer - Leading subsystems and reviewing patches * Hardware Vendor - Writing drivers for new hardware * Distribution Maintainer - Packaging kernels for distros * AI Coding Assistant - LLMs and AI-powered development tools For Specific Users ================== New Kernel Developer -------------------- Welcome! Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. Communication and Support ========================= * Mailing Lists: https://lore.kernel.org/ * IRC: #kernelnewbies on irc.oftc.net * Bugzilla: https://bugzilla.kernel.org/ * MAINTAINERS file: Lists subsystem maintainers and mailing lists * Email Clients: Documentation/process/email-clients.rst
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Pull x86 fix from Ingo Molnar:
"Work around S2RAM hang if the firmware unexpectedly re-enables the
x2apic hardware while it was disabled by the kernel.
Force-disable it again and issue a warning into the syslog"
* tag 'x86-urgent-2026-03-15' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
x86/apic: Disable x2apic on resume if the kernel expects so
Pull timer fix from Ingo Molnar:
"Fix function tracer recursion bug by marking jiffies_64_to_clock_t()
notrace"
* tag 'timers-urgent-2026-03-15' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
time/jiffies: Mark jiffies_64_to_clock_t() notrace
When resuming from s2ram, firmware may re-enable x2apic mode, which may have
been disabled by the kernel during boot either because it doesn't support IRQ
remapping or for other reasons. This causes the kernel to continue using the
xapic interface, while the hardware is in x2apic mode, which causes hangs.
This happens on defconfig + bare metal + s2ram.
Fix this in lapic_resume() by disabling x2apic if the kernel expects it to be
disabled, i.e. when x2apic_mode = 0.
The ACPI v6.6 spec, Section 16.3 [1] says firmware restores either the
pre-sleep configuration or initial boot configuration for each CPU, including
MSR state:
When executing from the power-on reset vector as a result of waking from an
S2 or S3 sleep state, the platform firmware performs only the hardware
initialization required to restore the system to either the state the
platform was in prior to the initial operating system boot, or to the
pre-sleep configuration state. In multiprocessor systems, non-boot
processors should be placed in the same state as prior to the initial
operating system boot.
(further ahead)
If this is an S2 or S3 wake, then the platform runtime firmware restores
minimum context of the system before jumping to the waking vector. This
includes:
CPU configuration. Platform runtime firmware restores the pre-sleep
configuration or initial boot configuration of each CPU (MSR, MTRR,
firmware update, SMBase, and so on). Interrupts must be disabled (for
IA-32 processors, disabled by CLI instruction).
(and other things)
So at least as per the spec, re-enablement of x2apic by the firmware is
allowed if "x2apic on" is a part of the initial boot configuration.
[1] https://uefi.org/specs/ACPI/6.6/16_Waking_and_Sleeping.html#initialization
[ bp: Massage. ]
Fixes: 6e1cb38a2aef ("x64, x2apic/intr-remap: add x2apic support, including enabling interrupt-remapping")
Co-developed-by: Rahul Bukte <rahul.bukte@sony.com>
Signed-off-by: Rahul Bukte <rahul.bukte@sony.com>
Signed-off-by: Shashank Balaji <shashank.mahadasyam@sony.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260306-x2apic-fix-v2-1-bee99c12efa3@sony.com
Pull scheduler fixes from Ingo Molnar:
"More MM-CID fixes, mostly fixing hangs/races:
- Fix CID hangs due to a race between concurrent forks
- Fix vfork()/CLONE_VM MMCID bug causing hangs
- Remove pointless preemption guard
- Fix CID task list walk performance regression on large systems
by removing the known-flaky and slow counting logic using
for_each_process_thread() in mm_cid_*fixup_tasks_to_cpus(), and
implementing a simple sched_mm_cid::node list instead"
* tag 'sched-urgent-2026-03-15' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
sched/mmcid: Avoid full tasklist walks
sched/mmcid: Remove pointless preempt guard
sched/mmcid: Handle vfork()/CLONE_VM correctly
sched/mmcid: Prevent CID stalls due to concurrent forks
The trace_clock_jiffies() function that handles the "uptime" clock for
tracing calls jiffies_64_to_clock_t(). This causes the function tracer to
constantly recurse when the tracing clock is set to "uptime". Mark it
notrace to prevent unnecessary recursion when using the "uptime" clock.
Fixes: 58d4e21e50ff3 ("tracing: Fix wraparound problems in "uptime" trace clock")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260306212403.72270bb2@robin
Pull objtool fixes from Ingo Molnar:
- Fix cross-build bug by using HOSTCFLAGS for HAVE_XXHASH test
- Fix klp bug by fixing detection of corrupt static branch/call entries
- Handle unsupported pr_debug() usage more gracefully
- Fix hypothetical klp bug by avoiding NULL pointer dereference when
printing code symbol name
- Fix data alignment bug in elf_add_data() causing mangled strings
- Fix confusing ERROR_INSN() error message
- Handle unexpected Clang RSP musical chairs causing false positive
warnings
- Fix another objtool stack overflow in validate_branch()
* tag 'objtool-urgent-2026-03-15' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
objtool: Fix another stack overflow in validate_branch()
objtool: Handle Clang RSP musical chairs
objtool: Fix ERROR_INSN() error message
objtool: Fix data alignment in elf_add_data()
objtool: Use HOSTCFLAGS for HAVE_XXHASH test
objtool/klp: Avoid NULL pointer dereference when printing code symbol name
objtool/klp: Disable unsupported pr_debug() usage
objtool/klp: Fix detection of corrupt static branch/call entries
Chasing vfork()'ed tasks on a CID ownership mode switch requires a full
task list walk, which is obviously expensive on large systems.
Avoid that by keeping a list of tasks using a mm MMCID entity in mm::mm_cid
and walk this list instead. This removes the proven to be flaky counting
logic and avoids a full task list walk in the case of vfork()'ed tasks.
Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260310202526.183824481@kernel.org
Pull EFI fix from Ard Biesheuvel:
"Fix for the x86 EFI workaround keeping boot services code and data
regions reserved until after SetVirtualAddressMap() completes:
deferred struct page initialization may result in some of this memory
being lost permanently"
* tag 'efi-fixes-for-v7.0-2' of git://git.kernel.org/pub/scm/linux/kernel/git/efi/efi:
x86/efi: defer freeing of boot services memory
Pull irq fixes from Ingo Molnar:
"Two fixes for the riscv-aplic irqchip driver:
- Fix probing dependency bug on probing failure
- Fix double register_syscore() bug"
* tag 'irq-urgent-2026-03-15' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
irqchip/riscv-aplic: Register syscore operations only once
irqchip/riscv-aplic: Do not clear ACPI dependencies on probe failure
The insn state is getting saved on the stack twice for each recursive
iteration. No need for that, once is enough.
Fixes the following reported stack overflow:
drivers/scsi/qla2xxx/qla_dbg.o: error: SIGSEGV: objtool stack overflow!
Segmentation fault
Fixes: 70589843b36f ("objtool: Add option to trace function validation")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/90956545-2066-46e3-b547-10c884582eb0@app.fastmail.com
Link: https://patch.msgid.link/8b97f62d083457f3b0a29a424275f7957dd3372f.1772821683.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
This is a leftover from the early versions of this function where it could
be invoked without mm::mm_cid::lock held.
Remove it and add lockdep asserts instead.
Fixes: 653fda7ae73d ("sched/mmcid: Switch over to the new mechanism")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260310202526.116363613@kernel.org
Pull i2c fix from Wolfram Sang:
"A revert for the i801 driver restoring old locking behaviour"
* tag 'i2c-for-7.0-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux:
i2c: i801: Revert "i2c: i801: replace acpi_lock with I2C bus lock"
efi_free_boot_services() frees memory occupied by EFI_BOOT_SERVICES_CODE
and EFI_BOOT_SERVICES_DATA using memblock_free_late().
There are two issue with that: memblock_free_late() should be used for
memory allocated with memblock_alloc() while the memory reserved with
memblock_reserve() should be freed with free_reserved_area().
More acutely, with CONFIG_DEFERRED_STRUCT_PAGE_INIT=y
efi_free_boot_services() is called before deferred initialization of the
memory map is complete.
Benjamin Herrenschmidt reports that this causes a leak of ~140MB of
RAM on EC2 t3a.nano instances which only have 512MB or RAM.
If the freed memory resides in the areas that memory map for them is
still uninitialized, they won't be actually freed because
memblock_free_late() calls memblock_free_pages() and the latter skips
uninitialized pages.
Using free_reserved_area() at this point is also problematic because
__free_page() accesses the buddy of the freed page and that again might
end up in uninitialized part of the memory map.
Delaying the entire efi_free_boot_services() could be problematic
because in addition to freeing boot services memory it updates
efi.memmap without any synchronization and that's undesirable late in
boot when there is concurrency.
More robust approach is to only defer freeing of the EFI boot services
memory.
Split efi_free_boot_services() in two. First efi_unmap_boot_services()
collects ranges that should be freed into an array then
efi_free_boot_services() later frees them after deferred init is complete.
Link: https://lore.kernel.org/all/ec2aaef14783869b3be6e3c253b2dcbf67dbc12a.camel@kernel.crashing.org
Fixes: 916f676f8dc0 ("x86, efi: Retain boot service code until after switching to virtual mode")
Cc: <stable@vger.kernel.org>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Pull i3c fixes from Alexandre Belloni:
"This introduces the I3C_OR_I2C symbol which is not a fix per se but is
affecting multiple subsystems so it is included to ease
synchronization.
Apart from that, Adrian is mostly fixing the mipi-i3c-hci driver DMA
handling, and I took the opportunity to add two fixes for the dw-i3c
driver.
Subsystem:
- simplify combined i3c/i2c dependencies
Drivers:
- dw: handle 2C properly, fix possible race condition
- mipi-i3c-hci: many DMA related fixes"
* tag 'i3c/fixes-for-7.0' of git://git.kernel.org/pub/scm/linux/kernel/git/i3c/linux:
i3c: dw-i3c-master: Set SIR_REJECT in DAT on device attach and reattach
i3c: master: dw-i3c: Fix missing of_node for virtual I2C adapter
i3c: mipi-i3c-hci: Fallback to software reset when bus disable fails
i3c: mipi-i3c-hci: Fix handling of shared IRQs during early initialization
i3c: mipi-i3c-hci: Fix race in DMA error handling in interrupt context
i3c: mipi-i3c-hci: Consolidate common xfer processing logic
i3c: mipi-i3c-hci: Restart DMA ring correctly after dequeue abort
i3c: mipi-i3c-hci: Add missing TID field to no-op command descriptor
i3c: mipi-i3c-hci: Correct RING_CTRL_ABORT handling in DMA dequeue
i3c: mipi-i3c-hci: Fix race between DMA ring dequeue and interrupt handler
i3c: mipi-i3c-hci: Fix race in DMA ring dequeue
i3c: mipi-i3c-hci: Fix race in DMA ring enqueue for parallel xfers
i3c: mipi-i3c-hci: Consolidate spinlocks
i3c: mipi-i3c-hci: Factor out DMA mapping from queuing path
i3c: mipi-i3c-hci: Fix Hot-Join NACK
i3c: mipi-i3c-hci: Use ETIMEDOUT instead of ETIME for timeout errors
i3c: simplify combined i3c/i2c dependencies
Since commit 95a8ddde3660 ("irqchip/riscv-aplic: Preserve APLIC
states across suspend/resume"), when multiple NUMA nodes exist
and AIA is not configured as "none", aplic_probe() is called
multiple times. This leads to register_syscore(&aplic_syscore)
being invoked repeatedly, causing the following Oops:
list_add double add: new=ffffffffb91461f0, prev=ffffffffb91461f0, next=ffffffffb915c408.
[<ffffffffb7b5c8ca>] __list_add_valid_or_report+0x60/0xc0
[<ffffffffb7cc3236>] register_syscore+0x3e/0x70
[<ffffffffb7b8d61c>] aplic_probe+0xc6/0x112
Fix this by registering syscore operations only once, using a static
variable aplic_syscore_registered to track registration.
[ tglx: Trim backtrace properly ]
Fixes: 95a8ddde3660 ("irqchip/riscv-aplic: Preserve APLIC states across suspend/resume")
Signed-off-by: Jessica Liu <liu.xuemei1@zte.com.cn>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260310141731145xMwLsyvXl9Gw-m6A4VRYj@zte.com.cn