linux.git/arch/loongarch/kernel/switch.S, branch master

LoongArch: Add THREAD_INFO_IN_TASK implementation

2026-06-25T05:03:47+00:00

Like other architectures such as x86, arm64, riscv, powerpc and s390, select THREAD_INFO_IN_TASK for LoongArch to move thread_info off the stack into task_struct. This follows modern kernel standards and also makes the system more secure. With this patch, thread_info is included in task_struct at an offset of 0 instead of being placed at the bottom of the kernel stack. Thus, the $tp register points to both thread_info and task_struct. To support this, introduce a per-CPU variable cpu_tasks to store the pointer to the current task_struct. This decouples the recovery of the $tp register from the stack pointer during exception entry. Then initialize cpu_tasks for the primary and secondary CPUs during arch-specific setup and SMP boot paths. To eliminate the dangerous windows during the early initialization where the cpu_tasks remains uninitialized, set_current() is invoked as early as possible in both setup_arch() and start_secondary(). This ensures the $tp recovery barrier is armed in case any early boot exceptions or kernel panics occur. Modify SAVE_SOME and handle_syscall to restore the $tp register from cpu_tasks, and also use the la_abs absolute addressing for cpu_tasks access in assembly to bypass the relocation limits within exception handling sections. By advancing the preservation of u0 in SAVE_SOME, we reuse the PERCPU_BASE_KS value in u0 for the cpu_tasks calculation, effectively eliminating a duplicate csrrd instruction execution on SMP platforms. Update and to fully support the CONFIG_THREAD_INFO_IN_TASK feature. Remove the obsolete next_ti argument from __switch_to(), which shifts the remaining arguments ahead in the calling convention (sched_ra from a3 to a2, and sched_cfa from a4 to a3). Under the new configuration, __switch_to() now directly derives the thread pointer ($tp) from the next task_struct pointer in a1. To preserve the optimal and clean "move tp, a1" path for 64-bit kernels, the thread pointer ($tp) is assigned directly from a1 in the core path. For 32-bit kernels, where a1 carries a 2000-byte structural pointer bias at entry, an explicit adjustment "PTR_ADDI tp, tp, -TASK_STRUCT_OFFSET" is introduced at the function exit. In the context of __switch_to(), local interrupts are disabled, and the kernel is in a critical switching phase where handling any synchronous exception is practically impossible and prohibited. If any synchronous exception or watchpoint does trigger in this narrow window, it constitutes a fatal double fault and the kernel is expected to die/panic immediately anyway. Therefore, the temporary biased value in $tp is safe and acceptable here. Additionally, evaluate the stack lookup as a single load instruction "LONG_LPTR t0, a1, (TASK_STACK - TASK_STRUCT_OFFSET)", this perfectly satisfies both 32-bit and 64-bit kernels. Using the "next" pointer in a1 as the base register, rather than $tp, effectively unchains the data dependency (RAW hazard) from the preceding move instruction, maximizing the instruction-level parallelism and superscalar execution efficiency while naturally adapting the structural shift. With CONFIG_THREAD_INFO_IN_TASK enabled, the kernel stack life cycle is decoupled from task_struct and can be freed concurrently. Currently, show_stacktrace() reads raw stack data via __get_addr() and subsequently calls show_backtrace() to unwind the frame, without holding any reference to the target task's stack. If show_stacktrace() is called on a concurrently exiting task, it could attempt to read from a freed or reallocated kernel stack. This introduces a severe use-after-free (UAF) read risk or kernel panics. Wrap the entire stack inspection process inside show_stacktrace() with a try_get_task_stack() and put_task_stack() pair. This ensures the task stack remains pinned safely during both the raw stack data dump loop and the subsequent stack unwinding phase. Also, ensure that the task pointer is initialized to "current" early if it is NULL, so that try_get_task_stack() always operates on a valid task reference. Signed-off-by: Tiezhu Yang Signed-off-by: Huacai Chen

LoongArch: Adjust process management for 32BIT/64BIT

2025-12-08T10:09:17+00:00

Adjust process management for both 32BIT and 64BIT, including: CPU context switching, FPU loading/restoring, process dumping and process tracing routines. Q: Why modify switch.S? A: LoongArch32 has no ldptr.d/stptr.d instructions, and asm offsets of thead_struct members are too large to be filled in the 12b immediate field of ld.w/st.w. Reviewed-by: Arnd Bergmann Signed-off-by: Jiaxun Yang Signed-off-by: Huacai Chen

LoongArch: Fix build errors for CONFIG_RANDSTRUCT

2025-12-06T02:39:40+00:00

When CONFIG_RANDSTRUCT enabled, members of task_struct are randomized. There is a chance that TASK_STACK_CANARY be out of 12bit immediate's range and causes build errors. TASK_STACK_CANARY is naturally aligned, so fix it by replacing ld.d/st.d with ldptr.d/stptr.d which have 14bit immediates. Cc: stable@vger.kernel.org Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-kbuild-all/202511240656.0NaPcJs1-lkp@intel.com/ Suggested-by: Rui Wang Signed-off-by: Huacai Chen

LoongArch: Correct the __switch_to() prototype in comments

2025-01-25T10:51:42+00:00

Correct the __switch_to() prototype in comments, keep it be the same as the declaration in switch_to.h. Signed-off-by: Huacai Chen

LoongArch: Add basic STACKPROTECTOR support

2022-12-14T00:41:53+00:00

Add basic stack protector support similar to other architectures. A constant canary value is set at boot time, and with help of compiler's -fstack-protector we can detect stack corruption. Signed-off-by: Huacai Chen

LoongArch: Remove unused kernel stack padding

2022-10-29T08:29:31+00:00

The current LoongArch kernel stack is padded as if obeying the MIPS o32 calling convention (32 bytes), signifying the port's MIPS lineage but no longer making sense. Remove the padding for clarity. Reviewed-by: WANG Xuerui Signed-off-by: Jinyang He Signed-off-by: Huacai Chen

LoongArch: Add STACKTRACE support

2022-08-12T05:10:11+00:00

1. Use common arch_stack_walk() infrastructure to avoid duplicated code and avoid taking care of the stack storage and filtering. 2. Add sched_ra (means sched return address) and sched_cfa (means sched call frame address) to thread_info, and store them in switch_to(). 3. Add __get_wchan() implementation. Now we can print the process stack and wait channel by cat /proc/*/stack and /proc/*/wchan. Signed-off-by: Qing Zhang Signed-off-by: Huacai Chen

LoongArch: Re-tab the assembly files

2022-07-29T10:22:32+00:00

Reflow the *.S files for better stylistic consistency, namely hard tabs after mnemonic position, and vertical alignment of the first operand with hard tabs. Tab width is obviously 8. Some pre-existing intra-block vertical alignments are preserved. Signed-off-by: WANG Xuerui Signed-off-by: Huacai Chen

LoongArch: Add process management

2022-06-03T12:09:28+00:00

Add process management support for LoongArch, including: thread info definition, context switch and process tracing. Reviewed-by: WANG Xuerui Reviewed-by: Jiaxun Yang Signed-off-by: Huacai Chen