对linux源码中thread_info的一点说明

       在读linux源码的时候，通常我们都把头文件放到include目录下，这样方便引用，那么我们可以在linux/sched.h中引用了linux/thread_info.h这个头文件，但是这个头文件中仍然没有结构体thread_info的定义，但是它引用了asm/thread_info.h，但是include目录下并没有asm这个目录，那么它在哪里呢?
     其实这里是和平台相关的，为了方便各个平台有各自的进程描述符管理方式，linux是根据平台的，比如以x86为例，它对应的文件地址就在arch\x86\include\asm\thread_info.h中。

      为了方便查看，下面是该文件的内容：

/* thread_info.h: low-level thread information
*
* Copyright (C) 2002  David Howells (dhowells@redhat.com)
* - Incorporating suggestions made by Linus Torvalds and Dave Miller
*/

#ifndef _ASM_X86_THREAD_INFO_H
#define _ASM_X86_THREAD_INFO_H

#include <linux/compiler.h>
#include <asm/page.h>
#include <asm/percpu.h>
#include <asm/types.h>

/*
* TOP_OF_KERNEL_STACK_PADDING is a number of unused bytes that we
* reserve at the top of the kernel stack.  We do it because of a nasty
* 32-bit corner case.  On x86_32, the hardware stack frame is
* variable-length.  Except for vm86 mode, struct pt_regs assumes a
* maximum-length frame.  If we enter from CPL 0, the top 8 bytes of
* pt_regs don't actually exist.  Ordinarily this doesn't matter, but it
* does in at least one case:
*
* If we take an NMI early enough in SYSENTER, then we can end up with
* pt_regs that extends above sp0.  On the way out, in the espfix code,
* we can read the saved SS value, but that value will be above sp0.
* Without this offset, that can result in a page fault.  (We are
* careful that, in this case, the value we read doesn't matter.)
*
* In vm86 mode, the hardware frame is much longer still, so add 16
* bytes to make room for the real-mode segments.
*
* x86_64 has a fixed-length stack frame.
*/
#ifdef CONFIG_X86_32
# ifdef CONFIG_VM86
#  define TOP_OF_KERNEL_STACK_PADDING 16
# else
#  define TOP_OF_KERNEL_STACK_PADDING 8
# endif
#else
# define TOP_OF_KERNEL_STACK_PADDING 0
#endif

/*
* low level task data that entry.S needs immediate access to
* - this struct should fit entirely inside of one cache line
* - this struct shares the supervisor stack pages
*/
#ifndef __ASSEMBLY__
struct task_struct;
#include <asm/processor.h>
#include <linux/atomic.h>

struct thread_info {
struct task_struct	*task;		/* main task structure */
__u32			flags;		/* low level flags */
__u32			status;		/* thread synchronous flags */
__u32			cpu;		/* current CPU */
mm_segment_t		addr_limit;
unsigned int		sig_on_uaccess_error:1;
unsigned int		uaccess_err:1;	/* uaccess failed */
};

#define INIT_THREAD_INFO(tsk)			\
{						\
.task		= &tsk,			\
.flags		= 0,			\
.cpu		= 0,			\
.addr_limit	= KERNEL_DS,		\
}

#define init_thread_info	(init_thread_union.thread_info)
#define init_stack		(init_thread_union.stack)

#else /* !__ASSEMBLY__ */

#include <asm/asm-offsets.h>

#endif

/*
* thread information flags
* - these are process state flags that various assembly files
*   may need to access
* - pending work-to-be-done flags are in LSW
* - other flags in MSW
* Warning: layout of LSW is hardcoded in entry.S
*/
#define TIF_SYSCALL_TRACE	0	/* syscall trace active */
#define TIF_NOTIFY_RESUME	1	/* callback before returning to user */
#define TIF_SIGPENDING		2	/* signal pending */
#define TIF_NEED_RESCHED	3	/* rescheduling necessary */
#define TIF_SINGLESTEP		4	/* reenable singlestep on user return*/
#define TIF_SYSCALL_EMU		6	/* syscall emulation active */
#define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */
#define TIF_SECCOMP		8	/* secure computing */
#define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
#define TIF_UPROBE		12	/* breakpointed or singlestepping */
#define TIF_NOTSC		16	/* TSC is not accessible in userland */
#define TIF_IA32		17	/* IA32 compatibility process */
#define TIF_FORK		18	/* ret_from_fork */
#define TIF_NOHZ		19	/* in adaptive nohz mode */
#define TIF_MEMDIE		20	/* is terminating due to OOM killer */
#define TIF_POLLING_NRFLAG	21	/* idle is polling for TIF_NEED_RESCHED */
#define TIF_IO_BITMAP		22	/* uses I/O bitmap */
#define TIF_FORCED_TF		24	/* true if TF in eflags artificially */
#define TIF_BLOCKSTEP		25	/* set when we want DEBUGCTLMSR_BTF */
#define TIF_LAZY_MMU_UPDATES	27	/* task is updating the mmu lazily */
#define TIF_SYSCALL_TRACEPOINT	28	/* syscall tracepoint instrumentation */
#define TIF_ADDR32		29	/* 32-bit address space on 64 bits */
#define TIF_X32			30	/* 32-bit native x86-64 binary */

#define _TIF_SYSCALL_TRACE	(1 << TIF_SYSCALL_TRACE)
#define _TIF_NOTIFY_RESUME	(1 << TIF_NOTIFY_RESUME)
#define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
#define _TIF_SINGLESTEP		(1 << TIF_SINGLESTEP)
#define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
#define _TIF_SYSCALL_EMU	(1 << TIF_SYSCALL_EMU)
#define _TIF_SYSCALL_AUDIT	(1 << TIF_SYSCALL_AUDIT)
#define _TIF_SECCOMP		(1 << TIF_SECCOMP)
#define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
#define _TIF_UPROBE		(1 << TIF_UPROBE)
#define _TIF_NOTSC		(1 << TIF_NOTSC)
#define _TIF_IA32		(1 << TIF_IA32)
#define _TIF_FORK		(1 << TIF_FORK)
#define _TIF_NOHZ		(1 << TIF_NOHZ)
#define _TIF_POLLING_NRFLAG	(1 << TIF_POLLING_NRFLAG)
#define _TIF_IO_BITMAP		(1 << TIF_IO_BITMAP)
#define _TIF_FORCED_TF		(1 << TIF_FORCED_TF)
#define _TIF_BLOCKSTEP		(1 << TIF_BLOCKSTEP)
#define _TIF_LAZY_MMU_UPDATES	(1 << TIF_LAZY_MMU_UPDATES)
#define _TIF_SYSCALL_TRACEPOINT	(1 << TIF_SYSCALL_TRACEPOINT)
#define _TIF_ADDR32		(1 << TIF_ADDR32)
#define _TIF_X32		(1 << TIF_X32)

/* work to do in syscall_trace_enter() */
#define _TIF_WORK_SYSCALL_ENTRY	\
(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU | _TIF_SYSCALL_AUDIT |	\
_TIF_SECCOMP | _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT |	\
_TIF_NOHZ)

/* work to do on any return to user space */
#define _TIF_ALLWORK_MASK						\
((0x0000FFFF & ~_TIF_SECCOMP) | _TIF_SYSCALL_TRACEPOINT |	\
_TIF_NOHZ)

/* flags to check in __switch_to() */
#define _TIF_WORK_CTXSW							\
(_TIF_IO_BITMAP|_TIF_NOTSC|_TIF_BLOCKSTEP)

#define _TIF_WORK_CTXSW_PREV (_TIF_WORK_CTXSW|_TIF_USER_RETURN_NOTIFY)
#define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)

#define STACK_WARN		(THREAD_SIZE/8)

/*
* macros/functions for gaining access to the thread information structure
*
* preempt_count needs to be 1 initially, until the scheduler is functional.
*/
#ifndef __ASSEMBLY__

static inline struct thread_info *current_thread_info(void)
{
return (struct thread_info *)(current_top_of_stack() - THREAD_SIZE);
}

static inline unsigned long current_stack_pointer(void)
{
unsigned long sp;
#ifdef CONFIG_X86_64
asm("mov %%rsp,%0" : "=g" (sp));
#else
asm("mov %%esp,%0" : "=g" (sp));
#endif
return sp;
}

#else /* !__ASSEMBLY__ */

#ifdef CONFIG_X86_64
# define cpu_current_top_of_stack (cpu_tss + TSS_sp0)
#endif

/* Load thread_info address into "reg" */
#define GET_THREAD_INFO(reg) \
_ASM_MOV PER_CPU_VAR(cpu_current_top_of_stack),reg ; \
_ASM_SUB $(THREAD_SIZE),reg ;

/*
* ASM operand which evaluates to a 'thread_info' address of
* the current task, if it is known that "reg" is exactly "off"
* bytes below the top of the stack currently.
*
* ( The kernel stack's size is known at build time, it is usually
*   2 or 4 pages, and the bottom  of the kernel stack contains
*   the thread_info structure. So to access the thread_info very
*   quickly from assembly code we can calculate down from the
*   top of the kernel stack to the bottom, using constant,
*   build-time calculations only. )
*
* For example, to fetch the current thread_info->flags value into %eax
* on x86-64 defconfig kernels, in syscall entry code where RSP is
* currently at exactly SIZEOF_PTREGS bytes away from the top of the
* stack:
*
*      mov ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS), %eax
*
* will translate to:
*
*      8b 84 24 b8 c0 ff ff      mov    -0x3f48(%rsp), %eax
*
* which is below the current RSP by almost 16K.
*/
#define ASM_THREAD_INFO(field, reg, off) ((field)+(off)-THREAD_SIZE)(reg)

#endif

/*
* Thread-synchronous status.
*
* This is different from the flags in that nobody else
* ever touches our thread-synchronous status, so we don't
* have to worry about atomic accesses.
*/
#define TS_COMPAT		0x0002	/* 32bit syscall active (64BIT)*/
#define TS_RESTORE_SIGMASK	0x0008	/* restore signal mask in do_signal() */

#ifndef __ASSEMBLY__
#define HAVE_SET_RESTORE_SIGMASK	1
static inline void set_restore_sigmask(void)
{
struct thread_info *ti = current_thread_info();
ti->status |= TS_RESTORE_SIGMASK;
WARN_ON(!test_bit(TIF_SIGPENDING, (unsigned long *)&ti->flags));
}
static inline void clear_restore_sigmask(void)
{
current_thread_info()->status &= ~TS_RESTORE_SIGMASK;
}
static inline bool test_restore_sigmask(void)
{
return current_thread_info()->status & TS_RESTORE_SIGMASK;
}
static inline bool test_and_clear_restore_sigmask(void)
{
struct thread_info *ti = current_thread_info();
if (!(ti->status & TS_RESTORE_SIGMASK))
return false;
ti->status &= ~TS_RESTORE_SIGMASK;
return true;
}

static inline bool is_ia32_task(void)
{
#ifdef CONFIG_X86_32
return true;
#endif
#ifdef CONFIG_IA32_EMULATION
if (current_thread_info()->status & TS_COMPAT)
return true;
#endif
return false;
}

/*
* Force syscall return via IRET by making it look as if there was
* some work pending. IRET is our most capable (but slowest) syscall
* return path, which is able to restore modified SS, CS and certain
* EFLAGS values that other (fast) syscall return instructions
* are not able to restore properly.
*/
#define force_iret() set_thread_flag(TIF_NOTIFY_RESUME)

#endif	/* !__ASSEMBLY__ */

#ifndef __ASSEMBLY__
extern void arch_task_cache_init(void);
extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
extern void arch_release_task_struct(struct task_struct *tsk);
#endif
#endif /* _ASM_X86_THREAD_INFO_H */