Linux kernel-<armv7a-switch_to>

1.__switch_to

File：entry_v7m.S

TI_CPU_SAVE 即上下文环境，在thread_info中cpu_context的偏移量

/*
* Register switch for ARMv7-M processors.
* r0 = previous task_struct, r1 = previous thread_info, r2 = next thread_info
* previous and next are guaranteed not to be the same.
*/
ENTRY(__switch_to)
.fnstart
.cantunwind
/* save field */
add ip, r1, #TI_CPU_SAVE
stmia   ip!, {r4 - r11}     @ Store most regs on stack
str sp, [ip], #4
str lr, [ip], #4
mov r5, r0
add r4, r2, #TI_CPU_SAVE
/* notifier to switch process */
ldr r0, =thread_notify_head
mov r1, #THREAD_NOTIFY_SWITCH
bl  atomic_notifier_call_chain
mov ip, r4
mov r0, r5
/* restore the new process field */
ldmia   ip!, {r4 - r11}     @ Load all regs saved previously
ldr sp, [ip]
ldr pc, [ip, #4]!
.fnend
ENDPROC(__switch_to)

2.thread_notify_head

File:process.c

ATOMIC_NOTIFIER_HEAD(thread_notify_head);

EXPORT_SYMBOL_GPL(thread_notify_head);

2.1 什么是notifier

* 摘抄源码中的注释*

/*
* Notifier chains are of four types:
*
*  Atomic notifier chains: Chain callbacks run in interrupt/atomic
*      context. Callouts are not allowed to block.
*  原子通知链：应用在中断或原子上下文
*  Blocking notifier chains: Chain callbacks run in process context.
*      Callouts are allowed to block.
*  阻塞通知链：应用在进程上下文
*  Raw notifier chains: There are no restrictions on callbacks,
*      registration, or unregistration.  All locking and protection
*      must be provided by the caller.
*  原始通知链：应用在不是很严格的条件下，比如回调，注册，注销的环境中。其中的锁保护需要由调用者提供。
*  SRCU notifier chains: A variant of blocking notifier chains, with
*      the same restrictions.
*  SRCU 通知链：是阻塞通知链的变体，一样的应用限制。
* atomic_notifier_chain_register() may be called from an atomic context,
* but blocking_notifier_chain_register() and srcu_notifier_chain_register()
* must be called from a process context.  Ditto for the corresponding
* _unregister() routines.
*
* atomic_notifier_chain_unregister(), blocking_notifier_chain_unregister(),
* and srcu_notifier_chain_unregister() _must not_ be called from within
* the call chain.
*
* SRCU notifier chains are an alternative form of blocking notifier chains.
* They use SRCU (Sleepable Read-Copy Update) instead of rw-semaphores for
* protection of the chain links.  This means there is _very_ low overhead
* in srcu_notifier_call_chain(): no cache bounces and no memory barriers.
* As compensation, srcu_notifier_chain_unregister() is rather expensive.
* SRCU notifier chains should be used when the chain will be called very
* often but notifier_blocks will seldom be removed.  Also, SRCU notifier
* chains are slightly more difficult to use because they require special
* runtime initialization.
* 补充一点，srcu通知链的花销是巨大的。当调用非常频繁，但是通知链的阻塞情况，很少被移除时，最好使用SRCU通知链。不过，用的时候会稍微麻烦一点，因为需要特殊的实时初始化。
*/

2.2 notifier

File: thread_notify.h

/*
* These are the reason codes for the thread notifier.
*/
#define THREAD_NOTIFY_FLUSH 0
#define THREAD_NOTIFY_EXIT  1
#define THREAD_NOTIFY_SWITCH    2
#define THREAD_NOTIFY_COPY  3