LINUX內核之普通自旋鎖

@CopyLeft by ICANTH，I Can do ANy THing that I CAN THink！~

Author：WenHui，WuHan University，2012-6-15

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PDF版閱讀地址：http://www.docin.com/p1-424285718.html

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普通自旋鎖

自旋鎖最常見的使用場景是創建一段臨界區 ：

static DEFINE_SPINLOCK(xxx_lock);

unsigned long flags;

spin_lock_irqsave(&xxx_lock, flags);

... critical section here ..

spin_unlock_irqrestore(&xxx_lock, flags);

自旋鎖使用時值得注意的是：對于采用使用自旋鎖以保證共享變量的存取安全時，僅當系統中所有涉及到存取該共享變量的程序部分都采用成對的spin_lock、和spin_unlock來進行操作才能保證其安全性。

NOTE! The spin-lock is safe only when you _also_ use the lock itself to do locking across CPU's, which implies that EVERYTHING that touches a shared variable has to agree about the spinlock they want to use.

在Linux2.6.15.5中，自旋體數據結構如下：

當配置CONFIG_SMP時，raw_spinlock_t才是一個含有slock變量的結構，該slock字段標識自旋鎖是否空閑狀態，用以處理多CPU處理器并發申請鎖的情況；當未配置CONFIG_SMP時，對于單CPU而言，不會發生發申請自旋鎖，故raw_lock為空結構體。

當配置CONFIG_SMP和CONFIG_PREEMPT時，spinlock_t才會有break_lock字段，break_lock字段用于標記自旋鎖競爭狀態，當break_lock = 0時表示沒有多于兩個的執行路徑，當break_lock = 1時表示沒有其它進程在忙等待該鎖。當在SMP多CPU體系架構下有可能出現申請不到自旋鎖、空等的情況，但LINUX內核必須保證在spin_lock的原子性，故在配置CONFIG_PREEMPT時必須禁止內核搶占。

字段	描述
spin_lock_init(lock)	一個自旋鎖時，可使用接口函數將其初始化為鎖定狀態
spin_lock(lock)	用于鎖定自旋鎖，如果成功則返回；否則循環等待自旋鎖變為空閑
spin_unlock(lock)	釋放自旋鎖lock，重新設置自旋鎖為鎖定狀態
spin_is_locked(lock)	判斷當前自旋鎖是否處于鎖定狀態
spin_unlock_wait(lock)	循環等待、直到自旋鎖lock變為可用狀態
spin_trylock(lock)	嘗試鎖定自旋鎖lock，如不成功則返回0；否則鎖定，并返回1
spin_can_lock(lock)	判斷自旋鎖lock是否處于空閑狀態

spin_lock和spin_unlock的關系如下：

可見，在UP體系架構中，由于沒有必要有實際的鎖以防止多CPU搶占，spin操作僅僅是禁止和開啟內核搶占。

LINUX 2.6.35版本，將spin lock實現更改為 ticket lock。spin_lock數據結構除了用于內核調試之外，字段為：raw_spinlock rlock。

ticket spinlock將rlock字段分解為如下兩部分：

Next是下一個票號，而Owner是允許使用自旋鎖的票號。加鎖時CPU取Next，并將rlock.Next + 1。將Next與Owner相比較，若相同，則加鎖成功；否則循環等待、直到Next = rlock.Owner為止。解鎖則直接將Owner + 1即可。

spin_lock和spin_unlock的調用關系如下：

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普通自旋鎖源碼分析

源程序文件目錄關系圖

在/include/linux/spinlock.h中通過是否配置CONFIG_SMP項判斷導入哪種自旋鎖定義及操作：

? 004 /* 005? * include/linux/spinlock.h - generic spinlock/rwlock declarations 007? * here's the role of the various spinlock/rwlock related include files: 009? * on SMP builds: 011? *? asm/spinlock_types.h: contains the arch_spinlock_t/arch_rwlock_t and the 012? *??????????????????????? initializers 014? *? linux/spinlock_types.h: 015? *??????????????????????? defines the generic type and initializers 017? *? asm/spinlock.h:?????? contains the arch_spin_*()/etc. lowlevel 018? *??????????????????????? implementations, mostly inline assembly code 022? *? linux/spinlock_api_smp.h: 023? *??????????????????????? contains the prototypes for the _spin_*() APIs. 025? *? linux/spinlock.h:???? builds the final spin_*() APIs. 027? * on UP builds: 029? *? linux/spinlock_type_up.h: 030? *??????????????????????? contains the generic, simplified UP spinlock type. 031? *??????????????????????? (which is an empty structure on non-debug builds) 033? *? linux/spinlock_types.h: 034? *??????????????????????? defines the generic type and initializers 036? *? linux/spinlock_up.h: 037? *??????????????????????? contains the arch_spin_*()/etc. version of UP 038? *??????????????????????? builds. (which are NOPs on non-debug, non-preempt 039? *??????????????????????? builds) 041? *?? (included on UP-non-debug builds:) 043? *? linux/spinlock_api_up.h: 044? *??????????????????????? builds the _spin_*() APIs. 046? *? linux/spinlock.h:???? builds the final spin_*() APIs. 047? */ ? 082 /* 083? * Pull the arch_spin*() functions/declarations (UP-nondebug doesnt need them): 084? */ 085 #ifdef CONFIG_SMP 086 # include <asm/spinlock.h> 087 #else 088 # include <linux/spinlock_up.h> 089 #endif 064 typedef struct spinlock { 065???????? union { 066???????????????? struct raw_spinlock rlock; 075???????? }; 076 } spinlock_t; 282 static inline void spin_lock(spinlock_t *lock) 283 { 284???????? raw_spin_lock(&lock->rlock); 285 } 169 #define raw_spin_lock(lock)???? _raw_spin_lock(lock) ? ? 322 static inline void spin_unlock(spinlock_t *lock) 323 { 324???????? raw_spin_unlock(&lock->rlock); 325 } 222 #define raw_spin_unlock(lock)?????????? _raw_spin_unlock(lock) ?

UP體系架構

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spin_lock函數在UP體系架構中最終實現方式為：

/include/linux/spinlock_api_up.h

052 #define _raw_spin_lock(lock)??????????????????? __LOCK(lock) 021 /* 022? * In the UP-nondebug case there's no real locking going on, so the 023? * only thing we have to do is to keep the preempt counts and irq 024? * flags straight, to suppress compiler warnings of unused lock 025? * variables, and to add the proper checker annotations: 026? */ 027 #define __LOCK(lock) \ 028?? do { preempt_disable(); __acquire(lock); (void)(lock); } while (0) 052 #define _raw_spin_lock(lock)??????????????????? __LOCK(lock) ? preempt_disable在未配置CONFIG_PREEMPT時為空函數，否則禁止內核搶占。而__acquire()用于內核編譯過程中靜態檢查。(void)(lock)則是為避免編譯器產生lock未被使用的警告。 ? spin_unlock函數在UP體系架構中最終實現方式為： 039 #define __UNLOCK(lock) \ 040?? do { preempt_enable(); __release(lock); (void)(lock); } while (0) ?

SMP體系架構-Tickect Spin Lock的實現方式

在Linux2.6.24中，自旋鎖由一個整數表示，當為1時表示鎖是空閑的，spin_lock()每次減少1，故 <=0時則表示有多個鎖在忙等待，但這將導致不公平性。自linux2.6.25開始，自旋鎖將整數拆為一個16位數，結構如下：

該實現機制稱為“Ticket spinlocks”，Next字節表示下一次請求鎖給其分配的票號，而Owner表示當前可以取得鎖的票號，Next和Owner初始化為0。當lock.Next = lock.Owner時，表示該鎖處于空閑狀態。spin_lock執行如下過程：

1、my_ticket = slock.next

2、slock.next++

3、wait until my_ticket = slock.owner

spin_unlock執行如下過程：

1、slock.owner++

但該鎖將導致一個問題：8個bit將只能最多表示255個CPU來競爭該鎖。故系統通過的方式，將實現兩個tickect_spin_lock和ticket_spin_unclock的版本： 058 #if (NR_CPUS < 256) 059 #define TICKET_SHIFT 8 106 #else 107 #define TICKET_SHIFT 16 ?

SMP體系架構-SPIN LOCK (ticket_shif 8)

046 #ifdef CONFIG_INLINE_SPIN_LOCK 047 #define _raw_spin_lock(lock) __raw_spin_lock(lock) 048 #endif

/include/linux/spinlock_api_smp.h：

140 static inline void __raw_spin_lock(raw_spinlock_t *lock) 141 { 142???????? preempt_disable(); 143???????? spin_acquire(&lock->dep_map, 0, 0, _RET_IP_); 144???????? LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock); 145 }

在__raw_spin_lock中，首先禁止內核搶占，調用LOCK_CONTENED宏

391 #define LOCK_CONTENDED(_lock, try, lock)??????????????????????? \ 392 do {??????????????????????????????????????????????????????????? \ 393???????? if (!try(_lock)) {????????????????????????????????????? \ 394???????????????? lock_contended(&(_lock)->dep_map, _RET_IP_);??? \ 395???????????????? lock(_lock);??????????????????????????????????? \ 396???????? }?????????????????????????????????????????????????????? \ 397???????? lock_acquired(&(_lock)->dep_map, _RET_IP_);???????????????????? \ 398 } while (0)

其中即在_raw_spin_lock中，即為首先調用do_raw_spin_trylock嘗試加鎖，若失敗則繼續調用do_raw_spin_lock進行加鎖。而do_raw_spin_xxx具體實現與平臺有關。

/include/linux/spinlock.h

136 static inline void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock) 137 { 138???????? __acquire(lock); 139???????? arch_spin_lock(&lock->raw_lock); 140 } ? 149 static inline int do_raw_spin_trylock(raw_spinlock_t *lock) 150 { 151???????? return arch_spin_trylock(&(lock)->raw_lock); 152 }

在X86平臺下，do_raw_spin_lock和do_raw_spin_trylock實現為兩個函數：

/arch/x86/include/asm/spinlock.h 188 static __always_inline void arch_spin_lock(arch_spinlock_t *lock) 189 { 190???????? __ticket_spin_lock(lock); 191 } 192 193 static __always_inline int arch_spin_trylock(arch_spinlock_t *lock) 194 { 195???????? return __ticket_spin_trylock(lock); 196 } 058 #if (NR_CPUS < 256) 059 #define TICKET_SHIFT 8 061 static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock) 062 { 063???????? short inc = 0x0100; 064 065???????? asm volatile ( 066???????????????? LOCK_PREFIX "xaddw %w0, %1\n" 067???????????????? "1:\t" 068???????????????? "cmpb %h0, %b0\n\t" 069???????????????? "je 2f\n\t" 070???????????????? "rep ; nop\n\t" 071???????????????? "movb %1, %b0\n\t" 072???????????????? /* don't need lfence here, because loads are in-order */ 073???????????????? "jmp 1b\n" 074???????????????? "2:" 075???????????????? : "+Q" (inc), "+m" (lock->slock) 076???????????????? : 077???????????????? : "memory", "cc"); 078 }

066行：LOCK_PREFIX在UP上為空定義，而在SMP上為Lock，用以保證從066行~074行為原子操作，強制所有CPU緩存失效。xaddw指令用法如下：

xaddw src, dsc ==

tmp = dsc

desc = dsc + src

src = tmp

XADDW語法驗證實驗：

xaddw使%0和%1按1個word長度交換相加，即：%0: inc → slock, %1: slock → slock + 0x0100。%1此時高字節Next + 1。xaddw使%0和%1內容改變如下：

068行：比較inc中自己的Next是否與Owner中ticket相等，若相等則獲取自旋鎖使用權、結束循環。

070行 ~ 073行：如果Owner不屬于自己，則執行空語句，并重新讀取slock中的Owner，跳回至068行進行判斷。

為什么要用LOCK_PREFIX宏來代替直接使用lock指令的方式呢？解釋如下：為了避免在配置了CONFIG_SMP項編譯產生的SMP內核、實際卻運行在UP系統上時系統執行lock命令所帶來的開銷，系統創建在.smp_locks一張SMP alternatives table用以保存系統中所有lock指令的指針。當實際運行時，若從SMP→UP時，可以根據.smp_locks lock 指針表通過熱補丁的方式將lock指令替換成nop指令。當然也可以實現系統運行時將鎖由UP→SMP的切換。具體應用可參見參考資料《Linux 內核 LOCK_PREFIX 的含義》。

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009 /* 010? * Alternative inline assembly for SMP. 011? * 012? * The LOCK_PREFIX macro defined here replaces the LOCK and 013? * LOCK_PREFIX macros used everywhere in the source tree. 014? * 015? * SMP alternatives use the same data structures as the other 016? * alternatives and the X86_FEATURE_UP flag to indicate the case of a 017? * UP system running a SMP kernel.? The existing apply_alternatives() 018? * works fine for patching a SMP kernel for UP. 019? * 020? * The SMP alternative tables can be kept after boot and contain both 021? * UP and SMP versions of the instructions to allow switching back to 022? * SMP at runtime, when hotplugging in a new CPU, which is especially 023? * useful in virtualized environments. 024? * 025? * The very common lock prefix is handled as special case in a 026? * separate table which is a pure address list without replacement ptr 027? * and size information.? That keeps the table sizes small. 028? */ 029 030 #ifdef CONFIG_SMP 031 #define LOCK_PREFIX_HERE \ 032???????????????? ".section .smp_locks,\"a\"\n"?? \ 033???????????????? ".balign 4\n"?????????????????? \ 034???????????????? ".long 671f - .\n" /* offset */ \ 035???????????????? ".previous\n"?????????????????? \ 036???????????????? "671:" 037 038 #define LOCK_PREFIX LOCK_PREFIX_HERE "\n\tlock; " 039 040 #else /* ! CONFIG_SMP */ 041 #define LOCK_PREFIX_HERE "" 042 #define LOCK_PREFIX "" 043 #endif

032行“.section .smp_locks, a”，表示以下代碼生成在.smp_locks段中，而“a”代表——allocatable。

033行~034行 “.balign 4 .long 571f”，表示以4字節對齊、將671標簽的地址置于.smp_locks段中，而標簽671的地址即為：代碼段lock指令的地址。（其實就是lock指令的指針啦~~~）

033行~034行 “.previous”偽指令，表示恢復以前section，即代碼段。故在038行將導致在代碼段生成lock指令。

LOCK_CONTENDED時首先嘗試使用__ticket_spin_trylock對lock進行加鎖，若失敗則繼續使用__ticket_spin_lock進行加鎖。不直接調用__ticket_spin_lock而使用__ticket_spin_trylock的原因是：

trylock首先不會修改lock.slock的ticket，它只是通過再次檢查，1）將slock讀出，并判斷slock是否處于空閑狀態；2）調用LOCK執行原子操作，判斷當前slock的Next是否已經被其它CPU修改，若未被修改則獲得該鎖，并將lock.slock.Next + 1。

spin_lock，無論如何，首先調用LOCK執行原子性操作、聲明ticket；而trylock則首先進行slock.Next == slock.Owner的判斷，降低第二次比較調用LOCK的概率。

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080 static __always_inline int __ticket_spin_trylock(arch_spinlock_t *lock) 081 { 082???????? int tmp, new; 083 084???????? asm volatile("movzwl %2, %0\n\t" 085????????????????????? "cmpb %h0,%b0\n\t" 086????????????????????? "leal 0x100(%" REG_PTR_MODE "0), %1\n\t" 087????????????????????? "jne 1f\n\t" 088????????????????????? LOCK_PREFIX "cmpxchgw %w1,%2\n\t" 089????????????????????? "1:" 090????????????????????? "sete %b1\n\t" 091????????????????????? "movzbl %b1,%0\n\t" 092????????????????????? : "=&a" (tmp), "=&q" (new), "+m" (lock->slock) 093????????????????????? : 094????????????????????? : "memory", "cc"); 095 096???????? return tmp; 097 }

084行將lock.slock的值賦給tmp。

085行 比較tmp.next == tmp.owner，判斷當前自旋鎖是否空閑。

086行 leal指令(Load effective address)，實際上是movl的變形，“leal 0x10 (%eax, %eax, 3), %edx” → “%edx = 0x10 + %eax + %eax * 3”，但leal卻不像movl那樣從內存取值、而直接讀取寄存器。086行語句，根據REG_PTR_MODE不同配置，在X86平臺下為：“leal 0x100(%k0), %1”，而在其它平臺為：“leal 0x100(%q0), %1”，忽略占位符修飾“k”或“q”，則該行語句等價于：

“movl (%0 + 0x100)，%1”，此時new = { tmp.Next + 1, tmp.Owner }。

087行 若tmp.next != tmp.owner，即自旋鎖不空閑，則跳到089行將0賦值給tmp并返回。

088行 原子性地執行操作cmpxchgw，用以檢測當前自旋鎖是否已被其它CPU修改lock.slock的Next域，若有競爭者則失敗、否則獲得該鎖并將Next + 1，這一系列操作是原子性的！cmpxchgw操作解釋如下：

the accumulator (8-32 bits) with "dest". If equal the "dest" is loaded with "src", otherwise the accumulator is loaded with "dest".(在IA32下，%EAX即為累加器。)

所以，“cmpxchgw %w1, %2”等效于：

“tmp.Next == lock.slock.Next ？ lock.slock = new : tmp = lock.slock”

若Next未發生變化，則將lock.slock更新為new，實質上是將slock的Next+1。

090行 執行sete指令，若cmpxchgw或cmpb成功則將new的最低字節%b1賦值為1，否則賦值為0. sete的解釋為：

Sets the byte in the operand to 1 if the Zero Flag is set, otherwise sets the operand to 0.

091行 movzbl(movz from byte to long)指令將%b1賦值給tmp最低字節，且其它位補0.即將tmp置為0或1.

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SMP體系架構-SPIN UNLOCK (ticket_shif 8)

/include/linux/spinlock_api_smp.h

046 #ifdef CONFIG_INLINE_SPIN_LOCK 047 #define _raw_spin_lock(lock) __raw_spin_lock(lock) 048 #endif 149 static inline void __raw_spin_unlock(raw_spinlock_t *lock) 150 { 151???????? spin_release(&lock->dep_map, 1, _RET_IP_); 152???????? do_raw_spin_unlock(lock); 153???????? preempt_enable(); 154 }

spin_unlock即最終調用do_raw_spin_unlock對自旋鎖進行釋放操作。

/include/linux/spinlock.h

136 static inline void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock) 137 { 138???????? __acquire(lock); 139???????? arch_spin_lock(&lock->raw_lock); 140 }

對于x86的IA32平臺，arch_spin_lock實現如下：

/arch/x86/include/asm/spinlock.h

198 static __always_inline void arch_spin_unlock(arch_spinlock_t *lock) 199 { 200???????? __ticket_spin_unlock(lock); 201 } 058 #if (NR_CPUS < 256) 059 #define TICKET_SHIFT 8 099 static __always_inline void __ticket_spin_unlock(arch_spinlock_t *lock) 100 { 101???????? asm volatile(UNLOCK_LOCK_PREFIX "incb %0" 102????????????????????? : "+m" (lock->slock) 103????????????????????? : 104????????????????????? : "memory", "cc"); 105 }

101行 將lock->slock的Owner + 1，表示可以讓下一個擁有牌號的CPU加鎖。

030 #if defined(CONFIG_X86_32) && \ 031???????? (defined(CONFIG_X86_OOSTORE) || defined(CONFIG_X86_PPRO_FENCE)) 032 /* 033? * On PPro SMP or if we are using OOSTORE, we use a locked operation to unlock 034? * (PPro errata 66, 92) 035? */ 036 # define UNLOCK_LOCK_PREFIX LOCK_PREFIX 037 #else 038 # define UNLOCK_LOCK_PREFIX 039 #endif ?

參考資料

自旋鎖

《spinlocks.txt》，/Documentation/spinlocks.txt

《Ticket spinlocks》，http://lwn.net/Articles/267968/

《Linux x86 spinlock實現之分析》，http://blog.csdn.net/david_henry/article/details/5405093

《Linux 內核 LOCK_PREFIX 的含義》，http://blog.csdn.net/ture010love/article/details/7663008

《The Intel 8086 / 8088/ 80186 / 80286 / 80386 / 80486 Instruction Set》：http://zsmith.co/intel.html

posted on 2012-06-15 11:12 我思我能 閱讀(...) 評論(...) 編輯 收藏

轉載于:https://www.cnblogs.com/icanth/archive/2012/06/15/2550379.html

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