December 2013
Intermediate to advanced
496 pages
8h 57m
Chinese
Content preview from LINUX系統程式設計 第二版
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第七章
此範例會使用
pthread_mutex_lock()
來取得一個排他鎖，最後會使用
pthread_mutex_
unlock()
來釋放它。這有助於排除爭用情況，但會為銀行帶來另一個爭用點：一次只有一
位客戶可以提款！這的確是一個瓶頸；對於一個大到不能倒的銀行來說，這是一個失敗。
因此鎖的使用通常會避開
全域鎖
（
global locks
），並把鎖關聯到資料結構的特定實例。
這稱為
細粒度上鎖
（
fine-grained locking
）。它可讓你的上鎖語義更為複雜，特別是在死
結的避免上，但它是把規模擴展到多核的關鍵。
就此例而言，我們不會定義全域鎖
the_mutex
，我們會在
account
結構裡定義一個排他
鎖，讓每個帳戶皆擁有自己的鎖。當關鍵區裡的資料只是
account
結構時，這將可運作
得很好。透過只鎖住要被扣錢的帳戶，我們可以讓銀行以並行的方式處理其他客戶所調
用的
withdraw
函式：
int withdraw (struct account *account, int amount)
{
pthread_mutex_lock (&account->mutex);
const int balance = account->balance;
if (balance < amount) {
pthread_mutex_unlock (&account->mutex);
return -1;
}
account->balance = balance - amount;
pthread_mutex_unlock ...
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