i

三種輸出可能：

ii

沒看懂題意說這個(gè)用戶內(nèi)核線程是想干啥。

Q5

(a) Our most basic synchronization primitive is a lock. When would we use a lock? When we use a lock, what concurrency bug are we trying to prevent?

Locks are used to guard a shared data variable, like the account balance shown here. If all accesses to a data variable are guarded (surrounded by a synchronized block) by the same lock object, then those accesses will be guaranteed to be atomic — uninterrupted by other threads.

Deadlock occurs when a set of threads is waiting on an event that can only be
generated by another thread in the set.
Race conditions occur when results change based on scheduling due to a failure to protect shared data.
上面是標(biāo)準(zhǔn)答案，其實(shí)就是死鎖bug和非死鎖bug
Two types of bugs – Deadlocks: threads cannot execute any further
and wait for each other – Non-deadlock bugs: non deadlock but incorrect
results when threads execute
Non deadlock and deadlock bugs.

Of the non-deadlock bugs, the main types are:
Atomicity Violation Bugs ： atomicity assumptions madeby programmer are violated during executionof concurrent threads。fix : Always use locks when accessing shared data
Order Violation Bugs:desired order of memory accesses is flipped during concurrent
execution – Fix: condition variables

b) Deadlock is an important concurrency bug that can be hard to spot. What are the four necessary and sufficient conditions for deadlock?

死鎖的四個(gè)必要條件 ：

Mutual exclusion
Hold-and-wait: They hold the lock while waiting for other resources
No preemption: Resources cannot be forcible removed from threads that hold them.
Circular wait: There is a circular chain of threads s.t. each holds one or more resources that are being requested by the next thread in the chain.
fix : destory one conditions.

? As we considered advanced synchronization techniques, we considered situations where we might want to acquire multiple locks. As we know, acquiring multiple locks can be a dangerous requirement, and so we studied conservative two-phase locking. What condition of deadlock does conservative two-phase locking break? Explain how.

兩階段鎖 two-phase locking
Conservative two-phased locking
Requires you to gather all locks before beginning your operations, and, if you
can’t, you must release any you have acquired. Breaks hold and wait.
也就是說，如果有兩把鎖，只有獲取兩把鎖才去操作資源，否則要放開鎖。破壞了四個(gè)條件中的hold and wait.
Conservative Two-Phase Locking
? A thread must:
– Acquire all locks it will need
? If all locks cannot be acquired, release any already acquired
and begin again
– Make necessary changes, commit, and release locks
? Thus B cannot see any of A’s changes until A
commits and releases the lock
? Provides serializability
? Prevents deadlock

(d) Consider the following code. Is it susceptible to deadlock? If so, fix the code so it is not. If not, explain why.

不會(huì)出現(xiàn)死鎖。主要是因?yàn)檫@兩個(gè)線程不會(huì)出現(xiàn)互相等待資源的情況。總共2個(gè)資源，都是按照順序鎖1和2的，如果一個(gè)先鎖1后鎖2，另一個(gè)先鎖2后鎖1，這種情況就會(huì)死鎖了。

下面是一些相關(guān)知識(shí)點(diǎn)
首先shared 鎖是讀鎖，excude 鎖是寫鎖。

A transaction is said to follow the Two-Phase Locking protocol if Locking and Unlocking can be done in two phases.

Growing Phase: New locks on data items may be acquired but none can be released.
Shrinking Phase: Existing locks may be released but no new locks can be acquired.

void herp ()flock_1. lock ();lock_2. lock ();void derp ()flock_1. lock ();lock_2. lock ();global_data. do_a_thing ();global_data. do_other_thing ();lock_1. unlock ();lock_2. unlock ();lock_2. unlock ();lock_1. unlock ();

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