CODEWITHSUNDEEP
multithreadingmutex
Vanilla Face
Vanilla Face

Reputation: 916

Does mutex tell processor not to interrupt thread?

I was thinking about this when using mutex.

If you lock the thread with mutex for instance

m.lock();
a += b;
m.unlock();

What happens when the the two threads collide? Is it like:

1) by locking thread A tells CPU that this must be executed uninterrupted until unlock and while the a += b statement is happening context switch never occurs.

2) thread A locks, while doing the a += b statement context switch happens, thread B sees that its locked, yelds control and everything is back to A which finishes a += b; operation and unlocks access?

If it's case 2 after all, is there a way to tell CPU that certain parts of the code shall never be interrupted by context switch?

Upvotes: 0

Views: 200

Answers (2)

Alexei Kaigorodov
Alexei Kaigorodov

Reputation: 13535

Both flavors of mutexes exist, but the first kind is used only in privileged programs (O/S kernel, hardware drivers). User is given only second kind, otherwise a user program could hang the whole system.

Upvotes: 0

500 - Internal Server Error
500 - Internal Server Error

Reputation: 28839

It is 2, and there is no direct way to say that code must run to completion without a context switch. You could probably get close by bumping the execution priority of the thread but such shenanigans are usually frowned upon and suggest a bad design.

Upvotes: 3

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