linux-kernel - Re: semaphore and mutex in current Linux kernel (3.2.2)

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Date:	Fri, 06 Apr 2012 12:10:25 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	"Chen, Dennis (SRDC SW)" <Dennis1.Chen@....com>
CC:	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: semaphore and mutex in current Linux kernel (3.2.2)

Chen, Dennis (SRDC SW) wrote:
> On Thu, Apr 5, 2012 at 10:15 PM, Clemens Ladisch <clemens@...isch.de> wrote:
>> It would not make sense to spin too long, especially if some other
>> process wants to run on the same CPU.
>>
>>> int mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
>>> {
>>>       ...
>>>       while (owner_running(lock, owner)) {
>>>               if (need_resched())
>>>                       break;
>>>
>>>               arch_mutex_cpu_relax();
>>>       }
>>>       ...
>>> }
>>>
>>> D+ means the App in CPU1 is sleeping in a UNINTERRUPTIBLE state. This is very interesting,
>>> How does this happen?
>>
>> Your experiment shows that there must be some condition that makes the
>> code break out of the spin loop ...
>
> I guess this is related with RCU component, but I don't found the right place where the code
> Located yet.

"On the internet, nobody can hear you being subtle."

If some other process wants to run on the same CPU, needs_resched() is set.
(This might happen to make the cursor blink, for keyboard input, or when
somebody starts a rogue process like ps.)


Regards,
Clemens
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