linux-kernel - Re: fs_struct refcounting: spinlock vs atomic

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Message-ID: <9cc188a2-406e-bf7c-e769-e982735ba5b6@metux.net>
Date:   Thu, 15 Feb 2018 14:46:19 +0100
From:   Enrico Weigelt <lkml@...ux.net>
To:     Richard Weinberger <richard.weinberger@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: fs_struct refcounting: spinlock vs atomic

On 15.02.2018 10:14, Richard Weinberger wrote:
> On Wed, Feb 14, 2018 at 10:13 PM, Enrico Weigelt <lkml@...ux.net> wrote:
>> Hi folks,
>>
>>
>> in fork.c, a spinlock is held for fs_struct refcounting, while other
>> places - eg. switch_task_namespaces uses atomic_dec_and_test() on
>> the nsproxy.
>>
>> What's the exact difference here ? Could the atomic counting also used
>> for fs_struct ?
> 
> Well, the spinlock protects more than just the counter. So atomic won't do it.

Okay. Is that needed in that case ?

See unshare() syscall:

if (new_fs) {
	fs = current->fs;
	spin_lock(&fs->lock);
	current->fs = new_fs;
	if (--fs->users)
		new_fs = NULL;
	else
		new_fs = fs;
	spin_unlock(&fs->lock);
}

Seems to me, that we're just refcounting here, and once it went dont to
zero, nobody else can access it anymore.


--mtx

-- 
Enrico Weigelt, metux IT consult
Free software and Linux embedded engineering
info@...ux.net -- +49-151-27565287