linux-kernel - Re: [PATCH v2] intelrdt: resctrl: recommend locking for resctrlfs

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Date:   Fri, 9 Dec 2016 10:35:20 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>
cc:     Fenghua Yu <fenghua.yu@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] intelrdt: resctrl: recommend locking for resctrlfs

On Fri, 2 Dec 2016, Marcelo Tosatti wrote:
> Actually, using flock(2) allows one to use LOCK_SH for readers and 
> this allows consistent writer/reader behaviour (say, a reader
> won't see a partially written directory).

Indeed.

> So the procedure would be:
> 
>     /var/lock/resctrl/fs.lock created previously in the filesystem.
> 
> WRITE LOCK:
> 
> A) Take flock(EXCLUSIVE) on /var/lock/resctrl/fs.lock
> B) If success, write pid of the program to the file.
> C) read/write the directory structure.
> D) funlock
> 
> READ LOCK:
> 
> A) Take flock(SHARED) on /var/lock/resctrl/fs.lock
> B) If success read the directory structure.
> C) funlock
> 
> 
> How about that?

Looks sane. Adding a small example in C and [ba]sh would be nice.

Thanks,

	tglx