linux-kernel - Re: [PATCH] introduce get_mm_hiwater_xxx(), fix taskstats->hiwater

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Date:	Sun, 7 Dec 2008 00:26:37 +0900
From:	"KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>
To:	"Oleg Nesterov" <oleg@...hat.com>
Cc:	"Hugh Dickins" <hugh@...itas.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"Jay Lan" <jlan@....com>, "Jiri Pirko" <jpirko@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] introduce get_mm_hiwater_xxx(), fix taskstats->hiwater_xxx accounting

Hi Oleg

> But the difference can be huge. Let's suppose the process has 2 threads
> T1 and T2.
>
> T1 exits, calls update_hiwater_vm(), notices that mm->hiwater_vm must be
> updated, and preempted right before mm->hiwater_vm = new_value.
>
> T2 does free(malloc(A_LOT)) and exits. It sets the correct value for
> ->hiwater_vm which takes A_LOT into account and disappears.
>
> T1 resumes, and "reverts" ->hiwater_vm to the "new_value" which was
> calculated before.
>
> Now, since T1 is the last exiting thread, the whole thread group
> exits and we report the wrong ->hiwater_vm to the userspace.
>
> Yes, this race is unlikely. But there is another reason (perhaps
> not very good) why I tried to remove update_hiwater_xxx() from
> do_exit().

This explain is very good clarification than current patch description, IMHO.
So, if possible, I hope to add it into the patch description.

thanks.
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