linux-kernel - Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm

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Message-Id: <201803270700.IJB35465.HJQFSFMVLFOtOO@I-love.SAKURA.ne.jp>
Date:   Tue, 27 Mar 2018 07:00:56 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     gorcunov@...il.com
Cc:     willy@...radead.org, yang.shi@...ux.alibaba.com,
        adobriyan@...il.com, mhocko@...nel.org, mguzik@...hat.com,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

Cyrill Gorcunov wrote:
> On Tue, Mar 27, 2018 at 06:10:09AM +0900, Tetsuo Handa wrote:
> > On 2018/03/27 4:21, Cyrill Gorcunov wrote:
> > > That said I think using read-lock here would be a bug.
> > 
> > If I understand correctly, the caller can't set both fields atomically, for
> > prctl() does not receive both fields at one call.
> > 
> >   prctl(PR_SET_MM, PR_SET_MM_ARG_START xor PR_SET_MM_ARG_END xor PR_SET_MM_ENV_START xor PR_SET_MM_ENV_END, new value, 0, 0);
> > 
> 
> True, but the key moment is that two/three/four system calls can
> run simultaneously. And while previously they are ordered by "write",
> with read lock they are completely unordered and this is really
> worries me.

Yes, we need exclusive lock when updating these fields.

>             To be fair I would prefer to drop this old per-field
> interface completely. This per-field interface was rather an ugly
> solution from my side.

But this is userspace visible API and thus we cannot change.

> 
> > Then, I wonder whether reading arg_start|end and env_start|end atomically makes
> > sense. Just retry reading if arg_start > env_end or env_start > env_end is fine?
> 
> Tetsuo, let me re-read this code tomorrow, maybe I miss something obvious.
> 

You are not missing my point. What I thought is

+retry:
-	down_read(&mm->mmap_sem);
 	arg_start = mm->arg_start;
 	arg_end = mm->arg_end;
 	env_start = mm->env_start;
 	env_end = mm->env_end;
-	up_read(&mm->mmap_sem);
 
-	BUG_ON(arg_start > arg_end);
-	BUG_ON(env_start > env_end);
+	if (unlikely(arg_start > arg_end || env_start > env_end)) {
+		cond_resched();
+		goto retry;
+	}

for reading these fields.

By the way, /proc/pid/ readers are serving as a canary who tells something
mm_mmap related problem is happening. On the other hand, it is sad that
such canary cannot be terminated by signal due to use of unkillable waits.
I wish we can use killable waits.