linux-kernel - Re: [PATCH] kthreads: kill CLONE_KERNEL, change kernel_thread(kernel_init) to avoid CLONE

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Message-ID: <537A2178.7070701@oracle.com>
Date:	Mon, 19 May 2014 11:21:28 -0400
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kthreads: kill CLONE_KERNEL, change kernel_thread(kernel_init)
 to avoid CLONE_SIGHAND

On 05/16/2014 11:35 AM, Oleg Nesterov wrote:
> On 05/16, Sasha Levin wrote:
>>
>> On 04/13/2014 03:56 PM, Oleg Nesterov wrote:
>>> 1. Remove CLONE_KERNEL, it has no users and it is dangerous.
>>>
>>>    The (old) comment says "List of flags we want to share for kernel
>>>    threads" but this is not true, we do not want to share ->sighand by
>>>    default. This flag can only be used if the caller is sure that both
>>>    parent/child will never play with signals (say, allow_signal/etc).
>>>
>>> 2. Change rest_init() to clone kernel_init() without CLONE_SIGHAND.
>>>
>>>    In this case CLONE_SIGHAND does not really hurt, and it looks like
>>>    optimization because copy_sighand() can avoid kmem_cache_alloc().
>>>
>>>    But in fact this only adds the minor pessimization. kernel_init()
>>>    is going to exec the init process, and de_thread() will need to
>>>    unshare ->sighand and do kmem_cache_alloc(sighand_cachep) anyway,
>>>    but it needs to do more work and take tasklist_lock and siglock.
>>>
>>> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>>
>> Hi Oleg,
>>
>> This patch triggers a hang during boot in my KVM guest.
> 
> Hmm... How??? ;)

I'm afraid this is just bisection gone bad. The real issue was the
new goldfish code added, and not this patch.

Since apparently the boot issue was probabilistic I ended up blaming
this commit by mistake. Sorry about that.


Thanks,
Sasha
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