From: Oleg Nesterov <oleg@redhat.com>
To: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: fix lockdep build in rcu-protected get_mm_exe_file()
Date: Tue, 24 Mar 2015 19:10:16 +0100 [thread overview]
Message-ID: <20150324181016.GA9678@redhat.com> (raw)
In-Reply-To: <55119B3B.5020403@yandex-team.ru>
On 03/24, Konstantin Khlebnikov wrote:
>
> On 23.03.2015 22:10, Oleg Nesterov wrote:
>> On 03/23, Davidlohr Bueso wrote:
>>>
>>> void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
>>> {
>>> struct file *old_exe_file = rcu_dereference_protected(mm->exe_file,
>>> - !atomic_read(&mm->mm_users) || current->in_execve ||
>>> - lock_is_held(&mm->mmap_sem));
>>> + !atomic_read(&mm->mm_users) || current->in_execve);
>>
>> Thanks, looks correct at first glance...
>>
>> But can't we remove the ->in_execve check above? and check
>>
>> atomic_read(&mm->mm_users) <= 1
>>
>> instead. OK, this is subjective, I won't insist. Just current->in_execve
>> looks a bit confusing, it means "I swear, the caller is flush_old_exec()
>> and this mm is actualy bprm->mm".
>>
>> "atomic_read(&mm->mm_users) <= 1" looks a bit more "safe". But again,
>> I won't insist.
>
> Not so safe: this will race with get_task_mm().
How?
If set_mm_exe_file() can race with get_task_mm() then we have a bug.
And it will be reported ;)
> A lot of proc files grab temporary reference to task mm.
> But this just a debug -- we can place here "true".
Yeees, probably rcu_dereference_raw() would be even better. set_mm_exe_file()
must be called only if nobody but us can access this mm.
Oleg.
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next prev parent reply other threads:[~2015-03-24 18:12 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-20 14:47 [PATCH] mm: fix lockdep build in rcu-protected get_mm_exe_file() Konstantin Khlebnikov
2015-03-23 18:11 ` Davidlohr Bueso
2015-03-23 19:10 ` Oleg Nesterov
2015-03-24 14:38 ` Davidlohr Bueso
2015-03-24 17:13 ` Konstantin Khlebnikov
2015-03-24 18:10 ` Oleg Nesterov [this message]
2015-03-24 18:15 ` Konstantin Khlebnikov
2015-03-24 19:02 ` Oleg Nesterov
2015-03-25 1:30 ` [PATCH v2] prctl: avoid using mmap_sem for exe_file serialization Davidlohr Bueso
2015-03-25 9:21 ` Konstantin Khlebnikov
2015-03-25 10:42 ` [PATCH v3] " Davidlohr Bueso
2015-03-25 11:08 ` Konstantin Khlebnikov
2015-03-25 12:50 ` Davidlohr Bueso
2015-03-25 12:53 ` Oleg Nesterov
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