Re: [PATCH] kernel: fix data race in put_pid

Dmitry Vyukov
2015-09-18T09:07:12Z

On Fri, Sep 18, 2015 at 10:51 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Sep 17, 2015 at 08:09:19PM +0200, Oleg Nesterov wrote:
>> On 09/17, Dmitry Vyukov wrote:
>> >
>> > I can update the patch description, but let me explain it here first.
>>
>> Yes thanks.
>>
>> > Here is the essence of what happens:
>>
>> Aha, so you really meant that 2 put_pid's can race with each other,
>>
>> > // thread 1
>> > 1: pid->foo = 1; // foo is the first word of pid object
>> > // then it does put_pid
>> > 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and
>> > returns false so the function returns
>> >
>> > // thread 2
>> > // executes put_pid
>> > 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free
>> > // then kmem_cache_free does:
>> > 4: *(void**)pid = head->freelist;
>> > 5: head->freelist = (void*)pid;
>> >
>> > This can be executed as:
>> >
>> > 4: *(void**)pid = head->freelist;
>> > 1: pid->foo = 1; // foo is the first word of pid object
>> > 2: atomic_dec_and_test(&pid->count) // decrements count to 1 and
>> > returns false so the function returns
>> > 3: atomic_load(&pid->count); // returns 1, so proceed to kmem_cache_free
>> > 5: head->freelist = (void*)pid;
>>
>> Unless I am totally confused, everything is simpler. We can forget
>> about the hoisting, freelist, etc.
>>
>> Thread 2 can see the result of atomic_dec_and_test(), but not the
>> result of "pid->foo = 1". In this case in can free the object which
>> can be re-allocated _before_ STORE(pid->foo) completes. Of course,
>> this would be really bad.
>>
>> I need to recheck, but afaics this is not possible. This optimization
>> is fine, but probably needs a comment.
>
> For sure, this code doesn't make any sense to me.
>
>> We rely on delayed_put_pid()
>> called by RCU. And note that nobody can write to this pid after it
>> is removed from the rcu-protected list.
>>
>> So I think this is false alarm, but I'll try to recheck tomorrow, it
>> is too late for me today.
>
> As an alternative patch, could we not do:
>
> void put_pid(struct pid *pid)
> {
> struct pid_namespace *ns;
>
> if (!pid)
> return;
>
> ns = pid->numbers[pid->level].ns;
> if ((atomic_read(&pid->count) == 1) ||
> atomic_dec_and_test(&pid->count)) {
>
> + smp_read_barrier_depends(); /* ctrl-dep */
>
> kmem_cache_free(ns->pid_cachep, pid);
> put_pid_ns(ns);
> }
> }
>
> That would upgrade the atomic_read() path to a full READ_ONCE_CTRL(),
> and thereby avoid any of the kmem_cache_free() stores from leaking out.
> And its free, except on Alpha. Whereas the atomic_read_acquire() will
> generate a full memory barrier on whole bunch of archs.

What you propose makes sense.

+Will, Paul

Can we have something along the lines of:

#define atomic_read_ctrl(v) READ_ONCE_CTRL(&(v)->counter)

then?

I've found a bunch of similar cases, e.g.:
https://groups.google.com/forum/#!topic/ktsan/YoU0yX2wQJU

They all would benefit from atomic_read_ctrl.

--
Dmitry Vyukov, Software Engineer, dvyukov@google.com
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