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

Oleg Nesterov
2015-09-17T18:12:17Z

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:
> 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. 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.

Oleg.