[CHECKER] races in 2.5.65/mm/swapfile.c?

[CHECKER] races in 2.5.65/mm/swapfile.c?

[Dawson Engler]

Hi All,

mm/swapfile.c seems to have three potential races.

The first two are in 
        linux-2.5.62/mm/swap_state.c:87:add_to_swap_cache

which seems reachable without a lock from the callchain:

        mm/swapfile.c:sys_swapoff:998->
              sys_swapoff:1026->
                try_to_unuse:591->
                        mm/swap_state.c:read_swap_cache_async:377->
                            add_to_swap_cache

add_to_swap_cache increments two global variables without a lock:
        INC_CACHE_INFO(add_total);
and
        INC_CACHE_INFO(exist_race);


The final one is in
        linux-2.5.62/mm/swapfile.c:213:swap_entry_free
which seems to increment
        nr_swap_pages++;
without a lock.

Are these real races?  Or are these just stats variables?  (Or is
there some implicit locking that protects these?)

Regards,
Dawson
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Re: [CHECKER] races in 2.5.65/mm/swapfile.c?

[Andrew Morton]

Dawson Engler <engler@csl.stanford.edu> wrote:
>
> Hi All,
> 
> mm/swapfile.c seems to have three potential races.
> 
> The first two are in 
>         linux-2.5.62/mm/swap_state.c:87:add_to_swap_cache
> 
> which seems reachable without a lock from the callchain:
> 
>         mm/swapfile.c:sys_swapoff:998->
>               sys_swapoff:1026->
>                 try_to_unuse:591->
>                         mm/swap_state.c:read_swap_cache_async:377->
>                             add_to_swap_cache
> 
> add_to_swap_cache increments two global variables without a lock:
>         INC_CACHE_INFO(add_total);
> and
>         INC_CACHE_INFO(exist_race);

These are just instrumentation.  If they're a bit inaccurate nobody cares,
and they're not worth locking.

So yes, that is a positive.

> The final one is in
>         linux-2.5.62/mm/swapfile.c:213:swap_entry_free
> which seems to increment
>         nr_swap_pages++;
> without a lock.

swap_entry_free() is called after swap_info_get(), which locks the swap
device list and the particular swap device.

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