Re: 2.6.35-rc2-git1 - lib/idr.c:605 invoked rcu_dereference_check() without protection!

[Miles Lane]

On Mon, Jun 7, 2010 at 8:12 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Jun 07, 2010 at 02:23:17PM -0400, Miles Lane wrote:
>> [    2.677955] [ INFO: suspicious rcu_dereference_check() usage. ]
>> [    2.679089] ---------------------------------------------------
>> [    2.680276] lib/idr.c:605 invoked rcu_dereference_check() without protection!
>> [    2.681499]
>> [    2.681500] other info that might help us debug this:
>> [    2.681501]
>> [    2.685509]
>> [    2.685510] rcu_scheduler_active = 1, debug_locks = 1
>> [    2.688221] 1 lock held by swapper/1:
>> [    2.689587]  #0:  (mtd_table_mutex){+.+...}, at:
>> [<ffffffff812bea45>] register_mtd_user+0x1a/0x69
>> [    2.691096]
>> [    2.691098] stack backtrace:
>> [    2.694059] Pid: 1, comm: swapper Not tainted 2.6.35-rc2-git1 #8
>> [    2.695601] Call Trace:
>> [    2.697243]  [<ffffffff81064e9c>] lockdep_rcu_dereference+0x9d/0xa5
>> [    2.698868]  [<ffffffff811b9c86>] idr_get_next+0x60/0x124
>> [    2.700556]  [<ffffffff812be779>] __mtd_next_device+0x1b/0x1d
>> [    2.702238]  [<ffffffff812bea7c>] register_mtd_user+0x51/0x69
>> [    2.703964]  [<ffffffff816cca45>] init_mtdchar+0xb3/0xd3
>> [    2.705686]  [<ffffffff816cc992>] ? init_mtdchar+0x0/0xd3
>> [    2.707470]  [<ffffffff810001ef>] do_one_initcall+0x59/0x14e
>> [    2.709255]  [<ffffffff816a768a>] kernel_init+0x144/0x1ce
>> [    2.711082]  [<ffffffff81003054>] kernel_thread_helper+0x4/0x10
>> [    2.712862]  [<ffffffff813ca480>] ? restore_args+0x0/0x30
>> [    2.714647]  [<ffffffff816a7546>] ? kernel_init+0x0/0x1ce
>> [    2.716415]  [<ffffffff81003050>] ? kernel_thread_helper+0x0/0x10
>
> This looks like a new one!  Does the following patch take care of it?
>
>                                                        Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 2d54a6c31b72c902b09d365e9c66205a5c07e549
> Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Date:   Mon Jun 7 17:09:45 2010 -0700
>
>    idr: fix RCU lockdep splat in idr_get_next()
>
>    Convert to rcu_dereference_raw() given that many callers may have many
>    different locking models.
>
>    Located-by: Miles Lane <miles.lane@xxxxxxxxx>
>    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> diff --git a/lib/idr.c b/lib/idr.c
> index 2eb1dca..f099f25 100644
> --- a/lib/idr.c
> +++ b/lib/idr.c
> @@ -599,7 +599,7 @@ void *idr_get_next(struct idr *idp, int *nextidp)
>        /* find first ent */
>        n = idp->layers * IDR_BITS;
>        max = 1 << n;
> -       p = rcu_dereference(idp->top);
> +       p = rcu_dereference_raw(idp->top);
>        if (!p)
>                return NULL;
>
> @@ -607,7 +607,7 @@ void *idr_get_next(struct idr *idp, int *nextidp)
>                while (n > 0 && p) {
>                        n -= IDR_BITS;
>                        *paa++ = p;
> -                       p = rcu_dereference(p->ary[(id >> n) & IDR_MASK]);
> +                       p = rcu_dereference_raw(p->ary[(id >> n) & IDR_MASK]);
>                }
>
>                if (p) {
>

Tested.  Looks good!
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