On Fri, Oct 29, 2010 at 03:55:50PM +0300, Sergey Senozhatsky wrote:
> Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for
> find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns.
> Assertion failed in sys_ioprio_get. The patch is fixing assertion
> failure in ioprio_set as well.
>
> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> kernel/pid.c:419 invoked rcu_dereference_check() without protection!
>
> rcu_scheduler_active = 1, debug_locks = 0
> 1 lock held by iotop/4254:
> #0: (tasklist_lock){.?.?..}, at: [<ffffffff811104b4>] sys_ioprio_get+0x22/0x2da
>
> stack backtrace:
> Pid: 4254, comm: iotop Not tainted
> Call Trace:
> [<ffffffff810656f2>] lockdep_rcu_dereference+0xaa/0xb2
> [<ffffffff81053c67>] find_task_by_pid_ns+0x4f/0x68
> [<ffffffff81053c9d>] find_task_by_vpid+0x1d/0x1f
> [<ffffffff811104e2>] sys_ioprio_get+0x50/0x2da
> [<ffffffff81002182>] system_call_fastpath+0x16/0x1b
>
>
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
>
> ---
>
> diff --git a/fs/ioprio.c b/fs/ioprio.c
> index 748cfb9..666343d 100644
> --- a/fs/ioprio.c
> +++ b/fs/ioprio.c
> @@ -113,8 +113,11 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
Interesting...
The task-list lock is read-held at this point, which should mean that
the PID mapping cannot change. The lockdep_tasklist_lock_is_held()
function does lockdep_is_held(&tasklist_lock), which must therefore
only be checking for write-holding the lock. The fix would be to
make lockdep_tasklist_lock_is_held() check for either read-holding or
write-holding tasklist lock.
Or is there some subtle reason that read-holding the tasklist lock is
not sufficient?
Thanx, Paul
> case IOPRIO_WHO_PROCESS:
> if (!who)
> p = current;
> - else
> + else {
> + rcu_read_lock();
> p = find_task_by_vpid(who);
> + rcu_read_unlock();
> + }
> if (p)
> ret = set_task_ioprio(p, ioprio);
> break;
> @@ -202,8 +205,11 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
> case IOPRIO_WHO_PROCESS:
> if (!who)
> p = current;
> - else
> + else {
> + rcu_read_lock();
> p = find_task_by_vpid(who);
> + rcu_read_unlock();
> + }
> if (p)
> ret = get_task_ioprio(p);
> break;
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@xxxxxxxxxxxxxxx
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
