On Thu, Nov 11, 2010 at 11:32:47PM -0800, Greg Thelen wrote:
> Using:
> - CONFIG_LOCKUP_DETECTOR=y
> - CONFIG_PREEMPT=y
> - CONFIG_LOCKDEP=y
> - CONFIG_PROVE_LOCKING=y
> - CONFIG_PROVE_RCU=y
> found a missing rcu lock during boot on a 512 MiB x86_64 ubuntu vm:
> ===================================================
> [ INFO: suspicious rcu_dereference_check() usage. ]
> ---------------------------------------------------
> kernel/pid.c:419 invoked rcu_dereference_check() without protection!
>
> other info that might help us debug this:
>
> rcu_scheduler_active = 1, debug_locks = 0
> 1 lock held by ureadahead/1355:
> #0: (tasklist_lock){.+.+..}, at: [<ffffffff8115bc09>] sys_ioprio_set+0x7f/0x29e
>
> stack backtrace:
> Pid: 1355, comm: ureadahead Not tainted 2.6.37-dbg-DEV #1
> Call Trace:
> [<ffffffff8109c10c>] lockdep_rcu_dereference+0xaa/0xb3
> [<ffffffff81088cbf>] find_task_by_pid_ns+0x44/0x5d
> [<ffffffff81088cfa>] find_task_by_vpid+0x22/0x24
> [<ffffffff8115bc3e>] sys_ioprio_set+0xb4/0x29e
> [<ffffffff8147cf21>] ? trace_hardirqs_off_thunk+0x3a/0x3c
> [<ffffffff8105c409>] sysenter_dispatch+0x7/0x2c
> [<ffffffff8147cee2>] ? trace_hardirqs_on_thunk+0x3a/0x3f
>
> The fix is to:
> a) grab rcu lock in sys_ioprio_{set,get}() and
> b) avoid grabbing tasklist_lock.
> Discussion in: http://marc.info/?l=linux-kernel&m=128951324702889
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx>
> ---
> fs/ioprio.c | 13 ++++---------
> 1 files changed, 4 insertions(+), 9 deletions(-)
>
> diff --git a/fs/ioprio.c b/fs/ioprio.c
> index 748cfb9..7da2a06 100644
> --- a/fs/ioprio.c
> +++ b/fs/ioprio.c
> @@ -103,12 +103,7 @@ SYSCALL_DEFINE3(ioprio_set, int, which, int, who, int, ioprio)
> }
>
> ret = -ESRCH;
> - /*
> - * We want IOPRIO_WHO_PGRP/IOPRIO_WHO_USER to be "atomic",
> - * so we can't use rcu_read_lock(). See re-copy of ->ioprio
> - * in copy_process().
> - */
> - read_lock(&tasklist_lock);
> + rcu_read_lock();
> switch (which) {
> case IOPRIO_WHO_PROCESS:
> if (!who)
> @@ -153,7 +148,7 @@ free_uid:
> ret = -EINVAL;
> }
>
> - read_unlock(&tasklist_lock);
> + rcu_read_unlock();
> return ret;
> }
>
> @@ -197,7 +192,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
> int ret = -ESRCH;
> int tmpio;
>
> - read_lock(&tasklist_lock);
> + rcu_read_lock();
> switch (which) {
> case IOPRIO_WHO_PROCESS:
> if (!who)
> @@ -250,6 +245,6 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
> ret = -EINVAL;
> }
>
> - read_unlock(&tasklist_lock);
> + rcu_read_unlock();
> return ret;
> }
> --
> 1.7.3.1
>
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