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SubjectRe: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call
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On Fri, 2010-10-29 at 15:55 +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@gmail.com>
>
> ---
>
> 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)
> 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;

If you add the rcu_read_lock/unlock() sections, we would also need to
update the comment above accordingly.

Thanks,
Davidlohr

From: Davidlohr Bueso <dave@gnu.org>
Subject: [PATCH] ioprio: remove comment to not use RCU

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
---
fs/ioprio.c | 5 -----
1 files changed, 0 insertions(+), 5 deletions(-)
diff --git a/fs/ioprio.c b/fs/ioprio.c
index 748cfb9..72d71de 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -103,11 +103,6 @@ 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);
switch (which) {
case IOPRIO_WHO_PROCESS:
--
1.7.1




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