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Subject: Re: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call
From: Davidlohr Bueso <dave@gnu.org>
Reply-To: dave@gnu.org
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>
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Date: Mon, 08 Nov 2010 13:37:20 -0300
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On Mon, 2010-11-08 at 18:18 +0200, Sergey Senozhatsky wrote:
> On (11/08/10 13:01), Davidlohr Bueso wrote:
> > 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.
> >

Ah, yes indeed, misread the cases, sorry about the noise.

Thanks,
Davidlohr

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