linux-kernel - Re: suspicious rcu_dereference

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Message-ID: <20101109202051.GT4032@linux.vnet.ibm.com>
Date:	Tue, 9 Nov 2010 12:20:51 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: suspicious rcu_dereference_check() usage splat

On Tue, Nov 09, 2010 at 11:09:31AM +0200, Sergey Senozhatsky wrote:
> On (11/08/10 12:24), Paul E. McKenney wrote:
> > Looking over the patch again, the scope of the RCU read-side critical
> > section needs to expand to cover the use of the pointer as well as the
> > call to find_task_by_vpid().  So, for example:
> > 
> > 		case IOPRIO_WHO_PROCESS:
> > 			rcu_read_lock();
> > 			if (!who)
> > 				p = current;
> > 			else
> > 				p = find_task_by_vpid(who);
> > 			if (p)
> > 				ret = set_task_ioprio(p, ioprio);
> > 			rcu_read_unlock();
> > 			break;
> > 		case IOPRIO_WHO_PGRP:
> > 
> > 
> > Otherwise, the task could go away before the last use.
> > 
> > Sergey, could you please make this change and re-post your patch?
> > 
> > 							Thanx, Paul
> > 
> 
> Hello,
> Should we protect set_task_ioprio? Critical operations are protected
> with rcu_read_lock/rcu_read_unlock in set_task_ioprio, the rest is protected
> via task_lock(task)/task_unlock(task).

At first glance, this function's locking is OK, but I must defer to
people who know this code better than do I.

							Thanx, Paul
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