INFO: suspicious rcu_dereference_check() usage - kernel/pid.c:419 invoked rcu_dereference_check() without protection!

[Greg Thelen]

I observe a failing rcu_dereference_check() in linux-next (found in
mmotm-2010-10-07-14-08).  An extra rcu assertion in
find_task_by_pid_ns() was added by:
  commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde
  Author: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
  Date:   Sat Jun 26 01:08:19 2010 +0900
  
      Add RCU check for find_task_by_vpid().

This extra assertion causes a rcu_dereference_check() failure during
boot in 512 MIB VM.  I would be happy to get out proposed patches to
this issue.  My config includes:
  CONFIG_PREEMPT=y
  CONFIG_LOCKDEP=y
  CONFIG_PROVE_LOCKING=y
  CONFIG_PROVE_RCU=y

The console error:

Begin: Running /scripts/local-bottom ...
Done.
Done.
Begin: Running /scripts/init-bottom ...
Done.
[    3.394348]
[    3.394349] ===================================================
[    3.395162] [ INFO: suspicious rcu_dereference_check() usage. ]
[    3.395786] ---------------------------------------------------
[    3.396452] kernel/pid.c:419 invoked rcu_dereference_check() without protection!
[    3.397483]
[    3.397484] other info that might help us debug this:
[    3.397485]
[    3.398363]
[    3.398364] rcu_scheduler_active = 1, debug_locks = 0
[    3.399073] 1 lock held by ureadahead/1438:
[    3.399515]  #0:  (tasklist_lock){.+.+..}, at: [<ffffffff811c1d1a>] sys_ioprio_set+0x8a/0x3f0
[    3.400500]
[    3.400501] stack backtrace:
[    3.401036] Pid: 1438, comm: ureadahead Not tainted 2.6.36-dbg-DEV #10
[    3.401717] Call Trace:
[    3.401996]  [<ffffffff810c720b>] lockdep_rcu_dereference+0xbb/0xc0
[    3.402742]  [<ffffffff810aebb1>] find_task_by_pid_ns+0x81/0x90
[    3.403445]  [<ffffffff810aebe2>] find_task_by_vpid+0x22/0x30
[    3.404146]  [<ffffffff811c2074>] sys_ioprio_set+0x3e4/0x3f0
[    3.404756]  [<ffffffff815c5919>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[    3.405455]  [<ffffffff8104331b>] system_call_fastpath+0x16/0x1b


ioprio_set() contains a comment warning against of usage of
rcu_read_lock() to avoid this warning:
	/*
	 * 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().
	 */

So I'm not sure what the best fix is.

Also I see that sys_ioprio_get() has a similar problem that might be
addressed with:

diff --git a/fs/ioprio.c b/fs/ioprio.c
index 748cfb9..02eed30 100644
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -197,6 +197,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 	int ret = -ESRCH;
 	int tmpio;
 
+	rcu_read_lock();
 	read_lock(&tasklist_lock);
 	switch (which) {
 		case IOPRIO_WHO_PROCESS:
@@ -251,5 +252,6 @@ SYSCALL_DEFINE2(ioprio_get, int, which, int, who)
 	}
 
 	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 	return ret;
 }

sys_ioprio_get() didn't have an explicit warning against usage of
rcu_read_lock(), but that doesn't mean this is a good patch.

--
Greg
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