[patch, 2.6.17-mm2] fix acct_collect() siglock bug

[Ingo Molnar <mingo@elte.hu>]

the patch below fixes a siglock usage bug introduced in -mm2. The lock 
validator found it during bootup.

---------------
Subject: fix acct_collect() siglock bug
From: Ingo Molnar <mingo@elte.hu>

the lock validator noticed the following bug:

 =========================================================
 [ INFO: possible irq lock inversion dependency detected ]
 ---------------------------------------------------------
 swapper/0 just changed the state of lock:
  (tasklist_lock){..-?}, at: [<c01247c8>] send_group_sig_info+0xe/0x2f
 but this lock took another, soft-irq-unsafe lock in the past:
  (&sighand->siglock){--..}

 and interrupts could create inverse lock ordering between them.
 ...
  -> (init_sighand.siglock){....} ops: 18 {
 ...
       hardirq-on-W at:
                       [<c0130577>] lock_acquire+0x56/0x73
                       [<c03ec59c>] _spin_lock+0x24/0x32
                       [<c01374bd>] acct_collect+0x81/0x115
                       [<c011b4fa>] do_exit+0x19a/0x7b3
                       [<c01273ce>] ____call_usermodehelper+0xdf/0xe1
                       [<c01007f1>] kernel_thread_helper+0x5/0xb

it turns out that acct_collect() takes current->sighand->siglock
with interrupts enabled - a big no-no. Fix it by disabling
interrupts.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/acct.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Index: linux/kernel/acct.c
===================================================================
--- linux.orig/kernel/acct.c
+++ linux/kernel/acct.c
@@ -549,7 +549,7 @@ void acct_collect(long exitcode, int gro
 		up_read(&current->mm->mmap_sem);
 	}
 
-	spin_lock(&current->sighand->siglock);
+	spin_lock_irq(&current->sighand->siglock);
 	if (group_dead)
 		pacct->ac_mem = vsize / 1024;
 	if (thread_group_leader(current)) {
@@ -567,7 +567,7 @@ void acct_collect(long exitcode, int gro
 	pacct->ac_stime = cputime_add(pacct->ac_stime, current->stime);
 	pacct->ac_minflt += current->min_flt;
 	pacct->ac_majflt += current->maj_flt;
-	spin_unlock(&current->sighand->siglock);
+	spin_unlock_irq(&current->sighand->siglock);
 }
 
 /**