Re: print_vma_addr possible deadlock (was Re: Jeste jeden bug)

[Ingo Molnar <mingo@elte.hu>]

* Jiri Kosina <jkosina@suse.cz> wrote:

> > > [   68.379054]
> > > [   68.379056] Call Trace:
> > > [   68.379061]  <#DB>  [<ffffffff81064883>] ? __debug_show_held_locks+0x13/0x30
> > > [   68.379109]  [<ffffffff81036765>] __might_sleep+0xe5/0x110
> > > [   68.379123]  [<ffffffff812f2240>] down_read+0x20/0x70
> > > [   68.379137]  [<ffffffff8109cdca>] print_vma_addr+0x3a/0x110
> > > [   68.379152]  [<ffffffff8100f435>] do_trap+0xf5/0x170
> > > [   68.379168]  [<ffffffff8100f52b>] do_int3+0x7b/0xe0
> > > [   68.379180]  [<ffffffff812f4a6f>] int3+0x9f/0xd0
> > > [   68.379203]  <<EOE>>
> > > [   68.379229]  in libglib-2.0.so.0.1505.0[3d2c800000+dc000]

ouch. Could you try the fix below? This makes print_vma_addr() more 
robust and should fix the immediate bug.

The longer-term fix will be to not run int3 exception handlers in a 
non-preemptible context (like 32-bit does) - but that will need more 
testing.

	Ingo

---------------->
Subject: x86: fix "BUG: sleeping function called from invalid context" in print_vma_addr()
From: Ingo Molnar <mingo@elte.hu>
Date: Wed Feb 13 20:21:06 CET 2008

Jiri Kosina reported the following deadlock scenario with
show_unhandled_signals enabled:

 [   68.379022] gnome-settings-[2941] trap int3 ip:3d2c840f34
 sp:7fff36f5d100 error:0<3>BUG: sleeping function called from invalid
 context at kernel/rwsem.c:21
 [   68.379039] in_atomic():1, irqs_disabled():0
 [   68.379044] no locks held by gnome-settings-/2941.
 [   68.379050] Pid: 2941, comm: gnome-settings- Not tainted 2.6.25-rc1 #30
 [   68.379054]
 [   68.379056] Call Trace:
 [   68.379061]  <#DB>  [<ffffffff81064883>] ? __debug_show_held_locks+0x13/0x30
 [   68.379109]  [<ffffffff81036765>] __might_sleep+0xe5/0x110
 [   68.379123]  [<ffffffff812f2240>] down_read+0x20/0x70
 [   68.379137]  [<ffffffff8109cdca>] print_vma_addr+0x3a/0x110
 [   68.379152]  [<ffffffff8100f435>] do_trap+0xf5/0x170
 [   68.379168]  [<ffffffff8100f52b>] do_int3+0x7b/0xe0
 [   68.379180]  [<ffffffff812f4a6f>] int3+0x9f/0xd0
 [   68.379203]  <<EOE>>
 [   68.379229]  in libglib-2.0.so.0.1505.0[3d2c800000+dc000]

and tracked it down to:

  commit 03252919b79891063cf99145612360efbdf9500b
  Author: Andi Kleen <ak@suse.de>
  Date:   Wed Jan 30 13:33:18 2008 +0100

      x86: print which shared library/executable faulted in segfault etc. messages

the problem is that we call down_read() from an atomic context.

Solve this by returning from print_vma_addr() if the preempt count is
elevated. Update preempt_conditional_sti / preempt_conditional_cli to
unconditionally lift the preempt count even on !CONFIG_PREEMPT.

Reported-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 arch/x86/kernel/traps_64.c |    4 ++--
 mm/memory.c                |    7 +++++++
 2 files changed, 9 insertions(+), 2 deletions(-)

Index: linux-x86.q/arch/x86/kernel/traps_64.c
===================================================================
--- linux-x86.q.orig/arch/x86/kernel/traps_64.c
+++ linux-x86.q/arch/x86/kernel/traps_64.c
@@ -84,7 +84,7 @@ static inline void conditional_sti(struc
 
 static inline void preempt_conditional_sti(struct pt_regs *regs)
 {
-	preempt_disable();
+	inc_preempt_count();
 	if (regs->flags & X86_EFLAGS_IF)
 		local_irq_enable();
 }
@@ -95,7 +95,7 @@ static inline void preempt_conditional_c
 		local_irq_disable();
 	/* Make sure to not schedule here because we could be running
 	   on an exception stack. */
-	preempt_enable_no_resched();
+	dec_preempt_count();
 }
 
 int kstack_depth_to_print = 12;
Index: linux-x86.q/mm/memory.c
===================================================================
--- linux-x86.q.orig/mm/memory.c
+++ linux-x86.q/mm/memory.c
@@ -2711,6 +2711,13 @@ void print_vma_addr(char *prefix, unsign
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
 
+	/*
+	 * Do not print if we are in atomic
+	 * contexts (in exception stacks, etc.):
+	 */
+	if (preempt_count())
+		return;
+
 	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, ip);
 	if (vma && vma->vm_file) {