lkml.org 
[lkml]   [2009]   [Apr]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 0/2] [GIT PULL] tracing: various bug fixes

On Thu, 23 Apr 2009, Steven Rostedt wrote:
>
> static inline void __native_flush_tlb_global(void)
> {
> unsigned long flags;
> unsigned long cr4;
>
> /*
> * Read-modify-write to CR4 - protect it from preemption and
> * from interrupts. (Use the raw variant because this code can
> * be called from deep inside debugging code.)
> */
> raw_local_irq_save(flags);
>
> cr4 = read_cr4();
> /* clear PGE */
> write_cr4(cr4 & ~X86_CR4_PGE);
> /* write old PGE again and flush TLBs */
> write_cr4(cr4);
>
> raw_local_irq_restore(flags);
> }
>
>
> Each of those read_cr4 and write_cr4 can be traced. But I still do not see
> how this can cause corruption in the current task struct.
>
> The TLB just caches the pages we are using, not the contents inside. I
> still do not see how this can corrupt a bit. Unless a register leaked :-/

Ah, this is a lockdep thing.

The raw_local_irq_save/restore in __native_flush_tlb_global does not
update hardirqs_enabled.

When we call into ftrace, when we cross page bounderies, we disable
interrupts using the normal local_irq_save/restore calls.

But when we restore, it detects that interrupts are not going to be
enabled, and keeps hardirqs_enabled off.

The printk solved the issue because it called local_irq_restore, which set
the variable back.

I guess there's two solutions here. One, we can change the
raw_local_irq_enable/disable variants in __native_flush_tlb_global to the
non-raw type (it should protect against recursion).

or we can try to make the ring buffer use the raw_local_irq variants too.
I tried this once before, and it did cause issues.

Note, there's a "check_flags" in lockdep, but it is only called on
locking, it is not called when we only disable/enable interrupts.

-- Steve



\
 
 \ /
  Last update: 2009-04-23 17:11    [W:0.114 / U:0.060 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site