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Message-ID: <alpine.DEB.2.00.0904231101030.24293@gandalf.stny.rr.com>
Date: Thu, 23 Apr 2009 11:08:22 -0400 (EDT)
From: Steven Rostedt <rostedt@...dmis.org>
To: Ingo Molnar <mingo@...e.hu>
cc: Frédéric Weisbecker <fweisbec@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Glauber de Oliveira Costa <gcosta@...hat.com>,
Chris Wright <chrisw@...s-sol.org>,
Jeremy Fitzhardinge <jeremy@...p.org>,
Rusty Russell <rusty@...tcorp.com.au>,
Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [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
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