linux-kernel - Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

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Message-ID: <20080819144211.GB21929@Krystal>
Date:	Tue, 19 Aug 2008 10:42:11 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Eran Liberty <liberty@...ricom.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <srostedt@...hat.com>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linuxppc-dev@...abs.org
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

* Eran Liberty (liberty@...ricom.com) wrote:
> Mathieu Desnoyers wrote:
>> Can you also give us
>>
>> objdump -S --start-address=0xC00BB724 vmlinux | head 20
>>
>> ?
>>
>> Then we could compare the result with the OOPS instruction dump :
>>
>> 7c0802a6 bf61000c 3f60c038 7c3f0b78 90010024 7c7c1b78 7c9d2378 83db32a0
>> 73c00001 7f83e378 7fa4eb78 4082002f <00000000> 2f830000 409e0030 801b32a0
>>
>> Mathieu
>>
>>   
>
> to give you more context I have run:
> > powerpc-linux-gnu-objdump -S --start-address=0xC00BB700 vmlinux | head -n 
> 60
>
> the discrepancy starts at address:
> c00bb720: 40 82 00 30 <=> 40 82 00 2f
> c00bb724: 4b ff fe 61 <=> 00 00 00 00
>

Ah !

I think I see what could be wrong :

First we have :

static unsigned int ftrace_nop = 0x60000000;

We probably replace the original function call by this nop.

Then we do :

notrace unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
{
        static unsigned int op;

        /*
         * It would be nice to just use create_function_call, but that will
         * update the code itself. Here we need to just return the
         * instruction that is going to be modified, without modifying the
         * code.
         */
        addr = GET_ADDR(addr);

        /* Set to "bl addr" */
        op = 0x48000001 | (ftrace_calc_offset(ip, addr) & 0x03fffffc);

        /*
         * No locking needed, this must be called via kstop_machine
         * which in essence is like running on a uniprocessor machine.
         */
        return (unsigned char *)&op;
}

And I guess we must be doing a 0x48000001 | 0x0; or something ?

Also, we have to consider that POWERPC 64 functions are :

/* PowerPC64's functions are data that points to the functions */

And this does not seem to hold for ppc32. Therefore, it is strange to me
that the same code is used for the update.. are we updating the correct
site ?

Mathieu

> vmlinux:     file format elf32-powerpc
>
> Disassembly of section .text:
>
> c00bb700 <d_lookup+0x1c>:
> * d_lookup() is protected against the concurrent renames in some unrelated
> * directory using the seqlockt_t rename_lock.
> */
>
> struct dentry * d_lookup(struct dentry * parent, struct qstr * name)
> {
> c00bb700:       7c 3f 0b 78     mr      r31,r1
> c00bb704:       90 01 00 24     stw     r0,36(r1)
> c00bb708:       7c 7c 1b 78     mr      r28,r3
> c00bb70c:       7c 9d 23 78     mr      r29,r4
> c00bb710:       83 db 32 a0     lwz     r30,12960(r27)
> {
>        unsigned ret;
>
> repeat:
>        ret = sl->sequence;
>        smp_rmb();
> c00bb714:       73 c0 00 01     andi.   r0,r30,1
>        struct dentry * dentry = NULL;
>        unsigned long seq;
>
>        do {
>                seq = read_seqbegin(&rename_lock);
>                dentry = __d_lookup(parent, name);
> c00bb718:       7f 83 e3 78     mr      r3,r28
> c00bb71c:       7f a4 eb 78     mr      r4,r29
>        if (unlikely(ret & 1)) {
> c00bb720:       40 82 00 30     bne-    c00bb750 <d_lookup+0x6c>
> c00bb724:       4b ff fe 61     bl      c00bb584 <__d_lookup>
>                if (dentry)
> c00bb728:       2f 83 00 00     cmpwi   cr7,r3,0
> c00bb72c:       40 9e 00 30     bne-    cr7,c00bb75c <d_lookup+0x78>
> *
> * If sequence value changed then writer changed data while in section.
> */
> static __always_inline int read_seqretry(const seqlock_t *sl, unsigned 
> start)
> {
>        smp_rmb();
> c00bb730:       80 1b 32 a0     lwz     r0,12960(r27)
>                        break;
>        } while (read_seqretry(&rename_lock, seq));
> c00bb734:       7f 80 f0 00     cmpw    cr7,r0,r30
> c00bb738:       41 9e 00 24     beq-    cr7,c00bb75c <d_lookup+0x78>
> /* Start of read calculation -- fetch last complete writer token */
> static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
> {
>        unsigned ret;
>
> repeat:
> c00bb73c:       7c 1e 03 78     mr      r30,r0
>        ret = sl->sequence;
>        smp_rmb();
> c00bb740:       73 c0 00 01     andi.   r0,r30,1
>        struct dentry * dentry = NULL;
>

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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