linux-kernel - Re: [PATCH v1 3/4] powerpc/code-patching: Use jump

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Date:   Thu, 19 May 2022 06:27:40 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v1 3/4] powerpc/code-patching: Use jump_label for testing
 freed initmem



Le 19/05/2022 à 04:17, Guenter Roeck a écrit :
> On Tue, Mar 22, 2022 at 04:40:20PM +0100, Christophe Leroy wrote:
>> Once init is done, initmem is freed forever so no need to
>> test system_state at every call to patch_instruction().
>>
>> Use jump_label.
>>
>> This reduces by 2% the time needed to activate ftrace on an 8xx.
>>
> 
> It also causes the qemu mpc8544ds emulation to crash.
> 
> BUG: Unable to handle kernel data access on write at 0xc122eb34
> Faulting instruction address: 0xc001b580
> Oops: Kernel access of bad area, sig: 11 [#1]
> BE PAGE_SIZE=4K MPC8544 DS
> Modules linked in:
> CPU: 0 PID: 1 Comm: swapper Not tainted 5.18.0-rc7-next-20220518 #1
> NIP:  c001b580 LR: c001b560 CTR: 00000003
> REGS: c5107dd0 TRAP: 0300   Not tainted  (5.18.0-rc7-next-20220518)
> MSR:  00009000 <EE,ME>  CR: 24000882  XER: 00000000
> DEAR: c122eb34 ESR: 00800000
> GPR00: c001b560 c5107ec0 c5120020 10000000 00000000 00000078 0c000000 cfffffff
> GPR08: c001e9ec 00000001 00000007 00000000 44000882 00000000 c0005178 00000000
> GPR16: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
> GPR24: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 c1230000
> NIP [c001b580] free_initmem+0x48/0xa8
> LR [c001b560] free_initmem+0x28/0xa8
> Call Trace:
> [c5107ec0] [c001b560] free_initmem+0x28/0xa8 (unreliable)
> [c5107ee0] [c00051b0] kernel_init+0x38/0x150
> [c5107f00] [c001626c] ret_from_kernel_thread+0x5c/0x64
> Instruction dump:
> 3fe0c123 912a00dc 90010024 48000665 3d20c218 8929fa65 2c090000 41820058
> 813feb34 2c090000 4082003c 39200001 <913feb34> 80010024 3cc0c114 83e1001c
> 
> Reverting this patch fixes the problem.
> 

That's strange.

I was able to reproduce the problem.

Removing the __ro_after_init in front of 
DEFINE_STATIC_KEY_FALSE(init_mem_is_free) fixes the problem.

I can't understand why, mark_readonly() is called after free_initmem().

Christophe