linux-kernel - Re: [PATCH] riscv: ftrace: no need to acquire text_mutex when executed in stop

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Date:   Mon, 21 Mar 2022 19:40:08 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     changbin.du@...il.com
CC:     rostedt@...dmis.org, changbin.du@...il.com,
        Paul Walmsley <paul.walmsley@...ive.com>, mingo@...hat.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject:     Re: [PATCH] riscv: ftrace: no need to acquire text_mutex when executed in stop_machine

On Sun, 13 Mar 2022 00:07:11 PST (-0800), changbin.du@...il.com wrote:
> On Thu, Mar 10, 2022 at 09:27:42AM -0500, Steven Rostedt wrote:
>> On Thu, 10 Mar 2022 12:54:54 +0800
>> Changbin Du <changbin.du@...il.com> wrote:
>>
>> > It's safe to patch text segment in stop_machine. No race is possible here.
>> > Besides, there is a false positive for the lock assertion in
>> > patch_insn_write() since the lock is not held by cpu migration thread.
>> >
>> > So we actually don't need our ftrace_arch_code_modify_prepare/post(). And
>> > the lock assertion in patch_insn_write() should be removed to avoid
>> > producing lots of false positive warnings.
>> >
>> > Signed-off-by: Changbin Du <changbin.du@...il.com>
>>
>> Ideally, RISC-V should try to get off of the stop_machine approach, and
>> move to the breakpoint modification.
>>
> yes, that's a further step. I can feel a obvious stall to enable ftrace running
> in QEMU. (maybe qemu-riscv tcg is too slow...)

Looks like we've had this exact discussion before, even with exactly the 
same patch and people and everything.  I guess I dropped the ball here, 
I got so distracted trying to figure out those sequences to avoid 
stop_machine() that I forgot to clean up the patch to fix stop machine.  
I just sent out a v2 <20220322022331.32136-1-palmer@...osinc.com>.  That 
should at least fix the crash, we can deal with getting rid of 
stop_machine() later.