linux-kernel - Re: [PATCH 37/49] x86/alternatives: Move text_poke_array completion from smp_text_poke_batch_finish() and smp_text_poke_batch_flush() to smp_text_poke_batch

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Message-ID: <b0b87bd7-e104-4c9b-b9e2-0682dfef28e9@suse.com>
Date: Thu, 3 Apr 2025 17:39:18 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
 "H . Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Borislav Petkov <bp@...en8.de>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 37/49] x86/alternatives: Move text_poke_array completion
 from smp_text_poke_batch_finish() and smp_text_poke_batch_flush() to
 smp_text_poke_batch_process()



On 3.04.25 г. 17:13 ч., Ingo Molnar wrote:
> 
> * Nikolay Borisov <nik.borisov@...e.com> wrote:
> 
>>
>>
>> On 3.04.25 г. 16:38 ч., Ingo Molnar wrote:
>>>
>>> * Nikolay Borisov <nik.borisov@...e.com> wrote:
>>>
>>>>
>>>>
>>>> On 28.03.25 г. 15:26 ч., Ingo Molnar wrote:
>>>>> Simplifies the code and improves code generation a bit:
>>>>>
>>>>>       text	   data	    bss	    dec	    hex	filename
>>>>>      14769	   1017	   4112	  19898	   4dba	alternative.o.before
>>>>>      14742	   1017	   4112	  19871	   4d9f	alternative.o.after
>>>>>
>>>>> Signed-off-by: Ingo Molnar <mingo@...nel.org>
>>>>> ---
>>>>>     arch/x86/kernel/alternative.c | 11 +++++------
>>>>>     1 file changed, 5 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>>>>> index 1df8fac6740d..5293929488f0 100644
>>>>> --- a/arch/x86/kernel/alternative.c
>>>>> +++ b/arch/x86/kernel/alternative.c
>>>>> @@ -2750,6 +2750,9 @@ static void smp_text_poke_batch_process(void)
>>>>>     		if (unlikely(!atomic_dec_and_test(refs)))
>>>>>     			atomic_cond_read_acquire(refs, !VAL);
>>>>>     	}
>>>>> +
>>>>> +	/* They are all completed: */
>>>>> +	text_poke_array.nr_entries = 0;
>>>>>     }
>>>>>     static void __smp_text_poke_batch_add(void *addr, const void *opcode, size_t len, const void *emulate)
>>>>> @@ -2857,20 +2860,16 @@ static bool text_poke_addr_ordered(void *addr)
>>>>>     void smp_text_poke_batch_finish(void)
>>>>>     {
>>>>> -	if (text_poke_array.nr_entries) {
>>>>> +	if (text_poke_array.nr_entries)
>>>>>     		smp_text_poke_batch_process();
>>>>> -		text_poke_array.nr_entries = 0;
>>>>> -	}
>>>>>     }
>>>>
>>>> This function becomes trivial, why not simply move the check into
>>>> smp_text_poke_batch_process and rename it to smp_text_poke_batch_finish ?
>>>
>>> Yeah, that's pretty much what happens in patch #47:
>>>
>>>     x86/alternatives: Remove 'smp_text_poke_batch_flush()'
>>
>> Well, that patch removes poke_batch_flush but still retains
>> poke_batch_finish + poke_batch_process. My suggestion is to eliminate
>> poke_batch_process name and turn it into poke_batch_finish by moving the
>> check from poke_batch_finish into poke_batch_process.
> 
> So, in the context of the full tree at:
> 
>    git://git.kernel.org/pub/scm/linux/kernel/git/mingo/tip.git WIP.x86/alternatives
> 
> Standalone smp_text_poke_batch_process() is still needed, because
> smp_text_poke_batch_add() uses it to drive the whole 'batching'
> machinery.
> 
> If smp_text_poke_batch_process() finishes for each call, if I
> understand your suggestion correctly, that reduces the amount of
> batching done, which is a disadvantage.
> 
> Note how right now it's possible to do something like this:
> 
> 	smp_text_poke_batch_add(1);
> 	smp_text_poke_batch_add(1);
> 	smp_text_poke_batch_add(1);
> 	smp_text_poke_batch_add(1);
> 	smp_text_poke_batch_finish();
> 
> This results in a single call to smp_text_poke_batch_process(), with 4
> entries, a single 4-entry flush in essence.

I meant doing this:

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 5b1a6252a4b9..b6a781b9de26 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -2587,12 +2587,16 @@ noinstr int 
smp_text_poke_int3_trap_handler(struct pt_regs *regs)
   *               replacing opcode
   *     - SMP sync all CPUs
   */
-static void smp_text_poke_batch_process(void)
+void smp_text_poke_batch_finish(void)
  {
         unsigned char int3 = INT3_INSN_OPCODE;
         unsigned int i;
         int do_sync;

+
+       if (!text_poke_array.nr_entries)
+               return;
+
         lockdep_assert_held(&text_mutex);

         /*
@@ -2832,12 +2836,6 @@ static bool text_poke_addr_ordered(void *addr)
         return true;
  }

-void smp_text_poke_batch_finish(void)
-{
-       if (text_poke_array.nr_entries)
-               smp_text_poke_batch_process();
-}
-
  /**
   * smp_text_poke_batch_add() -- update instruction on live kernel on 
SMP, batched
   * @addr:      address to patch
@@ -2854,7 +2852,7 @@ void smp_text_poke_batch_finish(void)
  void __ref smp_text_poke_batch_add(void *addr, const void *opcode, 
size_t len, const void *emulate)
  {
         if (text_poke_array.nr_entries == TEXT_POKE_ARRAY_MAX || 
!text_poke_addr_ordered(addr))
-               smp_text_poke_batch_process();
+               smp_text_poke_batch_finish();
         __smp_text_poke_batch_add(addr, opcode, len, emulate);
  }


AFAICS this doesn't change the semantics. I.e smp_text_poke_batch_add 
will call poke_batch_finish iff the address to be added violates the 
sorted order of text_poke_array. The net effect is we have 1 less 
function name to care about.

> 
> Thanks,
> 
> 	Ingo