linux-kernel - Re: [PATCH v4 02/10] objtool: Handle special cases of dead end insn

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Message-ID: <20241126064513.bf6yq56eklyo4xvt@jpoimboe>
Date: Mon, 25 Nov 2024 22:45:13 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Huacai Chen <chenhuacai@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 02/10] objtool: Handle special cases of dead end insn

On Fri, Nov 22, 2024 at 12:49:57PM +0800, Tiezhu Yang wrote:
> There are some "unreachable instruction" objtool warnings when compling
> with Clang on LoongArch, this is because the "break" instruction is set
> as dead end due to its type is INSN_BUG in decode_instructions() at the
> beginning, and it does not set insn->dead_end of the "break" instruction
> as false after checking ".rela.discard.reachable" in add_dead_ends(), so
> the next instruction of "break" is marked as unreachable.
> 
> Actually, it can find the reachable instruction after parsing the section
> ".rela.discard.reachable", in some cases, the "break" instruction may not
> be the first previous instruction with scheduling by Machine Instruction
> Scheduler of LLVM, it should find more times and then set insn->dead_end
> of the "break" instruction as false.
> 
> This is preparation for later patch on LoongArch, there is no effect for
> the other archs with this patch.
> 
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>

I'm having trouble understanding this commit log, is the problem that
the compiler is sometimes inserting code between 'break' and the
unreachable() inline asm?

If so, this sounds like a problem that was already solved for x86 with:

  bfb1a7c91fb7 ("x86/bug: Merge annotate_reachable() into _BUG_FLAGS() asm")

Can you check if that fixes it?

-- 
Josh