linux-kernel - Re: [PATCH 2/2] x86/MCE: Add command line option to extend MCE Records pool

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Date: Mon, 12 Feb 2024 21:10:38 +0100
From: Borislav Petkov <bp@...en8.de>
To: "Luck, Tony" <tony.luck@...el.com>
Cc: Yazen Ghannam <yazen.ghannam@....com>,
	"Naik, Avadhut" <avadnaik@....com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	"Mehta, Sohil" <sohil.mehta@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Avadhut Naik <avadhut.naik@....com>
Subject: Re: [PATCH 2/2] x86/MCE: Add command line option to extend MCE
 Records pool

On Mon, Feb 12, 2024 at 07:49:43PM +0000, Luck, Tony wrote:
> Yes. The question is whether a #MC that come in the middle of list_rcu_add()
> can safely do list_for_each_entry_rcu() on the same list.
> 
> RCU is black magic ... maybe it can do this? Adding Paul.

Yeah, the list traversal might be ok as this is what that list_add does
- you can't encounter an inconsistent list - but we still take
 a spinlock on addition and the commit which added it:

7f184275aa30 ("lib, Make gen_pool memory allocator lockless")

says

    The lockless operation only works if there is enough memory available.
    If new memory is added to the pool a lock has to be still taken.  So
    any user relying on locklessness has to ensure that sufficient memory
    is preallocated.

and this is exactly what we're doing - adding new memory.

So, until we're absolutely sure that it is ok to interrupt a context
holding a spinlock with a #MC which is non-maskable, I don't think we
wanna do this.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette