linux-kernel - Re: [RFC 20/20] mm/rmap: avoid potential races

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Message-ID: <87zgt8y4aj.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Mon, 23 Aug 2021 16:05:24 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        x86@...nel.org
Subject: Re: [RFC 20/20] mm/rmap: avoid potential races

Hi, Nadav,

Nadav Amit <nadav.amit@...il.com> writes:

> From: Nadav Amit <namit@...are.com>
>
> flush_tlb_batched_pending() appears to have a theoretical race:
> tlb_flush_batched is being cleared after the TLB flush, and if in
> between another core calls set_tlb_ubc_flush_pending() and sets the
> pending TLB flush indication, this indication might be lost. Holding the
> page-table lock when SPLIT_LOCK is set cannot eliminate this race.

Recently, when I read the corresponding code, I find the exact same race
too.  Do you still think the race is possible at least in theory?  If
so, why hasn't your fix been merged?

> The current batched TLB invalidation scheme therefore does not seem
> viable or easily repairable.

I have some idea to fix this without too much code.  If necessary, I
will send it out.

Best Regards,
Huang, Ying