linux-kernel - Re: [PATCH 4.19 057/191] sparc64: remove mm_cpumask clearing to fix kthread_use

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Message-ID: <20201105114408.GA9009@duo.ucw.cz>
Date:   Thu, 5 Nov 2020 12:44:08 +0100
From:   Pavel Machek <pavel@....cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Nicholas Piggin <npiggin@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 4.19 057/191] sparc64: remove mm_cpumask clearing to fix
 kthread_use_mm race

Hi!

> From: Nicholas Piggin <npiggin@...il.com>
> 
> [ Upstream commit bafb056ce27940c9994ea905336aa8f27b4f7275 ]
...
> io_uring 2b188cc1bb857 ("Add io_uring IO interface") added code which
> does a kthread_use_mm() from a mmget_not_zero() refcount.
...
> The basic fix for sparc64 is to remove its mm_cpumask clearing code. The
> optimisation could be effectively restored by sending IPIs to mm_cpumask
> members and having them remove themselves from mm_cpumask. This is more
> tricky so I leave it as an exercise for someone with a sparc64 SMP.
> powerpc has a (currently similarly broken) example.

So this removes optimalization from Sparc, because it clashes with
2b188cc1bb857 ("Add io_uring IO interface"). But that commit is not
present in 4.19... so this probably is not good idea.

Best regards,
								Pavel
-- 
http://www.livejournal.com/~pavelmachek

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