linux-kernel - Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state

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Message-ID: <20190405143942.xtpq5443qbc4gziy@linutronix.de>
Date:   Fri, 5 Apr 2019 16:39:43 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Julien Grall <julien.grall@....com>
Cc:     Dave Martin <Dave.Martin@....com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-rt-users@...r.kernel.org, catalin.marinas@....com,
        will.deacon@....com, ard.biesheuvel@...aro.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching
 FPSIMD/SVE state

On 2019-04-05 10:02:45 [+0100], Julien Grall wrote:
> RT folks already saw this corruption because local_bh_disable() does not
> preempt on RT. They are carrying a patch (see "arm64: fpsimd: use
> preemp_disable in addition to local_bh_disable()") to disable preemption
> along with local_bh_disable().
> 
> Alternatively, Julia suggested to introduce a per-cpu lock to protect the
> state. I am thinking to defer this for a follow-up patch. The changes in
> this patch should make it easier because we now have helper to mark the
> critical section.

A per-CPU lock? It has to be a raw_spinlock_t because a normal
spin_lock() / local_lock() would allow scheduling and might be taken as
part of the context switch or soon after.

Sebastian