linux-kernel - Re: [PATCH v6 2/3] x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation

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Message-ID: <alpine.DEB.2.21.1809122112480.1473@nanos.tec.linutronix.de>
Date:   Wed, 12 Sep 2018 21:14:48 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jiri Kosina <jikos@...nel.org>
cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v6 2/3] x86/speculation: Enable cross-hyperthread spectre
 v2 STIBP mitigation

On Wed, 12 Sep 2018, Jiri Kosina wrote:
>  	case X86_BUG_SPECTRE_V2:
> -		return sprintf(buf, "%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
> +		mutex_lock(&spec_ctrl_mutex);
> +		ret = sprintf(buf, "%s%s%s%s%s\n", spectre_v2_strings[spectre_v2_enabled],
>  			       boot_cpu_has(X86_FEATURE_USE_IBPB) ? ", IBPB" : "",
>  			       boot_cpu_has(X86_FEATURE_USE_IBRS_FW) ? ", IBRS_FW" : "",
> +			       (x86_spec_ctrl_base & SPEC_CTRL_STIBP) ? ", STIBP" : "",
>  			       spectre_v2_module_string());
> +		mutex_unlock(&spec_ctrl_mutex);

The mutex for this printing is overkill. It's a read after all and if there
is a concurrent SMT control fiddling going on then you have a chance of
getting the wrong information as well. I'll zap it.

Thanks,

	tglx