linux-kernel - Re: [PATCH 1/8] kcsan: Add Kernel Concurrency Sanitizer infrastructure

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Message-ID: <20191016184346.GT2328@hirez.programming.kicks-ass.net>
Date:   Wed, 16 Oct 2019 20:43:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     akiyks@...il.com, stern@...land.harvard.edu, glider@...gle.com,
        parri.andrea@...il.com, andreyknvl@...gle.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, arnd@...db.de, boqun.feng@...il.com,
        bp@...en8.de, dja@...ens.net, dlustig@...dia.com,
        dave.hansen@...ux.intel.com, dhowells@...hat.com,
        dvyukov@...gle.com, hpa@...or.com, mingo@...hat.com,
        j.alglave@....ac.uk, joel@...lfernandes.org, corbet@....net,
        jpoimboe@...hat.com, luc.maranget@...ia.fr, mark.rutland@....com,
        npiggin@...il.com, paulmck@...ux.ibm.com, tglx@...utronix.de,
        will@...nel.org, kasan-dev@...glegroups.com,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH 1/8] kcsan: Add Kernel Concurrency Sanitizer
 infrastructure

On Wed, Oct 16, 2019 at 10:39:52AM +0200, Marco Elver wrote:

> +bool __kcsan_check_watchpoint(const volatile void *ptr, size_t size,
> +			      bool is_write)
> +{
> +	atomic_long_t *watchpoint;
> +	long encoded_watchpoint;
> +	unsigned long flags;
> +	enum kcsan_report_type report_type;
> +
> +	if (unlikely(!is_enabled()))
> +		return false;
> +
> +	watchpoint = find_watchpoint((unsigned long)ptr, size, !is_write,
> +				     &encoded_watchpoint);
> +	if (watchpoint == NULL)
> +		return true;
> +
> +	flags = user_access_save();

Could use a comment on why find_watchpoint() is save to call without
user_access_save() on.

> +	if (!try_consume_watchpoint(watchpoint, encoded_watchpoint)) {
> +		/*
> +		 * The other thread may not print any diagnostics, as it has
> +		 * already removed the watchpoint, or another thread consumed
> +		 * the watchpoint before this thread.
> +		 */
> +		kcsan_counter_inc(kcsan_counter_report_races);
> +		report_type = kcsan_report_race_check_race;
> +	} else {
> +		report_type = kcsan_report_race_check;
> +	}
> +
> +	/* Encountered a data-race. */
> +	kcsan_counter_inc(kcsan_counter_data_races);
> +	kcsan_report(ptr, size, is_write, raw_smp_processor_id(), report_type);
> +
> +	user_access_restore(flags);
> +	return false;
> +}
> +EXPORT_SYMBOL(__kcsan_check_watchpoint);
> +
> +void __kcsan_setup_watchpoint(const volatile void *ptr, size_t size,
> +			      bool is_write)
> +{
> +	atomic_long_t *watchpoint;
> +	union {
> +		u8 _1;
> +		u16 _2;
> +		u32 _4;
> +		u64 _8;
> +	} expect_value;
> +	bool is_expected = true;
> +	unsigned long ua_flags = user_access_save();
> +	unsigned long irq_flags;
> +
> +	if (!should_watch(ptr))
> +		goto out;
> +
> +	if (!check_encodable((unsigned long)ptr, size)) {
> +		kcsan_counter_inc(kcsan_counter_unencodable_accesses);
> +		goto out;
> +	}
> +
> +	/*
> +	 * Disable interrupts & preemptions, to ignore races due to accesses in
> +	 * threads running on the same CPU.
> +	 */
> +	local_irq_save(irq_flags);
> +	preempt_disable();

Is there a point to that preempt_disable() here?