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

Peter Zijlstra
2019-10-16T18:45:05Z

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?