linux-kernel - Re: x86/mce: suspicious RCU usage in 4.13.4

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Date:   Tue, 10 Oct 2017 13:13:58 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Luck\, Tony" <tony.luck@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, Jeremy Cline <jcline@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-edac@...r.kernel.org, linux-kernel@...r.kernel.org,
        Laura Abbott <labbott@...hat.com>
Subject: Re: x86/mce: suspicious RCU usage in 4.13.4

"Luck, Tony" <tony.luck@...el.com> writes:

>>  	for (;;) {
>>  		entry = mce_log_get_idx_check(mcelog.next);
>
> Can't this get even simpler? Do we need the loop? The mutex
> will now protect us while we check to see if there is a slot
> to stash this new entry. Also just say:

IMHO the warning is just bogus. There's nothing here that actually
uses RCU. I would just remove it.

>>  			if (entry >= MCE_LOG_LEN) {
>> -				set_bit(MCE_OVERFLOW,
>> -					(unsigned long *)&mcelog.flags);
>> +				set_bit(MCE_OVERFLOW, (unsigned long *)&mcelog.flags);
>
> Need to mutex_unlock(&mce_chrdev_read_mutex); here.

And yes that too.

-Andi