linux-kernel - Re: [RFC][PATCH 5/8] x86/mm: fix exception table comments

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Date:   Mon, 10 Sep 2018 13:43:17 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, tglx@...utronix.de, x86@...nel.org,
        luto@...nel.org
Subject: Re: [RFC][PATCH 5/8] x86/mm: fix exception table comments

On Fri, 2018-09-07 at 14:51 -0700, Dave Hansen wrote:
> > 
> > > 
> > > +	 * Only do the expensive exception table search when we might be at
> > > +	 * risk of a deadlock:
> > > +	 * 1. We failed to acquire mmap_sem, and
> > > +	 * 2. The access was an explicit kernel-mode access
> > > +	 *    (X86_PF_USER=0).
> > Might be worth reminding the reader that X86_PF_USER will be set in
> > sw_error_code for implicit accesses.  I saw "explicit" and my mind
> > immediately jumped to hw_error_code for whatever reason.  E.g.:
> > 
> > 	* 2. The access was an explicit kernel-mode access (we set X86_PF_USER
> > 	*    in sw_error_code for implicit kernel-mode accesses).
> Yeah, that was not worded well.  Is this better?
> 
> > 
> >          * Only do the expensive exception table search when we might be at
> >          * risk of a deadlock:
> >          * 1. We failed to acquire mmap_sem, and
> >          * 2. The access was an explicit kernel-mode access.  An access
> >          *    from user-mode will X86_PF_USER=1 set via hw_error_code or
> >          *    set in sw_error_code if it were an implicit kernel-mode
> >          *    access that originated in user mode.

For me, mentioning hw_error_code just muddies the waters, e.g. why is
hw_error_code mentioned when it's not checked in the code?  Comments
alone won't help someone that's reading this code and doesn't understand
that hardware sets X86_PF_USER for user-mode accesses.  Maybe this?

	 * 2. The access was an explicit kernel-mode access.  X86_PF_USER
	 *    is set in sw_error_code for both user-mode accesses and
	 *    implicit kernel-mode accesses that originated in user mode.