linux-kernel - Re: [PATCH v8 06/14] lockdep: Detect and handle hist

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Message-ID: <20170811085201.GI20323@X58A-UD3R>
Date:   Fri, 11 Aug 2017 17:52:02 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     peterz@...radead.org, mingo@...nel.org, tglx@...utronix.de,
        walken@...gle.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, willy@...radead.org, npiggin@...il.com,
        kernel-team@....com
Subject: Re: [PATCH v8 06/14] lockdep: Detect and handle hist_lock ring
 buffer overwrite

On Fri, Aug 11, 2017 at 04:03:29PM +0800, Boqun Feng wrote:
> Thanks for taking a look at it ;-)

I rather appriciate it.

> > > @@ -5005,7 +5003,7 @@ static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock)
> > >  static void commit_xhlocks(struct cross_lock *xlock)
> > >  {
> > >  	unsigned int cur = current->xhlock_idx;
> > > -	unsigned int prev_hist_id = xhlock(cur).hist_id;
> > > +	unsigned int prev_hist_id = cur + 1;
> > 
> > I should have named it another. Could you suggest a better one?
> > 
> 
> I think "prev" is fine, because I thought the "previous" means the
> xhlock item we visit _previously_.
> 
> > >  	unsigned int i;
> > >  
> > >  	if (!graph_lock())
> > > @@ -5030,7 +5028,7 @@ static void commit_xhlocks(struct cross_lock *xlock)
> > >  			 * hist_id than the following one, which is impossible
> > >  			 * otherwise.
> > 
> > Or we need to modify the comment so that the word 'prev' does not make
> > readers confused. It was my mistake.
> > 
> 
> I think the comment needs some help, but before you do it, could you
> have another look at what Peter proposed previously? Note you have a
> same_context_xhlock() check in the commit_xhlocks(), so the your
> previous overwrite case actually could be detected, I think.

What is the previous overwrite case?

ppppppppppwwwwwwwwwwwwiiiiiiiii
iiiiiiiiiiiiiii................

Do you mean this one? I missed the check of same_context_xhlock(). Yes,
peterz's suggestion also seems to work.

> However, one thing may not be detected is this case:
> 
> 		ppppppppppppppppppppppppppppppppppwwwwwwww
> wrapped >	wwwwwww

To be honest, I think your suggestion is more natual, with which this
case would be also covered.

> 
> 	where p: process and w: worker.
> 
> , because p and w are in the same task_irq_context(). I discussed this
> with Peter yesterday, and he has a good idea: unconditionally do a reset
> on the ring buffer whenever we do a crossrelease_hist_end(XHLOCK_PROC).
> Basically it means we empty the lock history whenever we finished a
> worker function in a worker thread or we are about to return to
> userspace after we finish the syscall. This could further save some
> memory and so I think this may be better than my approach.

Do you mean reset _whenever_ hard irq exit, soft irq exit or work exit?
Why should we give up chances to check dependencies of remaining xhlocks
whenever each exit? Am I understanding correctly?

I am just curious. Does your approach have some problems?

Thanks,
Byungchul