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

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Message-ID: <20170811094448.GJ20323@X58A-UD3R>
Date:   Fri, 11 Aug 2017 18:44:48 +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 05:52:02PM +0900, Byungchul Park wrote:
> 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).

Ah, ok. You meant 'whenever _process_ context exit'.

I need more time to be sure, but anyway for now it seems to work with
giving up some chances for remaining xhlocks.

But, I am not sure if it's still true even in future and the code can be
maintained easily. I think your approach is natural and neat enough for
that purpose. What problem exists with yours?

> > 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