From: Boqun Feng <boqun.feng@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Andrea Parri <parri.andrea@gmail.com>
Subject: Re: [RFC tip/locking/lockdep v5 07/17] lockdep: Adjust check_redundant() for recursive read change
Date: Fri, 16 Mar 2018 16:20:09 +0800 [thread overview]
Message-ID: <20180316082009.r5rlfkedhnn3l7pa@tardis> (raw)
In-Reply-To: <20180222172906.GU25201@hirez.programming.kicks-ass.net>
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On Thu, Feb 22, 2018 at 06:29:06PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 22, 2018 at 03:08:54PM +0800, Boqun Feng wrote:
> > As we have four kinds of dependencies now, check_redundant() should only
> > report redundant if we have a dependency path which is equal or
> > _stronger_ than the current dependency. For example if in
> > check_prev_add() we have:
> >
> > prev->read == 2 && next->read != 2
> >
> > , we should only report redundant if we find a path like:
> >
> > prev--(RN)-->....--(*N)-->next
> >
> > and if we have:
> >
> > prev->read == 2 && next->read == 2
> >
> > , we could report redundant if we find a path like:
> >
> > prev--(RN)-->....--(*N)-->next
> >
> > or
> >
> > prev--(RN)-->....--(*R)-->next
> >
> > To do so, we need to pass the recursive-read status of @next into
> > check_redundant().
>
> Very hard to read that.
>
Right.. and I find a bug about this, let me explain below..
> > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> > ---
> > kernel/locking/lockdep.c | 13 ++++++++-----
> > 1 file changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > index e1be088a34c4..0b0ad3db78b4 100644
> > --- a/kernel/locking/lockdep.c
> > +++ b/kernel/locking/lockdep.c
> > @@ -1338,9 +1338,12 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth,
> > return 0;
> > }
> >
> > -static inline int class_equal(struct lock_list *entry, void *data)
> > +static inline int hlock_equal(struct lock_list *entry, void *data)
> > {
> > - return entry->class == data;
> > + struct held_lock *hlock = (struct held_lock *)data;
> > +
> > + return hlock_class(hlock) == entry->class &&
> > + (hlock->read == 2 || !entry->is_rr);
> > }
>
> So I guess @data = @next, and we're checking if @prev -> @next already
> exists.
>
> Since we only care about forward dependencies, @next->read==2 means *R
> and if so, any existing link is equal or stronger. If @next->read!=2, it
> means *N and we must regard *R as weaker and not match.
>
Yep, the idea if we find a path @prev -> .. -> @next is RN, and @prev ->
@next is RR, then we treat RR as weaker and redundant. Basically, if we
find a strong path that could replace the about-to-add dependency
(the path is stronger than or equal to the dependency), we report
redundant (a match).
But I miss something here, as you may see both the start and end of the
path are important, so say we find a strong path RN, but the
about-to-add dependency is NR, we can not report it as redundant,
because the path can not replace the dependency.
To make sure we find a path whose start point is stronger than @prev, we
need a trick, we should initialize the ->only_xr (or ->have_xr) of the
root (start point) of __bfs() to be @prev->read != 2, therefore if @prev
is N, __bfs() will pick N* for the first dependency, otherwise, __bfs()
can pick N* or R* for the first dependency.
I use a similar setup before check_noncircular(), which sets the initial
->only_xr to be @next->read == 2, because we need @prev -> @next ->
<path> to be strong. But I should use a opposite setup for
check_redundant() as I describe above.
Anyway, I will fix this and prove (hopefully) enough comments for those
tricks.
Regards,
Boqun
> OK, that seems to be fine, but again, that function _really_ could do
> with a comment.
>
>
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next prev parent reply other threads:[~2018-03-16 8:16 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-22 7:08 [RFC tip/locking/lockdep v5 00/17] lockdep: Support deadlock detection for recursive read locks Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 01/17] lockdep: Demagic the return value of BFS Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 02/17] lockdep: Make __bfs() visit every dependency until a match Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 03/17] lockdep: Redefine LOCK_*_STATE* bits Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 04/17] lockdep: Introduce lock_list::dep Boqun Feng
2018-02-23 11:55 ` Peter Zijlstra
2018-02-23 12:37 ` Boqun Feng
2018-02-24 5:32 ` Boqun Feng
2018-02-24 6:30 ` Boqun Feng
2018-02-24 8:38 ` Peter Zijlstra
2018-02-24 9:00 ` Boqun Feng
2018-02-24 9:26 ` Boqun Feng
2018-02-26 9:00 ` Peter Zijlstra
2018-02-26 10:15 ` Boqun Feng
2018-02-26 10:20 ` Boqun Feng
2018-02-24 7:31 ` Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 05/17] lockdep: Extend __bfs() to work with multiple kinds of dependencies Boqun Feng
2018-02-22 14:26 ` Peter Zijlstra
2018-02-22 15:12 ` Boqun Feng
2018-02-22 15:30 ` Peter Zijlstra
2018-02-22 15:51 ` Peter Zijlstra
2018-02-22 16:31 ` Boqun Feng
2018-02-23 5:02 ` Boqun Feng
2018-02-23 11:15 ` Peter Zijlstra
2018-02-22 16:08 ` Peter Zijlstra
2018-02-22 16:34 ` Boqun Feng
2018-02-22 16:32 ` Peter Zijlstra
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 06/17] lockdep: Support deadlock detection for recursive read in check_noncircular() Boqun Feng
2018-02-22 14:54 ` Peter Zijlstra
2018-02-22 15:16 ` Peter Zijlstra
2018-02-22 15:44 ` Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 07/17] lockdep: Adjust check_redundant() for recursive read change Boqun Feng
2018-02-22 17:29 ` Peter Zijlstra
2018-03-16 8:20 ` Boqun Feng [this message]
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 08/17] lockdep: Fix recursive read lock related safe->unsafe detection Boqun Feng
2018-02-22 17:41 ` Peter Zijlstra
2018-02-22 17:46 ` Peter Zijlstra
2018-02-23 8:21 ` Boqun Feng
2018-02-23 8:58 ` Boqun Feng
2018-02-23 11:36 ` Peter Zijlstra
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 09/17] lockdep: Add recursive read locks into dependency graph Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 10/17] lockdep/selftest: Add a R-L/L-W test case specific to chain cache behavior Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 11/17] lockdep: Take read/write status in consideration when generate chainkey Boqun Feng
2018-02-22 7:08 ` [RFC tip/locking/lockdep v5 12/17] lockdep/selftest: Unleash irq_read_recursion2 and add more Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 13/17] lockdep/selftest: Add more recursive read related test cases Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 14/17] Revert "locking/lockdep/selftests: Fix mixed read-write ABBA tests" Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 15/17] lockdep: Reduce the size of lock_list Boqun Feng
2018-02-23 11:38 ` Peter Zijlstra
2018-02-23 12:40 ` Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 16/17] lockdep: Documention for recursive read lock detection reasoning Boqun Feng
2018-02-24 22:53 ` Andrea Parri
2018-02-27 2:32 ` Boqun Feng
2018-02-22 7:09 ` [RFC tip/locking/lockdep v5 17/17] MAINTAINERS: Add myself as a LOCKING PRIMITIVES reviewer Boqun Feng
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