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 04/17] lockdep: Introduce lock_list::dep
Date: Sat, 24 Feb 2018 17:00:19 +0800 [thread overview]
Message-ID: <20180224090019.3smjampkk4zoacb3@tardis> (raw)
In-Reply-To: <20180224083807.GB25201@hirez.programming.kicks-ass.net>
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On Sat, Feb 24, 2018 at 09:38:07AM +0100, Peter Zijlstra wrote:
> On Sat, Feb 24, 2018 at 02:30:05PM +0800, Boqun Feng wrote:
> > On Sat, Feb 24, 2018 at 01:32:50PM +0800, Boqun Feng wrote:
>
> > > /*
> > > * DEP_*_BIT in lock_list::dep
> > > *
> > > * For dependency @prev -> @next:
> > > *
> > > * RR: both @prev and @next are recursive read locks, i.e. ->read == 2.
> > > * RN: @prev is recursive and @next is non-recursive.
> > > * NR: @prev is a not recursive and @next is recursive.
> > > * NN: both @prev and @next are non-recursive.
> > > *
> > > * Note that we define the value of DEP_*_BITs so that:
> > > * bit0 is prev->read != 2
> > > * bit1 is next->read != 2
> > > */
> > > #define DEP_RR_BIT 0
> > > #define DEP_RN_BIT 1
> > > #define DEP_NR_BIT 2
> > > #define DEP_NN_BIT 3
> > >
> > > #define DEP_RR_MASK (1U << (DEP_RR_BIT))
> > > #define DEP_RN_MASK (1U << (DEP_RN_BIT))
> > > #define DEP_NR_MASK (1U << (DEP_NR_BIT))
> > > #define DEP_NN_MASK (1U << (DEP_NN_BIT))
> > >
> > > static inline unsigned int
> > > __calc_dep_bit(struct held_lock *prev, struct held_lock *next)
> > > {
> > > return (prev->read != 2) + ((next->read != 2) << 1)
> > > }
> > >
> > > static inline u8 calc_dep(struct held_lock *prev, struct held_lock *next)
> > > {
> > > return 1U << __calc_dep_bit(prev, next);
> > > }
> > >
> > > static inline bool only_rx(u8 dep)
> > > {
> > > return !(dep & (DEP_NR_MASK | DEP_NN_MASK));
> > > }
> > >
> > > static inline bool only_xr(u8 dep)
> > > {
> > > return !(dep & (DEP_NR_MASK | DEP_NN_MASK));
> > > }
> > >
>
> > > > > if (have_xr && is_rx(entry->dep))
> > > > > continue;
> > > > >
> > > > > entry->have_xr = is_xr(entry->dep);
> > > > >
> >
> > Hmm.. I think this part also needs some tweak:
> >
> > /* if -> prev is *R, and we only have R* for prev -> this, * skip*/
> > if (have_xr && only_rx(entry->dep))
> > continue;
> >
> > /*
> > * we pick a *R for prev -> this only if:
> > * prev -> this dependencies are all *R
> > * or
> > * -> prev is *R, and we don't have NN for prev -> this
> > */
> > entry->have_xr = only_xr(entry->dep) || (have_xr && !is_nn(entry->dep));
> >
> > otherwise, we will wrongly set entry->have_xr to false if have_xr is
> > true and we have RN for prev -> this.
>
> OK, so its saturday morning and such, but what? Why should we set
> have_xr true when we have RN? Note that if we only had RN we'd already
> have bailed on the continue due to only_rx().
>
But what if we have RN and NR? only_rx() will return false, but due to
have_xr is true, we can not pick RN, so entry->have_xr should be set to
true (due to we have to pick NR), however only_xr() is false becuase we
have RN, so if we set entry->have_xr to only_xr(), we set it as false.
This is for case like:
TASK1:
read_lock(A);
read_lock(B);
TASK2:
write_lock(B);
read_lock(C);
TASK3:
read_lock(B);
write_lock(C);
TASK4:
read_lock(C);
write_lock(A);
, which is not a deadlock.
Am I missing something sublte?
Regards,
Boqun
> So can you elaborate a bit?
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next prev parent reply other threads:[~2018-02-24 8:56 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 [this message]
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
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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