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 13:32:50 +0800 [thread overview]
Message-ID: <20180224053250.ketrlbq4gtx573qo@tardis> (raw)
In-Reply-To: <20180223123732.acxbavnf2ktd4lzl@tardis>
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On Fri, Feb 23, 2018 at 08:37:32PM +0800, Boqun Feng wrote:
> On Fri, Feb 23, 2018 at 12:55:20PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 03:08:51PM +0800, Boqun Feng wrote:
> > > @@ -1012,6 +1013,33 @@ static inline bool bfs_error(enum bfs_result res)
> > > return res < 0;
> > > }
> > >
> > > +#define DEP_NN_BIT 0
> > > +#define DEP_RN_BIT 1
> > > +#define DEP_NR_BIT 2
> > > +#define DEP_RR_BIT 3
> > > +
> > > +#define DEP_NN_MASK (1U << (DEP_NN_BIT))
> > > +#define DEP_RN_MASK (1U << (DEP_RN_BIT))
> > > +#define DEP_NR_MASK (1U << (DEP_NR_BIT))
> > > +#define DEP_RR_MASK (1U << (DEP_RR_BIT))
> > > +
> > > +static inline unsigned int __calc_dep_bit(int prev, int next)
> > > +{
> > > + if (prev == 2 && next != 2)
> > > + return DEP_RN_BIT;
> > > + if (prev != 2 && next == 2)
> > > + return DEP_NR_BIT;
> > > + if (prev == 2 && next == 2)
> > > + return DEP_RR_BIT;
> > > + else
> > > + return DEP_NN_BIT;
> > > +}
> > > +
> > > +static inline unsigned int calc_dep(int prev, int next)
> > > +{
> > > + return 1U << __calc_dep_bit(prev, next);
> > > +}
> > > +
> > > static enum bfs_result __bfs(struct lock_list *source_entry,
> > > void *data,
> > > int (*match)(struct lock_list *entry, void *data),
> > > @@ -1921,6 +1949,16 @@ check_prev_add(struct task_struct *curr, struct held_lock *prev,
> > > if (entry->class == hlock_class(next)) {
> > > if (distance == 1)
> > > entry->distance = 1;
> > > + entry->dep |= calc_dep(prev->read, next->read);
> > > + }
> > > + }
> > > +
> > > + /* Also, update the reverse dependency in @next's ->locks_before list */
> > > + list_for_each_entry(entry, &hlock_class(next)->locks_before, entry) {
> > > + if (entry->class == hlock_class(prev)) {
> > > + if (distance == 1)
> > > + entry->distance = 1;
> > > + entry->dep |= calc_dep(next->read, prev->read);
> > > return 1;
> > > }
> > > }
> >
> > I think it all becomes simpler if you use only 2 bits. Such that:
> >
> > bit0 is the prev R (0) or N (1) value,
> > bit1 is the next R (0) or N (1) value.
> >
> > I think this should work because we don't care about the empty set
> > (currently 0000) and all the complexity in patch 5 is because we can
> > have R bits set when there's also N bits. The concequence of that is
> > that we cannot replace ! with ~ (which is what I kept doing).
> >
> > But with only 2 bits, we only track the strongest relation in the set,
> > which is exactly what we appear to need.
> >
>
> But if we only have RN and NR, both bits will be set, we can not check
> whether we have NN or not. Consider we have:
>
> A -(RR)-> B
> B -(NR)-> C and B -(RN)-> C
> C -(RN)-> A
>
> this is not a deadlock case, but with "two bits" approach, we can not
> differ this with:
>
> A -(RR)-> B
> B -(NN)-> C
> C -(RN)-> A
>
> , which is a deadlock.
>
> But maybe "three bits" (NR, RN and NN bits) approach works, that is if
> ->dep is 0, we indicates this is only RR, and is_rx() becomes:
>
> static inline bool is_rx(u8 dep)
> {
> return !(dep & (NR_MASK | NN_MASK));
> }
>
> and is_xr() becomes:
>
> static inline bool is_xr(u8 dep)
> {
> return !(dep & (RN_MASK | NN_MASK));
> }
>
> , with this I think your simplification with have_xr works, thanks!
>
Ah! I see. Actually your very first approach works, except the
definitions of is_rx() and ir_xr() are wrong. In that approach, you
define
static inline bool is_rx(u8 dep)
{
return !!(dep & (DEP_RR_MASK | DEP_RN_MASK);
}
, which means "whether we have a R* dependency?". But in fact, what we
need to check is "whether we _only_ have R* dependencies?", if so and
have_xr is true, that means we could only have a -(*R)-> A -(R*)-> if we
pick the next dependency, and that means we should skip. So my new
definition above works, and I think we better name it as only_rx() to
avoid confusion? Ditto for is_xr().
I also reorder bit number for each kind of dependency, so that we have a
simple __calc_dep_bit(), see the following:
/*
* 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));
}
Note that we actually don't need DEP_RR_BIT, but I leave it there for
implementation simplicity. With this, your check and set below works.
Thoughts?
Regards,
Boqun
> >
> >
> > if (have_xr && is_rx(entry->dep))
> > continue;
> >
> > entry->have_xr = is_xr(entry->dep);
> >
> >
> > Or did I mess that up somewhere?
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next prev parent reply other threads:[~2018-02-24 5:29 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 [this message]
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
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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