From: Pedro Alves <palves@redhat.com>
To: unlisted-recipients:; (no To-header on input)
Cc: Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Sasha Levin <levinsasha928@gmail.com>, Tejun Heo <tj@kernel.org>,
torvalds@linux-foundation.org, akpm@linux-foundation.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
paul.gortmaker@windriver.com, davem@davemloft.net, mingo@elte.hu,
ebiederm@xmission.com, aarcange@redhat.com, ericvh@gmail.com,
netdev@vger.kernel.org, josh@joshtriplett.org,
eric.dumazet@gmail.com, axboe@kernel.dk, agk@redhat.com,
dm-devel@redhat.com, neilb@suse.de, ccaulfie@redhat.com,
teigland@redhat.com, Trond.Myklebust@netapp.com,
bfields@fieldses.org, fweisbec@gmail.com, jesse@nicira.com,
venkat.x.venkatsubra@oracle.com, ejt@redhat.com,
snitzer@redhat.com, edumazet@google.com,
linux-nfs@vger.kernel.org, dev@openvswitch.org,
rds-devel@oss.oracle.com, lw@cn.fujitsu.com
Subject: Re: [PATCH v3 01/17] hashtable: introduce a small and naive hashtable
Date: Tue, 04 Sep 2012 17:40:08 +0100 [thread overview]
Message-ID: <50462EE8.1090903@redhat.com> (raw)
In-Reply-To: <50462C99.5000007@redhat.com>
On 09/04/2012 05:30 PM, Pedro Alves wrote:
> On 09/04/2012 04:35 PM, Steven Rostedt wrote:
>> On Tue, 2012-08-28 at 19:00 -0400, Mathieu Desnoyers wrote:
>>
>>> Looking again at:
>>>
>>> +#define hash_for_each_size(name, bits, bkt, node, obj, member) \
>>> + for (bkt = 0; bkt < HASH_SIZE(bits); bkt++) \
>>> + hlist_for_each_entry(obj, node, &name[bkt], member)
>>>
>>> you will notice that a "break" or "continue" in the inner loop will not
>>> affect the outer loop, which is certainly not what the programmer would
>>> expect!
>>>
>>> I advise strongly against creating such error-prone construct.
>>>
>>
>> A few existing loop macros do this. But they require a do { } while ()
>> approach, and all have a comment.
>>
>> It's used by do_each_thread() in sched.h and ftrace does this as well.
>> Look at kernel/trace/ftrace.c at do_for_each_ftrace_rec().
>>
>> Yes it breaks 'break' but it does not break 'continue' as it would just
>> go to the next item that would have been found (like a normal for
>> would).
>
> /*
> * This is a double for. Do not use 'break' to break out of the loop,
> * you must use a goto.
> */
> #define do_for_each_ftrace_rec(pg, rec) \
> for (pg = ftrace_pages_start; pg; pg = pg->next) { \
> int _____i; \
> for (_____i = 0; _____i < pg->index; _____i++) { \
> rec = &pg->records[_____i];
>
>
>
> You can make 'break' also work as expected if you can embed a little knowledge
> of the inner loop's condition in the outer loop's condition. Sometimes it's
> trivial, most often when the inner loop's iterator is a pointer that goes
> NULL at the end, but other times not so much. Something like (completely untested):
>
> #define do_for_each_ftrace_rec(pg, rec) \
> for (pg = ftrace_pages_start, rec = &pg->records[pg->index]; \
> pg && rec == &pg->records[pg->index]; \
> pg = pg->next) { \
> int _____i; \
> for (_____i = 0; _____i < pg->index; _____i++) { \
> rec = &pg->records[_____i];
>
>
> (other variants possible)
>
> IOW, the outer loop only iterates if the inner loop completes. If there's
> a break in the inner loop, then the outer loop breaks too. Of course, it
> all depends on whether the generated code looks sane or hideous, if
> the uses of the macro care for it over bug avoidance.
>
BTW, you can also go a step further and remove the need to close with double }},
with something like:
#define do_for_each_ftrace_rec(pg, rec) \
for (pg = ftrace_pages_start, rec = &pg->records[pg->index]; \
pg && rec == &pg->records[pg->index]; \
pg = pg->next) \
for (rec = pg->records; rec < &pg->records[pg->index]; rec++)
--
Pedro Alves
next prev parent reply other threads:[~2012-09-04 16:40 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-22 2:26 [PATCH v3 00/17] generic hashtable implementation Sasha Levin
2012-08-22 2:26 ` [PATCH v3 01/17] hashtable: introduce a small and naive hashtable Sasha Levin
2012-08-22 18:01 ` Tejun Heo
2012-08-22 23:54 ` Ryan Mallon
2012-08-23 0:24 ` Sasha Levin
2012-08-23 20:04 ` Tejun Heo
2012-08-24 19:47 ` Sasha Levin
2012-08-24 19:59 ` Tejun Heo
2012-08-24 20:11 ` Sasha Levin
2012-08-24 20:33 ` Tejun Heo
2012-08-24 20:53 ` Sasha Levin
2012-08-24 21:23 ` Tejun Heo
2012-08-24 22:59 ` Sasha Levin
2012-08-24 23:07 ` Tejun Heo
2012-08-25 4:24 ` Mathieu Desnoyers
2012-08-28 9:56 ` Sasha Levin
2012-08-28 10:11 ` Mathieu Desnoyers
2012-08-28 11:27 ` Sasha Levin
2012-08-28 11:56 ` Mathieu Desnoyers
2012-08-28 23:00 ` Mathieu Desnoyers
2012-09-04 15:35 ` Steven Rostedt
2012-09-04 16:30 ` Pedro Alves
2012-09-04 16:40 ` Pedro Alves [this message]
2012-09-04 17:01 ` Mathieu Desnoyers
2012-09-06 13:53 ` Sasha Levin
2012-09-06 14:19 ` Pedro Alves
2012-09-06 14:33 ` Mathieu Desnoyers
2012-09-06 14:36 ` David Laight
2012-09-06 14:55 ` Josh Triplett
2012-09-06 15:11 ` Steven Rostedt
2012-09-06 15:49 ` Sasha Levin
2012-09-06 16:00 ` Steven Rostedt
2012-09-06 16:21 ` Sasha Levin
2012-09-06 16:50 ` Mathieu Desnoyers
2012-09-06 17:01 ` Sasha Levin
2012-09-06 17:02 ` Mathieu Desnoyers
2012-09-06 17:15 ` Steven Rostedt
2012-09-04 17:17 ` Steven Rostedt
2012-09-04 17:21 ` Pedro Alves
2012-09-04 20:59 ` Steven Rostedt
2012-09-04 21:51 ` Pedro Alves
2012-09-04 22:41 ` Steven Rostedt
2012-09-04 22:58 ` Pedro Alves
2012-09-04 23:27 ` Steven Rostedt
2012-09-04 16:32 ` Mathieu Desnoyers
2012-08-22 2:26 ` [PATCH v3 02/17] userns: use new hashtable implementation Sasha Levin
2012-08-22 2:26 ` [PATCH v3 03/17] mm,ksm: " Sasha Levin
2012-08-22 2:26 ` [PATCH v3 04/17] workqueue: " Sasha Levin
2012-08-22 18:05 ` Tejun Heo
2012-08-22 2:27 ` [PATCH v3 05/17] mm/huge_memory: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 06/17] tracepoint: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 07/17] net,9p: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 08/17] block,elevator: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 09/17] SUNRPC/cache: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 10/17] dlm: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 11/17] net,l2tp: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 12/17] dm: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 13/17] lockd: " Sasha Levin
2012-08-22 11:47 ` J. Bruce Fields
2012-08-22 12:13 ` Sasha Levin
2012-08-22 13:12 ` J. Bruce Fields
2012-08-22 13:22 ` Mathieu Desnoyers
2012-08-22 17:32 ` Sasha Levin
2012-08-22 2:27 ` [PATCH v3 14/17] net,rds: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 15/17] openvswitch: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 16/17] tracing output: " Sasha Levin
2012-08-22 2:27 ` [PATCH v3 17/17] SUNRPC: use new hashtable implementation in auth Sasha Levin
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