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Date: Wed, 30 Mar 2011 13:25:30 +0800 From: Lai Jiangshan <laijs@...fujitsu.com> To: "H. Peter Anvin" <hpa@...ux.intel.com> CC: paulmck@...ux.vnet.ibm.com, Jan Beulich <JBeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>, Alexander van Heukelum <heukelum@...tmail.fm>, Dipankar Sarma <dipankar@...ibm.com>, Peter Zijlstra <peterz@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, Sam Ravnborg <sam@...nborg.org>, David Howells <dhowells@...hat.com>, Oleg Nesterov <oleg@...hat.com>, Roland McGrath <roland@...hat.com>, Serge Hallyn <serue@...ibm.com>, linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, Steven Rostedt <rostedt@...dmis.org> Subject: Re: [RFC PATCH 4/5] RCU: Add TASK_RCU_OFFSET On 03/30/2011 08:47 AM, Paul E. McKenney wrote: > On Tue, Mar 29, 2011 at 03:01:19PM -0700, H. Peter Anvin wrote: >> On 03/29/2011 02:47 PM, Paul E. McKenney wrote: >>> On Tue, Mar 29, 2011 at 02:32:30PM -0700, H. Peter Anvin wrote: >>>> On 03/29/2011 02:31 PM, Paul E. McKenney wrote: >>>>>> >>>>>> I have to say that if we have to use hardcoded offsets in C then we have >>>>>> bigger problems. >>>>> >>>>> In this case, the offsets are mechanically generated from the structure >>>>> definitions. >>>>> >>>>> Or am I missing your point? >>>> >>>> Yes. The point is if we have to pull out these kinds of hacks in *C* >>>> code, we are doing it wrong. Not just a little wrong, but completely >>>> and totally bonkers wrong. >>> >>> OK, maybe we are doing it wrong. >>> >>> But in that case, how do you suggest restructuring include/linux/sched.h >>> so that struct task_struct can be safely included everywhere >>> rcu_read_lock() and friends are invoked? Or, on the other hand, >>> what should we be doing so that we don't need to include task_struct >>> everywhere? >> >> Lai's text doesn't give any hint as to the specific nature of the >> conflict, which makes it hard to come up with a better alternative >> without having to rediscover the problem from first principles. >> However, a somewhat logical assumption is that the problem is that >> struct task_struct includes struct rcu_head, in which case the easiest >> thing to do is almost certainly to move the definition of struct >> rcu_head to its own header file, <linux/rcuhead.h>, and include that in >> <linux/sched.h>, which should make it possible to include >> <linux/sched.h> in <linux/rcupdate.h>. > > I believe that there are other circular dependencies -- there certainly > were a few years back -- but I will defer to Lai. > > Thanx, Paul > Yes, there are other circular dependencies, <linux/sched.h> includes many files which include or indirectly include <linux/rcupdate.h> for struct rcu_head or RCU apis. There are too many to be split all. Even we just create a <linux/task_struct_def.h>, it also needs to include many files which have included or indirectly included <linux/rcupdate.h> for struct rcu_head already. It is still not a easy work to split them, it still requires many subsystem maintainers to participate in. RCU is one of the most import subsystem, I think it is worth to do such "wrong" way. Thanks, Lai P.S. Circular dependencies is one of the original sins of C language. If it is a good change for lots of linux hackers participating in to split all un-split kernel headers, I'd like to do so too. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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