linux-kernel - Re: [PATCH 05/12] update FTRACE_SYSCALL

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Message-ID: <20090824145011.GA20517@linux-sh.org>
Date:	Mon, 24 Aug 2009 23:50:11 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	rostedt@...dmis.org, peterz@...radead.org,
	mathieu.desnoyers@...ymtl.ca, jiayingz@...gle.com,
	mbligh@...gle.com, lizf@...fujitsu.com
Subject: Re: [PATCH 05/12] update FTRACE_SYSCALL_MAX

On Mon, Aug 24, 2009 at 10:42:28AM -0400, Jason Baron wrote:
> On Mon, Aug 24, 2009 at 11:15:39PM +0900, Paul Mundt wrote:
> > On Mon, Aug 24, 2009 at 10:06:29AM -0400, Jason Baron wrote:
> > > On Mon, Aug 24, 2009 at 10:41:52PM +0900, Paul Mundt wrote:
> > > > I hope you can clarify what the meaning of this is supposed to be
> > > > exactly. Is this number supposed to be the last usable syscall, or is it
> > > > supposed to be the equivalent of NR_syscalls?
> > > > 
> > > 
> > > I am using as the equivalent of NR_syscalls.
> > > 
> > NR_syscalls has always been the total number of system calls, not the
> > last one.
> > 
> > > > Presently on SH we have this as NR_syscalls - 1, while on s390 I see it
> > > > is treated as NR_syscalls directly. s390 opencodes the NR_syscalls
> > > > directly and so presently blows up in -next due to a missing
> > > > FTRACE_SYSCALL_MAX definition:
> > > > 
> > > > 	http://kisskb.ellerman.id.au/kisskb/buildresult/1120523/
> > > > 
> > > > I was in the process of fixing that up when I noticed this difference.
> > > > x86 seems to also treat this as NR_syscalls - 1, but that looks to me
> > > > like there is an off-by-1 in arch_init_ftrace_syscalls() causing the last
> > > > syscall to be skipped?
> > > 
> > > I don't see how its used as 'NR_syscalls - 1' on x86,
> > > arch_init_ftrace_syscalls() does:
> > > 
> > >         for (i = 0; i < FTRACE_SYSCALL_MAX; i++) {
> > >                 meta = find_syscall_meta(psys_syscall_table[i]);
> > >                 syscalls_metadata[i] = meta;
> > >         }
> > > 
> > > So the last syscall should not be skipped.
> > > 
> > 
> > In today's -next:
> > 
> > #ifdef CONFIG_X86_64
> > # define FTRACE_SYSCALL_MAX     299
> > #else
> > # define FTRACE_SYSCALL_MAX     337
> > #endif
> > 
> > unistd_32.h:
> > 
> > #define __NR_reflinkat          337
> > 
> > unistd_64.h:
> > 
> > #define __NR_reflinkat          299
> > 
> > The first syscall starts at 0, but I don't see how this last syscall is
> > handled. If there were a __NR_syscalls 300 and 338 respectively, that
> > would seem to do the right thing. Or am I missing something?
> 
> No, you are right. When I changed the FTRACE_SYSCALL_MAX to 299, and
> 337, there was no reflinkat syscall in the tree. So, it was equivalent
> to NR_syscalls at that point in time. So that's where the confusion is.
> 
> Clearly, all the more reason to drop FTRACE_SYSCALL_MAX and change to
> NR_syscalls...
> 
If FTRACE_SYSCALL_MAX is dropped then s390 will be fixed, and I'll take
care of the sh update. If you want to hold off on adding NR_syscalls back
to x86, then s390 will need a #define FTRACE_SYSCALL_MAX __NR_syscalls in
arch/s390/include/asm/ftrace.h. Keeping FTRACE_SYSCALL_MAX around seems
to be asking for trouble, though (although I don't know what the original
rationale behind adding it was).
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