linux-kernel - Re: [PATCH 1/2] tracing: syscall_*regfunc() can race with copy

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Message-ID: <20130317190023.GA24023@redhat.com>
Date:	Sun, 17 Mar 2013 20:00:23 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] tracing: syscall_*regfunc() can race with
	copy_process()

On 03/17, Steven Rostedt wrote:
>
> On Sun, 2013-03-17 at 19:28 +0100, Oleg Nesterov wrote:
> > syscall_regfunc() and syscall_unregfunc() should set/clear
> > TIF_SYSCALL_TRACEPOINT system-wide, but do_each_thread() can race
> > with copy_process() and miss the new child which was not added to
> > init_task.tasks list yet.
> >
> > Change copy_process() to update the child's TIF_SYSCALL_TRACEPOINT
> > under tasklist.
>
> Is this because "p = dup_task_struct(current);" is outside the lock?
> Probably should state this in the change log.

Not only, syscall_regfunc/syscall_unregfunc can miss the new child.

Just suppose that syscall_regfunc() takes tasklist right before the
forking task tries to take it for writing and and the child to the
list.

> > +#ifdef CONFIG_TRACEPOINTS
> > +	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
> > +		set_tsk_thread_flag(p, TIF_SYSCALL_TRACEPOINT);
> > +	else
> > +		clear_tsk_thread_flag(p, TIF_SYSCALL_TRACEPOINT);
> > +#endif
>
> I hate seeing #ifdef code like this in C files. Can you add a function
> to set this in include/trace/syscalls.h:

It seems that everyone hates them, except me ;)

> #ifdef CONFIG_TRACEPOINTS
> static inline void syscall_tracepoint_update(struct task_struct *p)
> {
> 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
> 		set_tsk_thread_flag(p, TIF_SYSCALL_TRACEPOINT);
> 	else
> 		clear_tsk_thread_flag(p, TIF_SYSCALL_TRACEPOINT);
> }
> #else
> static inline void syscall_tracepoint_update(struct task_struct *p) {}
> #endif

OK, thanks, will do. But perhaps tracepoint_fork() would be better?

Oleg.

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