Re: [PATCH 1/2] tracing: syscall_*regfunc() can race withcopy_process()

[Steven Rostedt]

On Sun, 2013-03-17 at 20:00 +0100, Oleg Nesterov wrote:
> 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.

I'm a bit confused by the above. Maybe it's the typo with the "and and"
that's confusing me.

> 
> > > +#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?

tracepoint_fork() is similar to being called trace_fork() which would be
considered a tracepoint. Seeing tracepoint_fork() would make me think it
has something to do with the fork tracepoint.

Do we plan on doing anything other than updating the syscall tracepoint
flag here? I find the "syscall_tracepoint_update()" very descriptive to
what is actually happening. While reading the fork code, seeing
'syscall_tracepoint_update()' would tell me that this has something to
do with syscall tracepoints, which it does. But tracepoint_fork() would
have me think something completely different.

-- Steve


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