Re: [RFC][PATCH 1/2] tracing/ftrace: syscall tracing infrastructure

[Oleg Nesterov <oleg@redhat.com>]

On 03/16, Mathieu Desnoyers wrote:
>
> utrace_add_engine()
>   set_notify_resume(target);
>
> ok, so this is where the TIF_NOTIFY_RESUME thread flag is set. I notice
> that it is set asynchronously with the execution of the target thread
> (as I do with my TIF_KERNEL_TRACE thread flag).
>
> However, on x86_64, _TIF_DO_NOTIFY_MASK is only tested in
> entry_64.S
>
> int_signal:
> and
> retint_signal:
>
> code paths. However, if there is no syscall tracing to do upon syscall
> entry, the thread flags are not re-read at syscall exit and you will
> miss the syscall exit returning from your target thread if this thread
> was blocked while you set its TIF_NOTIFY_RESUME.

Afaics, TIF_NOTIFY_RESUME is not needed to trace syscall entry/exit.
If engine wants the syscall tracing, utrace_set_events(UTRACE_SYSCALL_xxx)
sets TIF_SYSCALL_TRACE. And syscall_trace_enter/syscall_trace_leave call
tracehook_report_syscall_xxx().

Oleg.