linux-kernel - Re: [PATCH -tip 3/3] Add get

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Date:	Mon, 16 Nov 2009 17:39:49 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	Roland McGrath <roland@...hat.com>
CC:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH -tip 3/3] Add get_signal tracepoint

Roland McGrath wrote:
>>>    - signal loss events (queue overflow)
>>
>> Perhaps, this event is only for rt-signals, since
>> legacy signals just overwritten if it was sent.
>
> Not exactly.  Nothing is ever "overwritten".  If a non-RT signal is already
> pending, then you just leave the existing queue elements alone--i.e. the
> first one isn't overwritten, rather the second one is dropped.  But this is
> not really the point.
>
> The "queue overflow" happens in two ways.  For RT signals it really is a
> "signal loss" event--but that's also reported to the sender as -EAGAIN.  So
> a signal-generation tracepoint that reports the return value would already
> cover that in a way.
>
> For non-RT signals, a new signal is never lost.  But __sigqueue_alloc() can
> still fail.  In this case, you get no queue element and thus no siginfo_t
> stored, so you can lose some information about the signal.  You don't lose
> the signal itself, but will later dequeue it with an all-zeros siginfo_t.
> While calling this a "signal loss" is inaccurate, it is indeed a silent
> failure of sorts (unlike the RT signal case, which the userland caller
> knows about from the return value).

Hmm, actually, trace_signal_send() doesn't record the return value.
So, what about trace_signal_overflow() for RT-signals and
trace_signal_loss_info() for non-RT?

e.g.
@@ -918,12 +918,15 @@ static int __send_signal(int sig, struct siginfo *info, struct task_struct *t,
                         break;
                 }
         } else if (!is_si_special(info)) {
-               if (sig >= SIGRTMIN && info->si_code != SI_USER)
+               if (sig >= SIGRTMIN && info->si_code != SI_USER) {
                 /*
                  * Queue overflow, abort.  We may abort if the signal was rt
                  * and sent by user using something other than kill().
                  */
+                       trace_signal_overflow(sig, t);
                         return -EAGAIN;
+               }
+               trace_signal_loss_info(sig, info);
         }

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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