Re: [PATCH v12 04/13] task_isolation: add initial support

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 18, 2016 at 12:34:22PM -0400, Chris Metcalf wrote:
> On 5/18/2016 9:34 AM, Peter Zijlstra wrote:
> >On Tue, Apr 05, 2016 at 01:38:33PM -0400, Chris Metcalf wrote:
> >>diff --git a/kernel/signal.c b/kernel/signal.c
> >>index aa9bf00749c1..53e4e62f2778 100644
> >>--- a/kernel/signal.c
> >>+++ b/kernel/signal.c
> >>@@ -34,6 +34,7 @@
> >>  #include <linux/compat.h>
> >>  #include <linux/cn_proc.h>
> >>  #include <linux/compiler.h>
> >>+#include <linux/isolation.h>
> >>  #define CREATE_TRACE_POINTS
> >>  #include <trace/events/signal.h>
> >>@@ -2213,6 +2214,9 @@ relock:
> >>  		/* Trace actually delivered signals. */
> >>  		trace_signal_deliver(signr, &ksig->info, ka);
> >>+		/* Disable task isolation when delivering a signal. */
> >Why !? Changelog is quiet on this.
> 
> There are really two reasons.
> 
> 1. If the task is receiving a signal, it will know it's not isolated
>    any more, so we don't need to worry about notifying it explicitly.
>    This behavior is easy to document and allows the application to decide
>    if the signal is unexpected and it should go straight to its error
>    handling path (likely outcome, and in that case you want task isolation
>    off anyway) or if it thinks it can plausibly re-enable isolation and
>    return to where the signal interrupted you at (hard to imagine how this
>    would ever make sense, but you could if you wanted to).
> 
> 2. When we are delivering a signal we may already be holding the lock
>    for the signal subsystem, and it gets hard to figure out whether it's
>    safe to send another signal to the application as a "task isolation
>    broken" notification.  For example, sending a signal to a task on
>    another core involves doing an IPI to that core to kick it; the IPI
>    normally is a generic point for notifying the remote core of broken
>    task isolation and sending a signal - except that at the point where
>    we would do that on the signal path we are already holding the lock,
>    so we end up deadlocked.  We could no doubt work around that, but it
>    seemed cleaner to decouple the existing signal mechanism from the
>    signal delivery for task isolation.
> 
> I will add more discussion of the rationale to the commit message.

Please also expand the in-code comment, as that is what we'll see first
when reading the code.