linux-kernel - Re: [RFC] TIF_NOTIFY_RESUME, arch/*/*/*signal*.c and all such

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Date:	Thu, 26 Apr 2012 15:52:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Tejun Heo <tj@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Roland McGrath <roland@...k.frob.com>
Subject: Re: [RFC] TIF_NOTIFY_RESUME, arch/*/*/*signal*.c and all such

On 04/26, Martin Schwidefsky wrote:
>
> On Wed, 25 Apr 2012 18:51:13 +0100
> Al Viro <viro@...IV.linux.org.uk> wrote:
>
> > do_signal()
> > {
> > 	if (we have any business doing restarts)
> > 		// note: we won't get here on subsequent calls of do_signal()
> > 		// due to the checks above; same logics that currently prevents
> > 		// double restarts
> > 		set NEED_RESTART flag
> > 	sig = get_signal_to_deliver(...)
> > 	if (sig) {
> > 		if (NEED_RESTART set) {
> > 			clear NEED_RESTART
> > 			same thing we do at that spot now - restart or EINTR
> > 			handle_signal(...)
> > 			...
> > 			return;
> > 		}
> > 	}
> > 	/* no handler */
> > 	if (test_and_clear_...(RESTORE_SIGMASK))
> > 		set_current_blocked(&current->saved_sigmask);
> > }
> > and in asm glue, *after* checking for SIGPENDING/NOTIFY_RESUME, check
> > NEED_RESTART and if it's set do what we currently do for restarts on
> > handlerless signal.
>
> You need to be careful with inferior calls there. gdb likes to play games
> with the registers inside the get_signal_to_deliver call, it wants to be
> able to jump out of an interrupted system call, do its inferior call in
> the debugee and then return to the interrupted system call.

Ah.

> You would have to to read, modify & restore the NEED_RESTART flag in gdb
> over an inferior call.

I am not sure, but perhaps this is not really needed...

But at least this means that "if (we have any business doing restarts)"
above is meaningless before get_signal_to_deliver().


And I am confused, off-topic question... How it is possible to
"then return to the interrupted system call" if that system call
returned -ERESTART_RESTARTBLOCK but the inferior call in turn
does the system call which changes restart_block->fn/etc ?

Oleg.

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