linux-kernel - Re: 3.14-rc2 XFS backtrace because irqs

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Date:	Fri, 14 Feb 2014 16:16:24 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>, Dave Jones <davej@...hat.com>,
	Eric Sandeen <sandeen@...deen.net>,
	Linux Kernel <linux-kernel@...r.kernel.org>, xfs@....sgi.com
Subject: Re: 3.14-rc2 XFS backtrace because irqs_disabled.

On Fri, Feb 14, 2014 at 08:13:02AM -0800, Christoph Hellwig wrote:
> On Wed, Feb 12, 2014 at 01:32:55PM -0800, Linus Torvalds wrote:
> > We'd have to teach each user of "dequeue_signal()" to free the siginfo
> > thing. Which shouldn't be too bad - I think we've collected all of
> > that into generic code, and there isn't the mass or architecture code
> > that knows about these things any more. But there are a few odd
> > drivers etc and signalfd.
> 
> The few odd drivers are nbd, jffs2 and the usb mass storage gadget.
> All of these have in common that they try to handle signals in a kernel
> thread (which we don't even allow by default), and that they ignore the
> siginfo.  I think they could mostly be replaced by an addition to the
> kthread API to allow a kthread to be killed by signals for legacy
> reasons.

FWIW, there's a funny situation - all users of dequeue_signal_lock()
actually ignore info completely.  I'm not saying that we ought to
stop returning it, but e.g. jbd part of that patch is simply
diff --git a/fs/jffs2/background.c b/fs/jffs2/background.c
index 2b60ce1..aefdff2 100644
--- a/fs/jffs2/background.c
+++ b/fs/jffs2/background.c
@@ -119,13 +119,14 @@ static int jffs2_garbage_collect_thread(void *_c)
                /* Put_super will send a SIGKILL and then wait on the sem.
                 */
                while (signal_pending(current) || freezing(current)) {
-                       siginfo_t info;
+                       ksiginfo_t info;
                        unsigned long signr;
 
                        if (try_to_freeze())
                                goto again;
 
                        signr = dequeue_signal_lock(current, &current->blocked, &info);
+                       dismiss_siginfo(&info);
 
                        switch(signr) {
                        case SIGSTOP:

Not complicated at all.  Where it does get complicated is ->last_siginfo and
PTRACE_SETSIGINFO - getting that reasonably clean is what I'm still fighting
right now...
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