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Re: 3.14-rc2 XFS backtrace because irqs_disabled.

To: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Subject: Re: 3.14-rc2 XFS backtrace because irqs_disabled.
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Date: Sat, 15 Feb 2014 18:43:45 +0100
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, Dave Chinner <david@xxxxxxxxxxxxx>, Dave Jones <davej@xxxxxxxxxx>, Eric Sandeen <sandeen@xxxxxxxxxxx>, Linux Kernel <linux-kernel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140215155838.GA18016@xxxxxxxxxxxxxxxxxx>
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User-agent: Mutt/1.5.18 (2008-05-17)
On 02/15, Al Viro wrote:
> > Ouch...  I think I see what you mean.  Let me see if I got it right:
> > timer->sigq is *not* freed by collect_signal(); it's done by
> > release_posix_timer() instead, under siglock.  Frankly, this
> >         /*
> >          * If it is queued it will be freed when dequeued,
> >          * like the "regular" sigqueue.
> >          */
> >         if (!list_empty(&q->list))
> >                 q = NULL;
> > in sigqueue_free() smells like it's asking for races.  Sigh...

This is protected by ->siglock, should be safe...

> So basically we want a different condition for "can we just go ahead and
> free that sucker", right?  Instead of "it's on the list, shan't free it"
> it ought to be something like "it's on the list or it is referenced by
> ksiginfo".  Locking will be interesting, though... ;-/

I guess yes... send_sigqueue() checks list_empty() too, probably nobody else.

> BTW, I really wonder how does that stuff interact with PTRACE_SETSIGINFO.
> What happens if tracer does PTRACE_GETSIGINFO, changes ->si_signo to
> something blocked, shoves it back with PTRACE_SETSIGINFO and does
> PTRACE_CONT with that new signal number?  Would we get two sigqueue instances
> with the same ->si_tid, one of them matching the timer->sigq and another
> - not?

Or the task sends a SI_TIMER info to itself via sys_rt_sigqueueinfo().

Afaics, nothing really bad can happen, I mean the kernel should not
crash or something like this. do_schedule_next_timer() can be fooled,
but at least lock_timer() can only succeed if this process actually
has a timer with the same timer_id. This sigqueue != timer->sigq, but
I think this doesn't matter, posix_timer_event() will use timer->sigq


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