linux-kernel - Re: [PATCH] coredump: fix unfreezable coredumping task

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Date:   Tue, 4 Oct 2016 18:13:05 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>, linux-pm@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] coredump: fix unfreezable coredumping task

On 10/04, Michal Hocko wrote:
>
> On Fri 30-09-16 14:47:41, Oleg Nesterov wrote:
> > On 09/30, Andrey Ryabinin wrote:
> > >
> > > @@ -423,7 +424,9 @@ static int coredump_wait(int exit_code, struct core_state *core_state)
> > >  	if (core_waiters > 0) {
> > >  		struct core_thread *ptr;
> > >
> > > +		freezer_do_not_count();
> > >  		wait_for_completion(&core_state->startup);
> > > +		freezer_count();
> >
> > Agreed... we could probably even do
> >
> > 	--- x/fs/coredump.c
> > 	+++ x/fs/coredump.c
> > 	@@ -423,7 +423,13 @@ static int coredump_wait(int exit_code, 
> > 		if (core_waiters > 0) {
> > 			struct core_thread *ptr;
> > 	 
> > 	-		wait_for_completion(&core_state->startup);
> > 	+		if (wait_for_completion_interruptible(&core_state->startup)) {
> > 	+			/* see the comment in dump_interrupted() */
> > 	+			down_write(&mm->mmap_sem);
> > 	+			coredump_finish(mm, false);
> > 	+			up_write(&mm->mmap_sem);
> > 	+			return -EINTR;
> > 	+		}
> > 			/*
> > 			 * Wait for all the threads to become inactive, so that
> > 			 * all the thread context (extended register state, like
>
> This looks like a very good idea to me. We really want to make the whole
> coredump_wait killable.

Well, it is already killable. And with the change above it can sleep
in down_write(mmap_sem) and we really need this lock to abort, so it
won't necessarily react to SIGKILL faster.

> I guess this should help us to remove the
> hackish sig->flags & SIGNAL_GROUP_COREDUMP check from
> __task_will_free_mem.

Why? This doesn't depend on "killable". __task_will_free_mem() checks
this flag to detect the CLONE_VM processes which won't exit soon because
they participate in the coredumping.

Oleg.