linux-kernel - Re: [PATCH 2/4] signal: Make flush_sigqueue() use free

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Message-ID: <20190322015208.GD19508@bombadil.infradead.org>
Date:   Thu, 21 Mar 2019 18:52:08 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        selinux@...r.kernel.org, Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 2/4] signal: Make flush_sigqueue() use free_q to release
 memory

On Thu, Mar 21, 2019 at 05:45:10PM -0400, Waiman Long wrote:
> It was found that if a process had many pending signals (e.g. millions),
> the act of exiting that process might cause its parent to have a hard
> lockup especially on a debug kernel with features like KASAN enabled.
> It was because the flush_sigqueue() was called in release_task() with
> tasklist_lock held and irq disabled.

This rather apocalyptic language is a bit uncalled for.  I appreciate the
warning is scary, but all that's really happening is that the debug code
is taking too long to execute.

> To avoid this dire condition and reduce lock hold time of tasklist_lock,
> flush_sigqueue() is modified to pass in a freeing queue pointer so that
> the actual freeing of memory objects can be deferred until after the
> tasklist_lock is released and irq re-enabled.

I think this is a really bad solution.  It looks kind of generic,
but isn't.  It's terribly inefficient, and all it's really doing is
deferring the debugging code until we've re-enabled interrupts.

We'd be much better off just having a list_head in the caller
and list_splice() the queue->list onto that caller.  Then call
__sigqueue_free() for each signal on the queue.