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Date: Fri, 22 Mar 2019 12:10:27 -0400 From: Waiman Long <longman@...hat.com> To: Oleg Nesterov <oleg@...hat.com>, Matthew Wilcox <willy@...radead.org> 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> Subject: Re: [PATCH 2/4] signal: Make flush_sigqueue() use free_q to release memory On 03/22/2019 07:16 AM, Oleg Nesterov wrote: > On 03/21, Matthew Wilcox wrote: >> On Thu, Mar 21, 2019 at 05:45:10PM -0400, Waiman Long wrote: >> >>> 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. > Agreed. Thanks for looking into that. As I am not knowledgeable enough about the signal handling code path, I choose the lowest risk approach of not trying to change the code flow while deferring memory deallocation after releasing the tasklist_lock. >> 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. > This won't work, note the comment which explains the race with sigqueue_free(). > > Let me think about it... at least we can do something like > > close_the_race_with_sigqueue_free(struct sigpending *queue) > { > struct sigqueue *q, *t; > > list_for_each_entry_safe(q, t, ...) { > if (q->flags & SIGQUEUE_PREALLOC) > list_del_init(&q->list); > } > > called with ->siglock held, tasklist_lock is not needed. > > After that flush_sigqueue() can be called lockless in release_task() release_task. > > I'll try to make the patch tomorrow. > > Oleg. > I am looking forward to it. Thanks, Longman
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