From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx112.postini.com [74.125.245.112]) by kanga.kvack.org (Postfix) with SMTP id 6EEBB6B0033 for ; Thu, 30 May 2013 17:57:20 -0400 (EDT) Date: Thu, 30 May 2013 18:56:57 -0300 From: Rafael Aquini Subject: Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion Message-ID: <20130530215656.GD13605@optiplex.redhat.com> References: <2434dea05a7fda7e7ccf48f70124bd65f2556b2d.1369935749.git.aquini@redhat.com> <20130530195539.GA27226@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130530195539.GA27226@cmpxchg.org> Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, hughd@google.com, shli@kernel.org, riel@redhat.com, lwoodman@redhat.com, kosaki.motohiro@jp.fujitsu.com, kamezawa.hiroyu@jp.fujitsu.com, stable@vger.kernel.org On Thu, May 30, 2013 at 03:55:39PM -0400, Johannes Weiner wrote: > On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote: > > read_swap_cache_async() can race against get_swap_page(), and stumble across > > a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the > > swapcache yet. This transient swap_map state is expected to be transitory, > > but the actual placement of discard at scan_swap_map() inserts a wait for > > I/O completion thus making the thread at read_swap_cache_async() to loop > > around its -EEXIST case, while the other end at get_swap_page() > > is scheduled away at scan_swap_map(). This can leave the system deadlocked > > if the I/O completion happens to be waiting on the CPU workqueue where > > waitqueue? > Ugh! I will repost this to fix it and the "compeletion" typo at subject... > > read_swap_cache_async() is busy looping and !CONFIG_PREEMPT. > > > > This patch introduces a cond_resched() call to make the aforementioned > > read_swap_cache_async() busy loop condition to bail out when necessary, > > thus avoiding the subtle race window. > > > > Signed-off-by: Rafael Aquini > > --- > > mm/swap_state.c | 14 +++++++++++++- > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > diff --git a/mm/swap_state.c b/mm/swap_state.c > > index b3d40dc..9ad9e3b 100644 > > --- a/mm/swap_state.c > > +++ b/mm/swap_state.c > > @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask, > > * Swap entry may have been freed since our caller observed it. > > */ > > err = swapcache_prepare(entry); > > - if (err == -EEXIST) { /* seems racy */ > > + if (err == -EEXIST) { > > radix_tree_preload_end(); > > + /* > > + * We might race against get_swap_page() and stumble > > + * across a SWAP_HAS_CACHE swap_map entry whose page > > + * has not been brought into the swapcache yet, while > > + * the other end is scheduled away waiting on discard > > + * I/O completion. > > + * In order to avoid turning this transitory state > > + * into a permanent loop around this -EEXIST case, > > + * lets just conditionally invoke the scheduler, > > + * if there are some more important tasks to run. > > + */ > > + cond_resched(); > > Might be worth mentioning the !CONFIG_PREEMPT deadlock scenario here, > especially since under CONFIG_PREEMPT the radix_tree_preload_end() is > already a scheduling point through the preempt_enable(). > Nice suggestion, will do it. Thanks for reviewing this patch! > Other than that, the patch looks good to me! > > Acked-by: Johannes Weiner -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org