From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx131.postini.com [74.125.245.131]) by kanga.kvack.org (Postfix) with SMTP id 51EF66B0032 for ; Thu, 30 May 2013 15:55:54 -0400 (EDT) Date: Thu, 30 May 2013 15:55:39 -0400 From: Johannes Weiner Subject: Re: [PATCH] swap: avoid read_swap_cache_async() race to deadlock while waiting on discard I/O compeletion Message-ID: <20130530195539.GA27226@cmpxchg.org> References: <2434dea05a7fda7e7ccf48f70124bd65f2556b2d.1369935749.git.aquini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2434dea05a7fda7e7ccf48f70124bd65f2556b2d.1369935749.git.aquini@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Rafael Aquini 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 03:05:00PM -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? > 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(). 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