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From: Kent Overstreet <kent.overstreet@gmail.com>
To: Coly Li <colyli@suse.de>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-bcache@vger.kernel.org
Subject: Re: [PATCH 12/12] closures: fix a race on wakeup from closure_sync
Date: Mon, 22 Jul 2019 13:22:09 -0400	[thread overview]
Message-ID: <20190722172209.GA25176@kmo-pixel> (raw)
In-Reply-To: <48527b97-e39a-0791-e038-d9f2ec28943e@suse.de>

On Thu, Jul 18, 2019 at 03:46:46PM +0800, Coly Li wrote:
> On 2019/7/16 6:47 下午, Coly Li wrote:
> > Hi Kent,
> > 
> > On 2019/6/11 3:14 上午, Kent Overstreet wrote:
> >> Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
> > Acked-by: Coly Li <colyli@suse.de>
> > 
> > And also I receive report for suspicious closure race condition in
> > bcache, and people ask for having this patch into Linux v5.3.
> > 
> > So before this patch gets merged into upstream, I plan to rebase it to
> > drivers/md/bcache/closure.c at this moment. Of cause the author is you.
> > 
> > When lib/closure.c merged into upstream, I will rebase all closure usage
> > from bcache to use lib/closure.{c,h}.
> 
> Hi Kent,
> 
> The race bug reporter replies me that the closure race bug is very rare
> to reproduce, after applying the patch and testing, they are not sure
> whether their closure race problem is fixed or not.
> 
> And I notice rcu_read_lock()/rcu_read_unlock() is used here, but it is
> not clear to me what is the functionality of the rcu read lock in
> closure_sync_fn(). I believe you have reason to use the rcu stuffs here,
> could you please provide some hints to help me to understand the change
> better ?

The race was when a thread using closure_sync() notices cl->s->done == 1 before
the thread calling closure_put() calls wake_up_process(). Then, it's possible
for that thread to return and exit just before wake_up_process() is called - so
we're trying to wake up a process that no longer exists.

rcu_read_lock() is sufficient to protect against this, as there's an rcu barrier
somewhere in the process teardown path.

  reply	other threads:[~2019-07-22 17:22 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-10 19:14 bcachefs status update (it's done cooking; let's get this sucker merged) Kent Overstreet
2019-06-10 19:14 ` [PATCH 01/12] Compiler Attributes: add __flatten Kent Overstreet
2019-06-12 17:16   ` Greg KH
2019-06-10 19:14 ` [PATCH 02/12] locking: SIX locks (shared/intent/exclusive) Kent Overstreet
2019-06-10 19:14 ` [PATCH 03/12] mm: pagecache add lock Kent Overstreet
2019-06-10 19:14 ` [PATCH 04/12] mm: export find_get_pages() Kent Overstreet
2019-06-10 19:14 ` [PATCH 05/12] fs: insert_inode_locked2() Kent Overstreet
2019-06-10 19:14 ` [PATCH 06/12] fs: factor out d_mark_tmpfile() Kent Overstreet
2019-06-10 19:14 ` [PATCH 07/12] Propagate gfp_t when allocating pte entries from __vmalloc Kent Overstreet
2019-06-10 19:14 ` [PATCH 08/12] block: Add some exports for bcachefs Kent Overstreet
2019-06-10 19:14 ` [PATCH 09/12] bcache: optimize continue_at_nobarrier() Kent Overstreet
2019-06-10 19:14 ` [PATCH 10/12] bcache: move closures to lib/ Kent Overstreet
2019-06-11 10:25   ` Coly Li
2019-06-13  7:28   ` Christoph Hellwig
2019-06-13 11:04     ` Kent Overstreet
2019-06-10 19:14 ` [PATCH 11/12] closures: closure_wait_event() Kent Overstreet
2019-06-11 10:25   ` Coly Li
2019-06-12 17:17   ` Greg KH
2019-06-10 19:14 ` [PATCH 12/12] closures: fix a race on wakeup from closure_sync Kent Overstreet
2019-07-16 10:47   ` Coly Li
2019-07-18  7:46     ` Coly Li
2019-07-22 17:22       ` Kent Overstreet [this message]
2020-11-24 23:07         ` Marc Smith
2020-11-25 18:10           ` Marc Smith
2019-06-10 20:46 ` bcachefs status update (it's done cooking; let's get this sucker merged) Linus Torvalds
2019-06-11  1:17   ` Kent Overstreet
2019-06-11  4:33     ` Dave Chinner
2019-06-12 16:21       ` Kent Overstreet
2019-06-12 23:02         ` Dave Chinner
2019-06-13 18:36           ` pagecache locking (was: bcachefs status update) merged) Kent Overstreet
2019-06-13 21:13             ` Andreas Dilger
2019-06-13 21:21               ` Kent Overstreet
2019-06-14  0:35                 ` Dave Chinner
2019-06-13 23:55             ` Dave Chinner
2019-06-14  2:30               ` Linus Torvalds
2019-06-14  7:30                 ` Dave Chinner
2019-06-15  1:15                   ` Linus Torvalds
2019-06-14  3:08               ` Linus Torvalds
2019-06-15  4:01                 ` Linus Torvalds
2019-06-17 22:47                   ` Dave Chinner
2019-06-17 23:38                     ` Linus Torvalds
2019-06-18  4:21                       ` Amir Goldstein
2019-06-19 10:38                         ` Jan Kara
2019-06-19 22:37                           ` Dave Chinner
2019-07-03  0:04                             ` pagecache locking Boaz Harrosh
     [not found]                               ` <DM6PR19MB250857CB8A3A1C8279D6F2F3C5FB0@DM6PR19MB2508.namprd19.prod.outlook.com>
2019-07-03  1:25                                 ` Boaz Harrosh
2019-07-05 23:31                               ` Dave Chinner
2019-07-07 15:05                                 ` Boaz Harrosh
2019-07-07 23:55                                   ` Dave Chinner
2019-07-08 13:31                                 ` Jan Kara
2019-07-09 23:47                                   ` Dave Chinner
2019-07-10  8:41                                     ` Jan Kara
2019-06-14 17:08               ` pagecache locking (was: bcachefs status update) merged) Kent Overstreet
2019-06-19  8:21           ` bcachefs status update (it's done cooking; let's get this sucker merged) Jan Kara
2019-07-03  1:04             ` [PATCH] mm: Support madvise_willneed override by Filesystems Boaz Harrosh
2019-07-03 17:21               ` Jan Kara
2019-07-03 18:03                 ` Boaz Harrosh
2019-06-11  4:55     ` bcachefs status update (it's done cooking; let's get this sucker merged) Linus Torvalds
2019-06-11 14:26       ` Matthew Wilcox
2019-06-11  4:10   ` Dave Chinner
2019-06-11  4:39     ` Linus Torvalds
2019-06-11  7:10       ` Dave Chinner
2019-06-12  2:07         ` Linus Torvalds
2019-07-03  5:59 ` Stefan K
2020-06-10 11:02 ` Stefan K
2020-06-10 11:32   ` Kent Overstreet

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