linux-kernel - Re: [PATCH -v2] f2fs: Remove lock from check_valid

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 09 Sep 2014 13:43:46 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Jaegeuk Kim <jaegeuk@...nel.org>
Cc:	huang ying <huang.ying.caritas@...il.com>,
	Changman Lee <cm224.lee@...sung.com>,
	linux-f2fs-devel@...ts.sourceforge.net,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2] f2fs: Remove lock from check_valid_map

On Mon, 2014-09-08 at 22:13 -0700, Jaegeuk Kim wrote:
> Hi Huang,
> 
> On Mon, Sep 08, 2014 at 03:36:35PM +0800, huang ying wrote:
> > Hi, Jaegeuk,
> > 
> > On Mon, Sep 8, 2014 at 11:50 AM, Jaegeuk Kim <jaegeuk@...nel.org> wrote:
> > 
> > > Hi,
> > >
> > > On Sun, Sep 07, 2014 at 11:38:30AM +0800, Huang Ying wrote:
> > > > Only one bit is read in check_valid_map, holding a lock to do that
> > > > doesn't help anything except decreasing performance.
> > > >
> > > > Signed-off-by: Huang, Ying <ying.huang@...el.com>
> > > > ---
> > > >
> > > > v2: Fixed a build warning.
> > > >
> > > > ---
> > > >  fs/f2fs/gc.c |    3 ---
> > > >  1 file changed, 3 deletions(-)
> > > >
> > > > --- a/fs/f2fs/gc.c
> > > > +++ b/fs/f2fs/gc.c
> > > > @@ -378,14 +378,11 @@ static void put_gc_inode(struct list_hea
> > > >  static int check_valid_map(struct f2fs_sb_info *sbi,
> > > >                               unsigned int segno, int offset)
> > > >  {
> > > > -     struct sit_info *sit_i = SIT_I(sbi);
> > > >       struct seg_entry *sentry;
> > > >       int ret;
> > > >
> > > > -     mutex_lock(&sit_i->sentry_lock);
> > > >       sentry = get_seg_entry(sbi, segno);
> > > >       ret = f2fs_test_bit(offset, sentry->cur_valid_map);
> > > > -     mutex_unlock(&sit_i->sentry_lock);
> > > >       return ret;
> > >
> > > The f2fs_test_bit is not atomic, so I'm not sure this is a good approach.
> > > How about introducing rw_semaphore?
> > >
> > 
> > IMO, f2fs_test_bit just read a global variable (a byte in cur_valid_map),
> > then check its value. The byte may be changed in another CPU concurrently.
> > But even protected with a mutex, it can be changed in another CPU
> > immediately after mutex_unlock.  So mutex does not help  here.  Here we
> > just read a global variable, not read/modify/write, so, we don't need
> > atomic too.
> 
> Hmm. This is a pretty hard corner case to allow the mutex removal under the
> following assumption.
> 
> 1. All the sit entries are cached in a global array, which means that it never
> happens that any sit entry pointers are changed.
> 
> 2. I agree that f2fs_gc tries to conduct the cleaning with best effort, and
> it triggers again when it detects there is something to do more.
> So, check_valid_bitmap doesn't need to make a precise decision.
> 
> But, what I concern is the consistent policy to use such the mutex.
> If we break the rule, it becomes harder to debug potential bugs.

Yes.  We definitely need a rule.  But I suggest to make a small tweak to
the rule.  If we just read one variable with fixed address, we need not
to use a mutex to protect that.

> Anyway, have you been facing with such the lock contention?

No, I just review the code and thinks the mutex is not necessary.

Best Regards,
Huang, Ying


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/