Re: [PATCH -v2] f2fs: Remove lock from check_valid_map

[Huang Ying]

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@xxxxxxxxxx> 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@xxxxxxxxx>
> > > > ---
> > > >
> > > > 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


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