Messages in this thread | | | Subject | Re: [PATCH -v2] f2fs: Remove lock from check_valid_map | From | Huang Ying <> | Date | Tue, 09 Sep 2014 13:43:46 +0800 |
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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@kernel.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@intel.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
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