Re: [f2fs-dev] [PATCH] f2fs: optimize fs_lock for better performance

From: Kim Jaegeuk
Date: Wed Sep 11 2013 - 09:19:21 EST

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Hi Russ,

The usage of fs_locks is for the recovery, so it doesn't matter
with stress-testing.
Actually what I've concerned is that we should not grab two or
more fs_locks in the same call path.
Thanks,

2013/9/11 Russ Knize <Russ.Knize@xxxxxxxxxxxx>:
> Hi Jaegeuk/Gu,
>
> I've removed the lock and have been stress-testing with SELinux and some
> additional xattr torture for 24+ hours. I have not encountered any issues
> yet.
>
> My previous suggestion about moving the lock is probably not a good idea
> without some significant code rework (thanks to the f2fs_balance_fs call in
> f2fs_setxattr).
>
> Russ
>
> On Tue, Sep 10, 2013 at 10:22 PM, Gu Zheng <guz.fnst@xxxxxxxxxxxxxx> wrote:
>>
>> Hi Jaegeuk,
>> On 09/10/2013 08:59 AM, Jaegeuk Kim wrote:
>>
>> > Hi,
>> >
>> > 2013-09-07 (토), 08:00 +0000, Chao Yu:
>> >> Hi Knize,
>> >>
>> >> Thanks for your reply, I think it's actually meaningless that it's
>> >> being named after "spin_lock",
>> >> it's better to rename this spinlock to "round_robin_lock".
>> >>
>> >> This patch can only resolve the issue of unbalanced fs_lock usage,
>> >> it can not fix the deadlock issue.
>> >> can we fix deadlock issue through this method:
>> >>
>> >> - vfs_create()
>> >> - f2fs_create() - takes an fs_lock and save current thread info into
>> >> thread_info[NR_GLOBAL_LOCKS]
>> >> - f2fs_add_link()
>> >> - __f2fs_add_link()
>> >> - init_inode_metadata()
>> >> - f2fs_init_security()
>> >> - security_inode_init_security()
>> >> - f2fs_initxattrs()
>> >> - f2fs_setxattr() - get fs_lock only if there is no current
>> >> thread info in thread_info
>> >>
>> >> So it keeps one thread can only hold one fs_lock to avoid deadlock.
>> >> Can we use this solution?
>> >
>> > It could be.
>> > But, I think we can avoid to grab the fs_lock at the f2fs_initxattrs()
>>
>> Agree. This fs_lock here is used to protect the xattr from parallel
>> modification,
>> but here is in the initxattrs routine, parallel modification can not
>> happen.
>> And in the normal setxattr routine the inode->i_mutex (vfs layer) is used
>> to
>> avoid parallel modification. So I think this fs_lock is needless.
>> Am I missing something?
>>
>> Regards,
>> Gu
>>
>> > level, since this case only happens when f2fs_initxattrs() is called.
>> > Let's think about ut in more detail.
>> > Thanks,
>> >
>> >>
>> >>
>> >>
>> >> thanks again!
>> >>
>> >>
>> >>
>> >> ------- Original Message -------
>> >>
>> >> Sender : Russ Knize<Russ.Knize@xxxxxxxxxxxx>
>> >>
>> >> Date : 九月 07, 2013 04:25 (GMT+09:00)
>> >>
>> >> Title : Re: [f2fs-dev] [PATCH] f2fs: optimize fs_lock for better
>> >> performance
>> >>
>> >>
>> >>
>> >> I encountered this same issue recently and solved it in much the same
>> >> way. Can we rename "spin_lock" to something more meaningful?
>> >>
>> >>
>> >> This race actually exposed a potential deadlock between f2fs_create()
>> >> and f2fs_initxattrs():
>> >>
>> >>
>> >> - vfs_create()
>> >> - f2fs_create() - takes an fs_lock
>> >> - f2fs_add_link()
>> >> - __f2fs_add_link()
>> >> - init_inode_metadata()
>> >> - f2fs_init_security()
>> >> - security_inode_init_security()
>> >> - f2fs_initxattrs()
>> >> - f2fs_setxattr() - also takes an fs_lock
>> >>
>> >>
>> >> If another CPU happens to have the same lock that f2fs_setxattr() was
>> >> trying to take because of the race around next_lock_num, we can get
>> >> into a deadlock situation if the two threads are also contending over
>> >> another resource (like bdi).
>> >>
>> >>
>> >> Another scenario is if the above happens while another thread is in
>> >> the middle of grabbing all of the locks via mutex_lock_all().
>> >> f2fs_create() is holding a lock that mutex_lock_all() is waiting for
>> >> and mutex_lock_all() is holding a lock that f2fs_setxattr() is waiting
>> >> for.
>> >>
>> >>
>> >> Russ
>> >>
>> >>
>> >> On Fri, Sep 6, 2013 at 4:48 AM, Chao Yu <chao2.yu@xxxxxxxxxxx> wrote:
>> >> Hi Kim:
>> >>
>> >> I think there is a performance problem: when all
>> >> sbi->fs_lock is holded,
>> >>
>> >> then all other threads may get the same next_lock value from
>> >> sbi->next_lock_num in function mutex_lock_op,
>> >>
>> >> and wait to get the same lock at position fs_lock[next_lock],
>> >> it unbalance the fs_lock usage.
>> >>
>> >> It may lost performance when we do the multithread test.
>> >>
>> >>
>> >>
>> >> Here is the patch to fix this problem:
>> >>
>> >>
>> >>
>> >> Signed-off-by: Yu Chao <chao2.yu@xxxxxxxxxxx>
>> >>
>> >> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>> >>
>> >> old mode 100644
>> >>
>> >> new mode 100755
>> >>
>> >> index 467d42d..983bb45
>> >>
>> >> --- a/fs/f2fs/f2fs.h
>> >>
>> >> +++ b/fs/f2fs/f2fs.h
>> >>
>> >> @@ -371,6 +371,7 @@ struct f2fs_sb_info {
>> >>
>> >> struct mutex fs_lock[NR_GLOBAL_LOCKS]; /* blocking FS
>> >> operations */
>> >>
>> >> struct mutex node_write; /* locking
>> >> node writes */
>> >>
>> >> struct mutex writepages; /* mutex for
>> >> writepages() */
>> >>
>> >> + spinlock_t spin_lock; /* lock for
>> >> next_lock_num */
>> >>
>> >> unsigned char next_lock_num; /* round-robin
>> >> global locks */
>> >>
>> >> int por_doing; /* recovery is
>> >> doing or not */
>> >>
>> >> int on_build_free_nids; /*
>> >> build_free_nids is doing */
>> >>
>> >> @@ -533,15 +534,19 @@ static inline void
>> >> mutex_unlock_all(struct f2fs_sb_info *sbi)
>> >>
>> >>
>> >>
>> >> static inline int mutex_lock_op(struct f2fs_sb_info *sbi)
>> >>
>> >> {
>> >>
>> >> - unsigned char next_lock = sbi->next_lock_num %
>> >> NR_GLOBAL_LOCKS;
>> >>
>> >> + unsigned char next_lock;
>> >>
>> >> int i = 0;
>> >>
>> >>
>> >>
>> >> for (; i < NR_GLOBAL_LOCKS; i++)
>> >>
>> >> if (mutex_trylock(&sbi->fs_lock[i]))
>> >>
>> >> return i;
>> >>
>> >>
>> >>
>> >> - mutex_lock(&sbi->fs_lock[next_lock]);
>> >>
>> >> + spin_lock(&sbi->spin_lock);
>> >>
>> >> + next_lock = sbi->next_lock_num % NR_GLOBAL_LOCKS;
>> >>
>> >> sbi->next_lock_num++;
>> >>
>> >> + spin_unlock(&sbi->spin_lock);
>> >>
>> >> +
>> >>
>> >> + mutex_lock(&sbi->fs_lock[next_lock]);
>> >>
>> >> return next_lock;
>> >>
>> >> }
>> >>
>> >>
>> >>
>> >> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>> >>
>> >> old mode 100644
>> >>
>> >> new mode 100755
>> >>
>> >> index 75c7dc3..4f27596
>> >>
>> >> --- a/fs/f2fs/super.c
>> >>
>> >> +++ b/fs/f2fs/super.c
>> >>
>> >> @@ -657,6 +657,7 @@ static int f2fs_fill_super(struct
>> >> super_block *sb, void *data, int silent)
>> >>
>> >> mutex_init(&sbi->cp_mutex);
>> >>
>> >> for (i = 0; i < NR_GLOBAL_LOCKS; i++)
>> >>
>> >> mutex_init(&sbi->fs_lock[i]);
>> >>
>> >> + spin_lock_init(&sbi->spin_lock);
>> >>
>> >> mutex_init(&sbi->node_write);
>> >>
>> >> sbi->por_doing = 0;
>> >>
>> >> spin_lock_init(&sbi->stat_lock);
>> >>
>> >> (END)
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
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>> >> _______________________________________________
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>> >> Linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
>> >> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >
>>
>>
>
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Next message: Kim Jaegeuk: "Re: [f2fs-dev] [PATCH] f2fs: optimize fs_lock for better performance"
Previous message: Rafael J. Wysocki: "Re: "cpufreq: fix serialization issues with freq change notifiers" breaks cpufreq too"
In reply to: Gu Zheng: "Re: [f2fs-dev] [PATCH] f2fs: optimize fs_lock for better performance"
Next in thread: Russ Knize: "Re: [f2fs-dev] [PATCH] f2fs: optimize fs_lock for better performance"
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