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

From: Russ Knize
Date: Wed Sep 11 2013 - 11:43:15 EST

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Jaegeuk,

My tests include forced kernel panics while fsstress is running, which
generates a lot of recovery activity. Sorry I wasn't more clear.

I understand your concern, which is why I first tried to keep the
fs_lock in the xattr_handler->set() path from VFS while removing it
from the call path I shared earlier. Doing this requires reworking
the xattr code a bit. Based on what Gu said about the inode mutex, it
seems like we can just remove it.

Russ

On Wed, Sep 11, 2013 at 8:19 AM, Kim Jaegeuk <jaegeuk.kim@xxxxxxxxx> wrote:
> 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 (í), 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 : äæ 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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>>> >> Linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx
>>> >> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>>> >>
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>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >>
>>> >
>>>
>>>
>>
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Next message: Mark Brown: "Re: [PATCH v2 7/9] ASoC: codecs: convert existing I2C client driversto use I2C core runtime PM"
Previous message: James Morris: "Re: [GIT PULL] Keyrings patches"
In reply to: Kim Jaegeuk: "Re: [f2fs-dev] [PATCH] f2fs: optimize fs_lock for better performance"
Next in thread: Jin Xu: "Re: [f2fs-dev] [PATCH] f2fs: optimize fs_lock for better performance"
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