On 10/04/2011 01:57 AM, Dmitry Monakhov wrote:
On Mon, 03 Oct 2011 12:00:00 -0700, Allison Henderson<achender@xxxxxxxxxxxxxxxxxx> wrote:
Hi all,
I've been working on locating all the existing uses of i_mutex in the
current ext4 code because I know we are planning to reduce the usage of
i_mutex in ext4. So I've gone through the ext4 code and also the vfs
code and come up with a list of ext4 items that appear to be protected
under i_mutex. I'm thinking about doing a patch to replace i_mutex with
a private ext4 mutex, and I wanted to update folks on this idea and pick
up any feed back people might have.
I'm thinking maybe we can have a separate mutex for functions that only
modify meta data like ext4_ioctl and ext4_setattr to help relieve
unneeded contention.
Are you going to change vfs core locking?
Hi there,
No, I initially had only thought about adding private locks to ext4, and
removing any occurrence of i_mutex locking in ext4, but it sounds like
Christoph has some more ideas to share to make this more generic.
And then the rest of functions that are modifying
data can go under a data mutex (including truncate since sometimes
ext4_ioctl and ext4_setattr will call ext4_truncate if they modify i_size).
So these are ext4 functions that currently lock i_mutex:
ext4_sync_file
ext4_fallocate
ext4_move_extents via two helper routines:
mext_inode_double_lock and mext_inode_double_unlock
ext4_ioctl (for the EXT4_IOC_SETFLAGS ioctl)
ext4_quota_write
We can easily avoid i_mutex on quota write because quota file can not
be truncated, and grows only in case of new dquot added.
I'll send you a patch.
Ah, alrighty then, thx! Any place we can currently remove i_mutex where
it is not needed is certainly helpful. :)
ext4_llseek
ext4_end_io_work
ext4_evict_inode (only while calling ext4_flush_completed_IO)
ext4_ind_direct_IO (only while calling ext4_flush_completed_IO)
And these are ext4 functions that have i_mutex locked by the vfs layer.
So we will need to lock the new private mutex here too if we want them
to be synchronous with the above functions.
ext4_setattr
ext4_da_writepages
ext4_rmdir
ext4_unlink
ext4_symlink
ext4_link
ext4_rename
And one unique case:
ext4_fiemap calls generic_block_fiemap and passes it a function pointer
to ext4_get_block. generic_block_fiemap will lock i_mutex before
calling the pointer. I dont think ext4_get_block needs i_mutex locked
all the time, so I think we can just make a wrapper for ext4_get_block
that locks the new private mutex and then we can pass a pointer to the
wrapper.
That's my list so far, if anyone knows of one I missed please let me
know, and also if you spot any other places where we can reduce unneeded
contention by using a separate lock. Thx!
Allison Henderson
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