Re: linux-next: next-20090609 hangs in early user mode

From: Theodore Tso
Date: Wed Jun 10 2009 - 12:10:20 EST

On Wed, Jun 10, 2009 at 04:08:39PM +0100, Al Viro wrote:
> We can, but... it's again a matter of combining things with different
> locking. i_flags is protected by i_mutex, so if you put another
> unsigned short next to it, you'd better make sure that i_mutex
> is necessary and sufficient for modifying it.
> Depending on the target, gcc may turn 16bit read-modify-store into 32bit one,
> so if you have two 16bit fields next to each other, you can run into
> CPU1: CPU2:
> r1 = *(u32 *)p; r2 = *(u32 *)p;
> r1 |= 1; r2 |= 1 << 16;
> *(u32 *)p = r1; *(u32 *)p = r2;
> with obvious results. So we need the same locking for both such fields...

Yelch.... good point. I'll look and see if there's some other 8 or
16-bit type to combine it with, but we may have started to hit
diminishing returns with this this approach to sliming the inode slab
caches. I'm beginning to think if I want to make the inodes smaller,
I'm going to have to create a separate substructure for fields only
used when a file descriptor is opened on that inode, both in struct
inode and in struct ext4_inode_info. (Lifetime management of the
substructure is going to be non-trivial, though.)

- Ted
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