In message <20080501170819.bdcb9035.akpm@xxxxxxxxxxxxxxxxxxxx>, Andrew Morton writes:
> On Thu, 1 May 2008 19:44:18 -0400
> Erez Zadok <ezk@xxxxxxxxxxxxx> wrote:
[...]
> i_size is much more important because glitches in there can result in
> incorrect data being returned from read() and things like that. i_blocks
> is just a beancounting curiosity.
>
> >
> > 2. I've rewritten your suggested code a bit to reduce stack use. Modulo
> > having 32-bit spin_lock/unlock variants, do you see any problem with this
> > code below? My testing of it so far on 32/64-bit SMP/UMP have all
> > passed.
> >
> > void fsstack_copy_inode_size(struct inode *dst, struct inode *src)
> > {
> > #if BITS_PER_LONG == 32
> > blkcnt_t i_blocks;
> >
> > spin_lock(&src->i_lock);
> > i_blocks = src->i_blocks;
> > spin_unlock(&src->i_lock);
> > spin_lock(&dst->i_lock);
> > dst->i_blocks = i_blocks;
> > spin_unlock(&dst->i_lock);
> > #else
> > dst->i_blocks = src->i_blocks;
> > #endif
> > i_size_write(dst, i_size_read(src));
> > }
>
> That looks sane, as long as we don't care about i_size-vs-i_blocks
> coherency.
> However I expect that approximately zero of the sites which modify i_blocks
> are taking i_lock to do so.
If i_blocks is indeed less important than i_size, then we can live with some
incoherency b/t i_size and i_blocks, for now. Nevertheless, I propose
adding this to linux/fs.h:
static inline blkcnt_t i_blocks_read(const struct inode *inode)
{
#if BITS_PER_LONG == 32
blkcnt_t i_blocks;
spin_lock(&src->i_lock);
i_blocks = src->i_blocks;
spin_unlock(&src->i_lock);
return i_blocks;
#else
return src->i_blocks;
#endif
}
and a matching i_blocks_write function. We can then gradually convert those
"unsafe" users of i_blocks to use the new i_blocks_read/write helpers.
The nice thing about these two helpers is fsstack_copy_inode_size becomes a
lot cleaner and more elegant:
void fsstack_copy_inode_size(struct inode *dst, struct inode *src)
{
i_blocks_write(dst, i_blocks_read(src));
i_size_write(dst, i_size_read(src));
}
And, if we ever wanted to ensure coherency b/t i_blocks and i_size, we'll
need to create helpers that merge the functionality of i_size_read/write and
i_blocks_read/write.
What do you think?
Erez.
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