Re: [PATCH v2] 9p: use inode->i_lock to protect i_size_write()

[Dominique Martinet <asmadeus@codewreck.org>]

Hou Tao wrote on Wed, Jan 23, 2019:
> > write_end() has a comment that i_size cannot change under it because it
> > has the i_mutex, but it's obviously not sufficient given the stat2inode
> > code does not have it, so it needs to do the same dance as write_iter.
>
> OK, will do that in v3
> 
> How about adding a helper as shown in the following lines ?
>
> static inline void v9fs_i_size_write(struct inode *inode, loff_t i_size)
> {
>     spin_lock(&inode->i_lock);
>     i_size_write(inode, i_size);
>     spin_unlock(&inode->i_lock);
> }

Sure. I'm actually surprise no other part of the kernel has such helper,
cifs seems to be using that pattern a lot too.
Actually, looking a bit deeper fs/stack.c has this code:
        /*
         * If CONFIG_SMP or CONFIG_PREEMPT on 32-bit, it's vital for
         * fsstack_copy_inode_size() to hold some lock around
         * i_size_write(), otherwise i_size_read() may spin forever (see
         * include/linux/fs.h).  We don't necessarily hold i_mutex when this
         * is called, so take i_lock for that case.
         *
         * And if CONFIG_LBDAF (on 32-bit), continue our effort to keep the
         * two halves of i_blocks in sync despite SMP or PREEMPT: use i_lock
         * for that case too, and do both at once by combining the tests.
         *
         * There is none of this locking overhead in the 64-bit case.
         */
        if (sizeof(i_size) > sizeof(long) || sizeof(i_blocks) > sizeof(long))
                spin_lock(&dst->i_lock);
        i_size_write(dst, i_size);
        dst->i_blocks = i_blocks;
        if (sizeof(i_size) > sizeof(long) || sizeof(i_blocks) > sizeof(long))
                spin_unlock(&dst->i_lock);

It might make sense to do the same in our little helper ?

(it looks like i_blocks has the same problem? speaking of which we
probably do not want to update i_blocks either in the KEEP_SIZE
case...?)

> > As a nitpick I don't really like foo() vs foo_flags() as
> > foo-that-takes-extra-flags.
> > There are a few such examples in the kernel already but I think it does
> > not really convery information; it's better to have the base function
> > take flags and just use it, or if you want wrappers then just never
> > expose the flags but make a static _v9fs_stat2inode take flags,
> > v9fs_stat2inode behave as the old one and a new
> > v9fs_stat2inode_keepisize for the update with cache.
> > I'd personally go with the former are there only are four call sites.
>
> I agree with you. I will add a new flags parameter to v9fs_stat2inode() and use
> it directly instead of creating inline wrappers around it.

Thanks.

-- 
Dominique