On Mon, Nov 08, 2010 at 10:28:29AM -0500, Jeff Moyer wrote:
> Dave Chinner <david@xxxxxxxxxxxxx> writes:
>
> > +struct dio_zero_block {
> > + struct list_head dio_list; /* list of io in progress */
> > + sector_t zero_block; /* block being zeroed */
> > + struct dio *dio; /* owner dio */
> > + wait_queue_head_t wq; /* New IO block here */
> New IOs block here, or new IO blocks here?
>
> > +/*
> > + * Add a filesystem block to the list of blocks we are tracking.
> > + */
> > +static void
> > +dio_start_zero_block(struct dio *dio, sector_t zero_block)
> > +{
> > + struct dio_zero_block *zb;
> > +
> > + zb = kmalloc(sizeof(*zb), GFP_NOIO);
> > + if (!zb)
> > + return;
> > + INIT_LIST_HEAD(&zb->dio_list);
> > + init_waitqueue_head(&zb->wq);
> > + zb->zero_block = zero_block;
> > + zb->dio = dio;
> > + atomic_set(&zb->ref, 1);
> > +
> > + spin_lock(&dio_zero_block_lock);
> > + list_add(&zb->dio_list, &dio_zero_block_list);
> > + spin_unlock(&dio_zero_block_lock);
>
> What protects from two processes getting here at the same time, and
> hence adding the same block to the list? i_mutex?
The wait in dio_zero_block() called before this function is called is
supposed to serialise them, but now that you point it out, it's not
an atomic wait-and-add so ther eis a small window where two IOs
could pass through here. Easy enoug to fix by combining the search
and inÑert - I'll restructure it along those lines.
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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