[compendium reply]
On Wed, Nov 07, 2018 at 06:43:03PM -0500, Josef Bacik wrote:
> On Thu, Nov 08, 2018 at 10:37:40AM +1100, Dave Chinner wrote:
> > On Wed, Nov 07, 2018 at 03:10:55PM -0500, Josef Bacik wrote:
> > > If we failed to writeout a xfs_buf we'll grab a ref for it and put it on
> > > li->li_buf. Then when submitting the failed bufs we'll clear LI_FAILED
> > > on the li, which clears the LI_FAILED flag, but also drops the ref on
> > > the buf. Since it isn't on a IO list at this point this could very well
> > > be the last ref on the buf, which wreaks havoc when we go to add the buf
> > > to the delwrite list. Fix this by holding a ref on the buf before we
> > > call xfs_buf_resubmit_failed_buffers in order to make sure the buf
> > > doesn't disappear before we're able to clear the error and add it to the
> > > delwri list. This fixes the panics I was seeing with error injection.
....
> > Perhaps something like the patch below?
> >
>
> I thought about this, but I was worried that clearing the XFS_LI_FAILED may race
> with submitting the IO and having it fail again, so we end up clearing it when
> we need it set to resubmit again. But you are the expert here, if that isn't
> possible then I'm happy with this patch. Thanks,
The buffer cannot be submitted while we are clearing the failed
flags because a) the caller holds the buffer locked and so owns it
completely, and b) the caller owns the buffer_list that the buffer
is queued to and so controls when the list of buffers is submitted
for IO.
IOWs, there is no possibility of racing with clearing the
XFS_LI_FAILED flags because we own everything in that context.
> The other question, is it possible for the buffer to be submitted in another
> thread immediately after it is queued for IO?
See a) above - you have to hold the buffer lock to submit it for IO.
Hence holding the buffer lock over queueing means nothing can submit
it for IO at the same time. And you have to hold the buffer lock to
submit it to the delwri list:
bool
xfs_buf_delwri_queue(
struct xfs_buf *bp,
struct list_head *list)
{
>>>>> ASSERT(xfs_buf_islocked(bp));
ASSERT(!(bp->b_flags & XBF_READ));
Cheers,
Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
