Re: [PATCH] xfs: remove xfs_buf_cache.bc_lock

[Christoph Hellwig <hch@lst.de>]

On Tue, Jan 28, 2025 at 07:19:11AM +1100, Dave Chinner wrote:
> Ok, so now we can get racing inserts, which means this can find
> the buffer that has just been inserted by another thread in this
> same function. Or, indeed, and xfs_buf_lookup() call.

Yes.

> What prevents
> those racing tasks from using this buffer before the task that
> inserted it can use it?
> 
> I think that the the buffer lock being initialised to "held" and
> b_hold being initialised to 1 make this all work correctly,

Exactly, the buffer is inserted with the b_sema held and b_hold
initializes 1, aka locked and held.

> but
> comments that explicitly spell out why RCU inserts are safe
> (both in xfs_buf_alloc() for the init values and here) would be
> appreciated.

Sure.

> >  struct xfs_buf_cache {
> > -	spinlock_t		bc_lock;
> >  	struct rhashtable	bc_hash;
> >  };
> 
> At this point, the struct xfs_buf_cache structure can go away,
> right?  (separate patch and all that...)

Yes.  And in fact I think the per-pag hash should also go away, as with
the per-bucket locking there is no point in it.  I've had this patch in
my testing runs for a while, which I think is where we should be
going:

http://git.infradead.org/?p=users/hch/xfs.git;a=commitdiff;h=890cd2cd255710ee5d3408bc60792b9cdad3adfb