Hmm.. seems like something is wrong either with a mailer (yours or mine) or the list. I initially thought I replied to this and accidentally dropped the list cc, but now that I try again, mutt drops the list on reply-to-all. It shows the following for the original mail, so somehow this ends up garbled in the cc list for whatever reason.
To: unlisted-recipients: no To-header on input <; Anyways, here's a resend of my previous reply that unintentionally dropped the list. On Wed, Sep 13, 2023 at 08:37:46PM -0400, Kent Overstreet wrote: > bucket_lock() previously open coded a spinlock, because we need to cram > a spinlock into a single byte. > > But it turns out not all archs support xchg() on a single byte; since we > need struct bucket to be small, this means we have to play fun games > with casts and ifdefs for endianness. > > This fixes building on 32 bit arm, and likely other architectures. > > Signed-off-by: Kent Overstreet <[email protected]> > Cc: [email protected] > --- > fs/bcachefs/buckets.h | 26 +++++++++++++++++++++++--- > 1 file changed, 23 insertions(+), 3 deletions(-) > > diff --git a/fs/bcachefs/buckets.h b/fs/bcachefs/buckets.h > index f192809f50cf..e055c1076e63 100644 > --- a/fs/bcachefs/buckets.h > +++ b/fs/bcachefs/buckets.h > @@ -40,15 +40,35 @@ static inline size_t sector_to_bucket_and_offset(const > struct bch_dev *ca, secto > for (_b = (_buckets)->b + (_buckets)->first_bucket; \ > _b < (_buckets)->b + (_buckets)->nbuckets; _b++) > > +/* > + * Ugly hack alert: > + * > + * We need to cram a spinlock in a single byte, because that's what we have > left > + * in struct bucket, and we care about the size of these - during fsck, we > need > + * in memory state for every single bucket on every device. > + * > + * We used to do > + * while (xchg(&b->lock, 1) cpu_relax(); > + * but, it turns out not all architectures support xchg on a single byte. > + * > + * So now we use bit_spin_lock(), with fun games since we can't burn a whole > + * ulong for this. > + */ > + Oof. :P Well I think I understand what this is doing, but it would be helpful if this last sentence were a little more direct. For example: "So now we use bit_spin_lock(). We can't burn a whole ulong for this, so cast and define the lock bit such that it always lands in the b->lock byte." ... but feel free to reword that, of course. > +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ > +#define BUCKET_LOCK_BITNR 0 > +#else > +#define BUCKET_LOCK_BITNR (BITS_PER_LONG - 1) > +#endif > + > static inline void bucket_unlock(struct bucket *b) > { > - smp_store_release(&b->lock, 0); > + bit_unspin_lock(BUCKET_LOCK_BITNR, (void *) &b->lock); This doesn't compile.. bit_spin_unlock() I assume. Also, is there any good way to add a simple debug mode check here just to confirm the external code does what we expect on whatever obscure/otherwise untested arch somebody might try to use? I.e. EBUG_ON(b->lock != 1) or some such after acquiring the lock..? Brian > } > > static inline void bucket_lock(struct bucket *b) > { > - while (xchg(&b->lock, 1)) > - cpu_relax(); > + bit_spin_lock(BUCKET_LOCK_BITNR, (void *) &b->lock); > } > > static inline struct bucket_array *gc_bucket_array(struct bch_dev *ca) > -- > 2.40.1 >
