On Fri 15-11-19 18:54:20, Sebastian Siewior wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>
> Bit spinlocks are problematic if PREEMPT_RT is enabled, because they
> disable preemption, which is undesired for latency reasons and breaks when
> regular spinlocks are taken within the bit_spinlock locked region because
> regular spinlocks are converted to 'sleeping spinlocks' on RT. So RT
> replaces the bit spinlocks with regular spinlocks to avoid this problem.
> Bit spinlocks are also not covered by lock debugging, e.g. lockdep.
>
> Substitute the BH_Uptodate_Lock bit spinlock with a regular spinlock.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> [bigeasy: remove the wrapper and use always spinlock_t and move it into
> the padding hole]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>
> v1…v2: Move the spinlock_t to the padding hole as per Jan Kara. pahole says
> its total size remained unchanged, before
>
> | atomic_t b_count; /* 96 4 */
> |
> | /* size: 104, cachelines: 2, members: 12 */
> | /* padding: 4 */
> | /* last cacheline: 40 bytes */
>
> after
>
> | atomic_t b_count; /* 96 4 */
> | spinlock_t uptodate_lock; /* 100 4 */
> |
> | /* size: 104, cachelines: 2, members: 13 */
> | /* last cacheline: 40 bytes */
To follow the naming of other members in struct buffer_head, please name
this b_uptodate_lock (note the b_ prefix). Otherwise the patch looks good
to me so feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
after fixing this.
Honza
>
> fs/buffer.c | 19 +++++++------------
> fs/ext4/page-io.c | 8 +++-----
> fs/ntfs/aops.c | 9 +++------
> include/linux/buffer_head.h | 6 +++---
> 4 files changed, 16 insertions(+), 26 deletions(-)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 0ac4ae15ea4dc..2c6f2b41a88e4 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -277,8 +277,7 @@ static void end_buffer_async_read(struct buffer_head *bh, int uptodate)
> * decide that the page is now completely done.
> */
> first = page_buffers(page);
> - local_irq_save(flags);
> - bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
> + spin_lock_irqsave(&first->uptodate_lock, flags);
> clear_buffer_async_read(bh);
> unlock_buffer(bh);
> tmp = bh;
> @@ -291,8 +290,7 @@ static void end_buffer_async_read(struct buffer_head *bh, int uptodate)
> }
> tmp = tmp->b_this_page;
> } while (tmp != bh);
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&first->uptodate_lock, flags);
>
> /*
> * If none of the buffers had errors and they are all
> @@ -304,8 +302,7 @@ static void end_buffer_async_read(struct buffer_head *bh, int uptodate)
> return;
>
> still_busy:
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&first->uptodate_lock, flags);
> return;
> }
>
> @@ -333,8 +330,7 @@ void end_buffer_async_write(struct buffer_head *bh, int uptodate)
> }
>
> first = page_buffers(page);
> - local_irq_save(flags);
> - bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
> + spin_lock_irqsave(&first->uptodate_lock, flags);
>
> clear_buffer_async_write(bh);
> unlock_buffer(bh);
> @@ -346,14 +342,12 @@ void end_buffer_async_write(struct buffer_head *bh, int uptodate)
> }
> tmp = tmp->b_this_page;
> }
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&first->uptodate_lock, flags);
> end_page_writeback(page);
> return;
>
> still_busy:
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&first->uptodate_lock, flags);
> return;
> }
> EXPORT_SYMBOL(end_buffer_async_write);
> @@ -3422,6 +3416,7 @@ struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
> struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
> if (ret) {
> INIT_LIST_HEAD(&ret->b_assoc_buffers);
> + spin_lock_init(&ret->uptodate_lock);
> preempt_disable();
> __this_cpu_inc(bh_accounting.nr);
> recalc_bh_state();
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 893913d1c2fe3..592816713e0d6 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -125,11 +125,10 @@ static void ext4_finish_bio(struct bio *bio)
> }
> bh = head = page_buffers(page);
> /*
> - * We check all buffers in the page under BH_Uptodate_Lock
> + * We check all buffers in the page under uptodate_lock
> * to avoid races with other end io clearing async_write flags
> */
> - local_irq_save(flags);
> - bit_spin_lock(BH_Uptodate_Lock, &head->b_state);
> + spin_lock_irqsave(&head->uptodate_lock, flags);
> do {
> if (bh_offset(bh) < bio_start ||
> bh_offset(bh) + bh->b_size > bio_end) {
> @@ -141,8 +140,7 @@ static void ext4_finish_bio(struct bio *bio)
> if (bio->bi_status)
> buffer_io_error(bh);
> } while ((bh = bh->b_this_page) != head);
> - bit_spin_unlock(BH_Uptodate_Lock, &head->b_state);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&head->uptodate_lock, flags);
> if (!under_io) {
> fscrypt_free_bounce_page(bounce_page);
> end_page_writeback(page);
> diff --git a/fs/ntfs/aops.c b/fs/ntfs/aops.c
> index 7202a1e39d70c..14ca433b3a9e4 100644
> --- a/fs/ntfs/aops.c
> +++ b/fs/ntfs/aops.c
> @@ -92,8 +92,7 @@ static void ntfs_end_buffer_async_read(struct buffer_head *bh, int uptodate)
> "0x%llx.", (unsigned long long)bh->b_blocknr);
> }
> first = page_buffers(page);
> - local_irq_save(flags);
> - bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
> + spin_lock_irqsave(&first->uptodate_lock, flags);
> clear_buffer_async_read(bh);
> unlock_buffer(bh);
> tmp = bh;
> @@ -108,8 +107,7 @@ static void ntfs_end_buffer_async_read(struct buffer_head *bh, int uptodate)
> }
> tmp = tmp->b_this_page;
> } while (tmp != bh);
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&first->uptodate_lock, flags);
> /*
> * If none of the buffers had errors then we can set the page uptodate,
> * but we first have to perform the post read mst fixups, if the
> @@ -142,8 +140,7 @@ static void ntfs_end_buffer_async_read(struct buffer_head *bh, int uptodate)
> unlock_page(page);
> return;
> still_busy:
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + spin_unlock_irqrestore(&first->uptodate_lock, flags);
> return;
> }
>
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 7b73ef7f902d4..e3f8421d17bab 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -22,9 +22,6 @@ enum bh_state_bits {
> BH_Dirty, /* Is dirty */
> BH_Lock, /* Is locked */
> BH_Req, /* Has been submitted for I/O */
> - BH_Uptodate_Lock,/* Used by the first bh in a page, to serialise
> - * IO completion of other buffers in the page
> - */
>
> BH_Mapped, /* Has a disk mapping */
> BH_New, /* Disk mapping was newly created by get_block */
> @@ -76,6 +73,9 @@ struct buffer_head {
> struct address_space *b_assoc_map; /* mapping this buffer is
> associated with */
> atomic_t b_count; /* users using this buffer_head */
> + spinlock_t uptodate_lock; /* Used by the first bh in a page, to
> + * serialise IO completion of other
> + * buffers in the page */
> };
>
> /*
> --
> 2.24.0
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
