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Date: Tue, 19 Nov 2019 10:30:01 +0100
From: Jan Kara <jack@...e.cz>
To: Sebastian Siewior <bigeasy@...utronix.de>
Cc: Jan Kara <jack@...e.cz>, Thomas Gleixner <tglx@...utronix.de>,
Matthew Wilcox <willy@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Anna-Maria Gleixner <anna-maria@...utronix.de>,
Steven Rostedt <rostedt@...dmis.org>,
Julia Cartwright <julia@...com>, Theodore Tso <tytso@....edu>,
Alexander Viro <viro@...iv.linux.org.uk>,
Jan Kara <jack@...e.com>, Mark Fasheh <mark@...heh.com>,
Joseph Qi <joseph.qi@...ux.alibaba.com>,
Christoph Hellwig <hch@...radead.org>,
Joel Becker <jlbec@...lplan.org>
Subject: Re: [PATCH v3] fs/buffer: Make BH_Uptodate_Lock bit_spin_lock a
regular spinlock_t
On Mon 18-11-19 14:28:24, Sebastian Siewior wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
>
> 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.
>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> [bigeasy: remove the wrapper and use always spinlock_t and move it into
> the padding hole]
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
OK, how do we push this? Do you plan to push this through tip tree?
Honza
> ---
> v2…v3: rename uptodate_lock to b_uptodate_lock.
>
> 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 */
>
> 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 86a38b9793235..4baea587981e0 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -275,8 +275,7 @@ 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->b_uptodate_lock, flags);
> clear_buffer_async_read(bh);
> unlock_buffer(bh);
> tmp = bh;
> @@ -289,8 +288,7 @@ 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->b_uptodate_lock, flags);
>
> /*
> * If none of the buffers had errors and they are all
> @@ -302,8 +300,7 @@ 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->b_uptodate_lock, flags);
> return;
> }
>
> @@ -331,8 +328,7 @@ 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->b_uptodate_lock, flags);
>
> clear_buffer_async_write(bh);
> unlock_buffer(bh);
> @@ -344,14 +340,12 @@ 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->b_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->b_uptodate_lock, flags);
> return;
> }
> EXPORT_SYMBOL(end_buffer_async_write);
> @@ -3368,6 +3362,7 @@ 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->b_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 12ceadef32c5a..64d4c06fbf836 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -87,11 +87,10 @@ 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 b_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->b_uptodate_lock, flags);
> do {
> if (bh_offset(bh) < bio_start ||
> bh_offset(bh) + bh->b_size > bio_end) {
> @@ -103,8 +102,7 @@ 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->b_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..554b744f41bf8 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->b_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->b_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->b_uptodate_lock, flags);
> return;
> }
>
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 7b73ef7f902d4..e0b020eaf32e2 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 b_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@...e.com>
SUSE Labs, CR
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