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Date: Wed, 31 Jul 2019 16:46:39 +0200
From: Jan Kara <jack@...e.cz>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Peter Zijlstra <peterz@...radead.org>,
Matthew Wilcox <willy@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Anna-Maria Gleixner <anna-maria@...utronix.de>,
Sebastian Siewior <bigeasy@...utronix.de>,
Theodore Tso <tytso@....edu>, Julia Cartwright <julia@...com>,
Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [patch 2/4] fs/buffer: Move BH_Uptodate_Lock locking into
wrapper functions
On Tue 30-07-19 13:24:54, Thomas Gleixner wrote:
> 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.
>
> To avoid ifdeffery at the source level, wrap all BH_Uptodate_Lock bitlock
> operations with inline functions, so the spinlock substitution can be done
> at one place.
>
> Using regular spinlocks can also be enabled for lock debugging purposes so
> the lock operations become visible to lockdep.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: "Theodore Ts'o" <tytso@....edu>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: linux-fsdevel@...r.kernel.org
Looks good to me. You can add:
Reviewed-by: Jan Kara <jack@...e.cz>
BTW, it should be possible to get rid of BH_Uptodate_Lock altogether using
bio chaining (which was non-existent when this bh code was written) to make
sure IO completion function gets called only once all bios used to fill in
/ write out the page are done. It would be also more efficient. But I guess
that's an interesting cleanup project for some other time...
Honza
> ---
> fs/buffer.c | 20 ++++++--------------
> fs/ext4/page-io.c | 6 ++----
> fs/ntfs/aops.c | 10 +++-------
> include/linux/buffer_head.h | 16 ++++++++++++++++
> 4 files changed, 27 insertions(+), 25 deletions(-)
>
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -275,8 +275,7 @@ static void end_buffer_async_read(struct
> * 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);
> + flags = bh_uptodate_lock_irqsave(first);
> clear_buffer_async_read(bh);
> unlock_buffer(bh);
> tmp = bh;
> @@ -289,8 +288,7 @@ static void end_buffer_async_read(struct
> }
> tmp = tmp->b_this_page;
> } while (tmp != bh);
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + bh_uptodate_unlock_irqrestore(first, flags);
>
> /*
> * If none of the buffers had errors and they are all
> @@ -302,9 +300,7 @@ static void end_buffer_async_read(struct
> return;
>
> still_busy:
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> - return;
> + bh_uptodate_unlock_irqrestore(first, flags);
> }
>
> /*
> @@ -331,8 +327,7 @@ void end_buffer_async_write(struct buffe
> }
>
> first = page_buffers(page);
> - local_irq_save(flags);
> - bit_spin_lock(BH_Uptodate_Lock, &first->b_state);
> + flags = bh_uptodate_lock_irqsave(first);
>
> clear_buffer_async_write(bh);
> unlock_buffer(bh);
> @@ -344,15 +339,12 @@ void end_buffer_async_write(struct buffe
> }
> tmp = tmp->b_this_page;
> }
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + bh_uptodate_unlock_irqrestore(first, flags);
> end_page_writeback(page);
> return;
>
> still_busy:
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> - return;
> + bh_uptodate_unlock_irqrestore(first, flags);
> }
> EXPORT_SYMBOL(end_buffer_async_write);
>
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -90,8 +90,7 @@ static void ext4_finish_bio(struct bio *
> * We check all buffers in the page under BH_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);
> + flags = bh_uptodate_lock_irqsave(head);
> 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 *
> 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);
> + bh_uptodate_unlock_irqrestore(head, flags);
> if (!under_io) {
> fscrypt_free_bounce_page(bounce_page);
> end_page_writeback(page);
> --- a/fs/ntfs/aops.c
> +++ b/fs/ntfs/aops.c
> @@ -92,8 +92,7 @@ static void ntfs_end_buffer_async_read(s
> "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);
> + flags = bh_uptodate_lock_irqsave(first);
> clear_buffer_async_read(bh);
> unlock_buffer(bh);
> tmp = bh;
> @@ -108,8 +107,7 @@ static void ntfs_end_buffer_async_read(s
> }
> tmp = tmp->b_this_page;
> } while (tmp != bh);
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> + bh_uptodate_unlock_irqrestore(first, 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
> @@ -142,9 +140,7 @@ static void ntfs_end_buffer_async_read(s
> unlock_page(page);
> return;
> still_busy:
> - bit_spin_unlock(BH_Uptodate_Lock, &first->b_state);
> - local_irq_restore(flags);
> - return;
> + bh_uptodate_unlock_irqrestore(first, flags);
> }
>
> /**
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -78,6 +78,22 @@ struct buffer_head {
> atomic_t b_count; /* users using this buffer_head */
> };
>
> +static inline unsigned long bh_uptodate_lock_irqsave(struct buffer_head *bh)
> +{
> + unsigned long flags;
> +
> + local_irq_save(flags);
> + bit_spin_lock(BH_Uptodate_Lock, &bh->b_state);
> + return flags;
> +}
> +
> +static inline void
> +bh_uptodate_unlock_irqrestore(struct buffer_head *bh, unsigned long flags)
> +{
> + bit_spin_unlock(BH_Uptodate_Lock, &bh->b_state);
> + local_irq_restore(flags);
> +}
> +
> /*
> * macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
> * and buffer_foo() functions.
>
>
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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