[3/4] fs/buffer: Substitute BH_Uptodate_Lock for RT and bit spinlock debugging

Bit spinlocks are problematic if PREEMPT_RT is enabled. 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.

Substitute the BH_Uptodate_Lock bit spinlock with a regular spinlock for
PREEMPT_RT enabled kernels.

Bit spinlocks are also not covered by lock debugging, e.g. lockdep. With
the spinlock substitution in place, they can be exposed via
CONFIG_DEBUG_BIT_SPINLOCKS.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
---
 fs/buffer.c                 |    1 +
 include/linux/buffer_head.h |   31 +++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+)

On Tue 30-07-19 13:24:55, Thomas Gleixner wrote:
> Bit spinlocks are problematic if PREEMPT_RT is enabled. 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.
> 
> Substitute the BH_Uptodate_Lock bit spinlock with a regular spinlock for
> PREEMPT_RT enabled kernels.
> 
> Bit spinlocks are also not covered by lock debugging, e.g. lockdep. With
> the spinlock substitution in place, they can be exposed via
> CONFIG_DEBUG_BIT_SPINLOCKS.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: "Theodore Ts'o" <tytso@mit.edu>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: linux-fsdevel@vger.kernel.org

Looks good to me. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/buffer.c                 |    1 +
>  include/linux/buffer_head.h |   31 +++++++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
> 
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -3360,6 +3360,7 @@ struct buffer_head *alloc_buffer_head(gf
>  	struct buffer_head *ret = kmem_cache_zalloc(bh_cachep, gfp_flags);
>  	if (ret) {
>  		INIT_LIST_HEAD(&ret->b_assoc_buffers);
> +		buffer_head_init_locks(ret);
>  		preempt_disable();
>  		__this_cpu_inc(bh_accounting.nr);
>  		recalc_bh_state();
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -76,8 +76,35 @@ struct buffer_head {
>  	struct address_space *b_assoc_map;	/* mapping this buffer is
>  						   associated with */
>  	atomic_t b_count;		/* users using this buffer_head */
> +
> +#if defined(CONFIG_PREEMPT_RT) || defined(CONFIG_DEBUG_BIT_SPINLOCKS)
> +	spinlock_t b_uptodate_lock;
> +#endif
>  };
>  
> +#if defined(CONFIG_PREEMPT_RT) || defined(CONFIG_DEBUG_BIT_SPINLOCKS)
> +
> +static inline unsigned long bh_uptodate_lock_irqsave(struct buffer_head *bh)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&bh->b_uptodate_lock, flags);
> +	return flags;
> +}
> +
> +static inline void
> +bh_uptodate_unlock_irqrestore(struct buffer_head *bh, unsigned long flags)
> +{
> +	spin_unlock_irqrestore(&bh->b_uptodate_lock, flags);
> +}
> +
> +static inline void buffer_head_init_locks(struct buffer_head *bh)
> +{
> +	spin_lock_init(&bh->b_uptodate_lock);
> +}
> +
> +#else /* PREEMPT_RT || DEBUG_BIT_SPINLOCKS */
> +
>  static inline unsigned long bh_uptodate_lock_irqsave(struct buffer_head *bh)
>  {
>  	unsigned long flags;
> @@ -94,6 +121,10 @@ bh_uptodate_unlock_irqrestore(struct buf
>  	local_irq_restore(flags);
>  }
>  
> +static inline void buffer_head_init_locks(struct buffer_head *bh) { }
> +
> +#endif /* !PREEMPT_RT && !DEBUG_BIT_SPINLOCKS */
> +
>  /*
>   * macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
>   * and buffer_foo() functions.
> 
> 
>