Re: [PATCH v8 4/4] xfs: replace mrlock_t with rw_semaphores

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Oct 05, 2020 at 11:38:52PM +0200, Pavel Reichl wrote:
> Remove mrlock_t as it does not provide any extra value over
> rw_semaphores. Make i_lock and i_mmaplock native rw_semaphores and
> replace mr*() functions with native rwsem calls.
> 
> Release the lock in xfs_btree_split() just before the work-queue
> executing xfs_btree_split_worker() is scheduled and make
> xfs_btree_split_worker() to acquire the lock as a first thing and
> release it just before returning from the function. This it done so the
> ownership of the lock is transfered between kernel threads and thus
> lockdep won't complain about lock being held by a different kernel
> thread.
> 
> Signed-off-by: Pavel Reichl <preichl@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_btree.c | 10 +++++
>  fs/xfs/mrlock.h           | 78 ---------------------------------------
>  fs/xfs/xfs_inode.c        | 36 ++++++++++--------
>  fs/xfs/xfs_inode.h        |  4 +-
>  fs/xfs/xfs_iops.c         |  4 +-
>  fs/xfs/xfs_linux.h        |  2 +-
>  fs/xfs/xfs_super.c        |  6 +--
>  7 files changed, 37 insertions(+), 103 deletions(-)
>  delete mode 100644 fs/xfs/mrlock.h
> 
> diff --git a/fs/xfs/libxfs/xfs_btree.c b/fs/xfs/libxfs/xfs_btree.c
> index 2d25bab68764..d798d288eed1 100644
> --- a/fs/xfs/libxfs/xfs_btree.c
> +++ b/fs/xfs/libxfs/xfs_btree.c
> @@ -2816,6 +2816,7 @@ xfs_btree_split_worker(
>  	unsigned long		pflags;
>  	unsigned long		new_pflags = PF_MEMALLOC_NOFS;
>  
> +	rwsem_acquire(&args->cur->bc_ino.ip->i_lock.dep_map, 0, 0, _RET_IP_);
>  	/*
>  	 * we are in a transaction context here, but may also be doing work
>  	 * in kswapd context, and hence we may need to inherit that state
> @@ -2832,6 +2833,7 @@ xfs_btree_split_worker(
>  	complete(args->done);
>  
>  	current_restore_flags_nested(&pflags, new_pflags);
> +	rwsem_release(&args->cur->bc_ino.ip->i_lock.dep_map, _THIS_IP_);
>  }
>  
>  /*
> @@ -2863,8 +2865,16 @@ xfs_btree_split(
>  	args.done = &done;
>  	args.kswapd = current_is_kswapd();
>  	INIT_WORK_ONSTACK(&args.work, xfs_btree_split_worker);
> +
> +	/* Release the lock so it can be acquired in the kernel thread which

Strange comment style.

Also, we're not really releasing the lock itself here, right?  We're
merely updating lockdep's bookkeepping so that the worker can make
itself look like the lock owner (to lockdep, anyway).

Does this exist as a helper anywhere in the kernel?  I don't really like
XFS poking into the rw_semaphore innards, though I concede that this
lock transferring dance is probably pretty rare.

--D

> +	 * executes the xfs_btree_split_worker.
> +	 */
> +	rwsem_release(&cur->bc_ino.ip->i_lock.dep_map, _THIS_IP_);
>  	queue_work(xfs_alloc_wq, &args.work);
> +
>  	wait_for_completion(&done);
> +	/* Re-aqcquire the lock previously released for the work queue. */
> +	rwsem_acquire(&cur->bc_ino.ip->i_lock.dep_map, 0, 0, _RET_IP_);
>  	destroy_work_on_stack(&args.work);
>  	return args.result;
>  }
> diff --git a/fs/xfs/mrlock.h b/fs/xfs/mrlock.h
> deleted file mode 100644
> index 79155eec341b..000000000000
> --- a/fs/xfs/mrlock.h
> +++ /dev/null
> @@ -1,78 +0,0 @@
> -// SPDX-License-Identifier: GPL-2.0
> -/*
> - * Copyright (c) 2000-2006 Silicon Graphics, Inc.
> - * All Rights Reserved.
> - */
> -#ifndef __XFS_SUPPORT_MRLOCK_H__
> -#define __XFS_SUPPORT_MRLOCK_H__
> -
> -#include <linux/rwsem.h>
> -
> -typedef struct {
> -	struct rw_semaphore	mr_lock;
> -#if defined(DEBUG) || defined(XFS_WARN)
> -	int			mr_writer;
> -#endif
> -} mrlock_t;
> -
> -#if defined(DEBUG) || defined(XFS_WARN)
> -#define mrinit(mrp, name)	\
> -	do { (mrp)->mr_writer = 0; init_rwsem(&(mrp)->mr_lock); } while (0)
> -#else
> -#define mrinit(mrp, name)	\
> -	do { init_rwsem(&(mrp)->mr_lock); } while (0)
> -#endif
> -
> -#define mrlock_init(mrp, t,n,s)	mrinit(mrp, n)
> -#define mrfree(mrp)		do { } while (0)
> -
> -static inline void mraccess_nested(mrlock_t *mrp, int subclass)
> -{
> -	down_read_nested(&mrp->mr_lock, subclass);
> -}
> -
> -static inline void mrupdate_nested(mrlock_t *mrp, int subclass)
> -{
> -	down_write_nested(&mrp->mr_lock, subclass);
> -#if defined(DEBUG) || defined(XFS_WARN)
> -	mrp->mr_writer = 1;
> -#endif
> -}
> -
> -static inline int mrtryaccess(mrlock_t *mrp)
> -{
> -	return down_read_trylock(&mrp->mr_lock);
> -}
> -
> -static inline int mrtryupdate(mrlock_t *mrp)
> -{
> -	if (!down_write_trylock(&mrp->mr_lock))
> -		return 0;
> -#if defined(DEBUG) || defined(XFS_WARN)
> -	mrp->mr_writer = 1;
> -#endif
> -	return 1;
> -}
> -
> -static inline void mrunlock_excl(mrlock_t *mrp)
> -{
> -#if defined(DEBUG) || defined(XFS_WARN)
> -	mrp->mr_writer = 0;
> -#endif
> -	up_write(&mrp->mr_lock);
> -}
> -
> -static inline void mrunlock_shared(mrlock_t *mrp)
> -{
> -	up_read(&mrp->mr_lock);
> -}
> -
> -static inline void mrdemote(mrlock_t *mrp)
> -{
> -#if defined(DEBUG) || defined(XFS_WARN)
> -	mrp->mr_writer = 0;
> -#endif
> -	downgrade_write(&mrp->mr_lock);
> -}
> -
> -#endif /* __XFS_SUPPORT_MRLOCK_H__ */
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 49d296877494..dbc1b80629c5 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -191,14 +191,15 @@ xfs_ilock(
>  	}
>  
>  	if (lock_flags & XFS_MMAPLOCK_EXCL)
> -		mrupdate_nested(&ip->i_mmaplock, XFS_MMAPLOCK_DEP(lock_flags));
> +		down_write_nested(&ip->i_mmaplock,
> +				XFS_MMAPLOCK_DEP(lock_flags));
>  	else if (lock_flags & XFS_MMAPLOCK_SHARED)
> -		mraccess_nested(&ip->i_mmaplock, XFS_MMAPLOCK_DEP(lock_flags));
> +		down_read_nested(&ip->i_mmaplock, XFS_MMAPLOCK_DEP(lock_flags));
>  
>  	if (lock_flags & XFS_ILOCK_EXCL)
> -		mrupdate_nested(&ip->i_lock, XFS_ILOCK_DEP(lock_flags));
> +		down_write_nested(&ip->i_lock, XFS_ILOCK_DEP(lock_flags));
>  	else if (lock_flags & XFS_ILOCK_SHARED)
> -		mraccess_nested(&ip->i_lock, XFS_ILOCK_DEP(lock_flags));
> +		down_read_nested(&ip->i_lock, XFS_ILOCK_DEP(lock_flags));
>  }
>  
>  /*
> @@ -242,27 +243,27 @@ xfs_ilock_nowait(
>  	}
>  
>  	if (lock_flags & XFS_MMAPLOCK_EXCL) {
> -		if (!mrtryupdate(&ip->i_mmaplock))
> +		if (!down_write_trylock(&ip->i_mmaplock))
>  			goto out_undo_iolock;
>  	} else if (lock_flags & XFS_MMAPLOCK_SHARED) {
> -		if (!mrtryaccess(&ip->i_mmaplock))
> +		if (!down_read_trylock(&ip->i_mmaplock))
>  			goto out_undo_iolock;
>  	}
>  
>  	if (lock_flags & XFS_ILOCK_EXCL) {
> -		if (!mrtryupdate(&ip->i_lock))
> +		if (!down_write_trylock(&ip->i_lock))
>  			goto out_undo_mmaplock;
>  	} else if (lock_flags & XFS_ILOCK_SHARED) {
> -		if (!mrtryaccess(&ip->i_lock))
> +		if (!down_read_trylock(&ip->i_lock))
>  			goto out_undo_mmaplock;
>  	}
>  	return 1;
>  
>  out_undo_mmaplock:
>  	if (lock_flags & XFS_MMAPLOCK_EXCL)
> -		mrunlock_excl(&ip->i_mmaplock);
> +		up_write(&ip->i_mmaplock);
>  	else if (lock_flags & XFS_MMAPLOCK_SHARED)
> -		mrunlock_shared(&ip->i_mmaplock);
> +		up_read(&ip->i_mmaplock);
>  out_undo_iolock:
>  	if (lock_flags & XFS_IOLOCK_EXCL)
>  		up_write(&VFS_I(ip)->i_rwsem);
> @@ -309,14 +310,14 @@ xfs_iunlock(
>  		up_read(&VFS_I(ip)->i_rwsem);
>  
>  	if (lock_flags & XFS_MMAPLOCK_EXCL)
> -		mrunlock_excl(&ip->i_mmaplock);
> +		up_write(&ip->i_mmaplock);
>  	else if (lock_flags & XFS_MMAPLOCK_SHARED)
> -		mrunlock_shared(&ip->i_mmaplock);
> +		up_read(&ip->i_mmaplock);
>  
>  	if (lock_flags & XFS_ILOCK_EXCL)
> -		mrunlock_excl(&ip->i_lock);
> +		up_write(&ip->i_lock);
>  	else if (lock_flags & XFS_ILOCK_SHARED)
> -		mrunlock_shared(&ip->i_lock);
> +		up_read(&ip->i_lock);
>  
>  	trace_xfs_iunlock(ip, lock_flags, _RET_IP_);
>  }
> @@ -335,9 +336,9 @@ xfs_ilock_demote(
>  		~(XFS_IOLOCK_EXCL|XFS_MMAPLOCK_EXCL|XFS_ILOCK_EXCL)) == 0);
>  
>  	if (lock_flags & XFS_ILOCK_EXCL)
> -		mrdemote(&ip->i_lock);
> +		downgrade_write(&ip->i_lock);
>  	if (lock_flags & XFS_MMAPLOCK_EXCL)
> -		mrdemote(&ip->i_mmaplock);
> +		downgrade_write(&ip->i_mmaplock);
>  	if (lock_flags & XFS_IOLOCK_EXCL)
>  		downgrade_write(&VFS_I(ip)->i_rwsem);
>  
> @@ -385,11 +386,14 @@ xfs_isilocked(
>  	uint			lock_flags)
>  {
>  	if (lock_flags & (XFS_ILOCK_EXCL | XFS_ILOCK_SHARED)) {
> +		ASSERT(!(lock_flags & ~(XFS_ILOCK_EXCL | XFS_ILOCK_SHARED)));
>  		return __xfs_rwsem_islocked(&ip->i_lock,
>  				(lock_flags >> XFS_ILOCK_FLAG_SHIFT));
>  	}
>  
>  	if (lock_flags & (XFS_MMAPLOCK_EXCL | XFS_MMAPLOCK_SHARED)) {
> +		ASSERT(!(lock_flags &
> +			~(XFS_MMAPLOCK_EXCL | XFS_MMAPLOCK_SHARED)));
>  		return __xfs_rwsem_islocked(&ip->i_mmaplock,
>  				(lock_flags >> XFS_MMAPLOCK_FLAG_SHIFT));
>  	}
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 77d5655191ab..02c98ecfe4c5 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -39,8 +39,8 @@ typedef struct xfs_inode {
>  
>  	/* Transaction and locking information. */
>  	struct xfs_inode_log_item *i_itemp;	/* logging information */
> -	mrlock_t		i_lock;		/* inode lock */
> -	mrlock_t		i_mmaplock;	/* inode mmap IO lock */
> +	struct rw_semaphore	i_lock;		/* inode lock */
> +	struct rw_semaphore	i_mmaplock;	/* inode mmap IO lock */
>  	atomic_t		i_pincount;	/* inode pin count */
>  
>  	/*
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 80a13c8561d8..66cca3e599c7 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -1336,9 +1336,9 @@ xfs_setup_inode(
>  		 */
>  		lockdep_set_class(&inode->i_rwsem,
>  				  &inode->i_sb->s_type->i_mutex_dir_key);
> -		lockdep_set_class(&ip->i_lock.mr_lock, &xfs_dir_ilock_class);
> +		lockdep_set_class(&ip->i_lock, &xfs_dir_ilock_class);
>  	} else {
> -		lockdep_set_class(&ip->i_lock.mr_lock, &xfs_nondir_ilock_class);
> +		lockdep_set_class(&ip->i_lock, &xfs_nondir_ilock_class);
>  	}
>  
>  	/*
> diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
> index ab737fed7b12..ba37217f86d2 100644
> --- a/fs/xfs/xfs_linux.h
> +++ b/fs/xfs/xfs_linux.h
> @@ -22,7 +22,6 @@ typedef __u32			xfs_nlink_t;
>  #include "xfs_types.h"
>  
>  #include "kmem.h"
> -#include "mrlock.h"
>  
>  #include <linux/semaphore.h>
>  #include <linux/mm.h>
> @@ -61,6 +60,7 @@ typedef __u32			xfs_nlink_t;
>  #include <linux/ratelimit.h>
>  #include <linux/rhashtable.h>
>  #include <linux/xattr.h>
> +#include <linux/rwsem.h>
>  
>  #include <asm/page.h>
>  #include <asm/div64.h>
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index 71ac6c1cdc36..00be9cfa29fa 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -708,10 +708,8 @@ xfs_fs_inode_init_once(
>  	atomic_set(&ip->i_pincount, 0);
>  	spin_lock_init(&ip->i_flags_lock);
>  
> -	mrlock_init(&ip->i_mmaplock, MRLOCK_ALLOW_EQUAL_PRI|MRLOCK_BARRIER,
> -		     "xfsino", ip->i_ino);
> -	mrlock_init(&ip->i_lock, MRLOCK_ALLOW_EQUAL_PRI|MRLOCK_BARRIER,
> -		     "xfsino", ip->i_ino);
> +	init_rwsem(&ip->i_mmaplock);
> +	init_rwsem(&ip->i_lock);
>  }
>  
>  /*
> -- 
> 2.26.2
>