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Date:   Fri, 7 Jun 2019 03:52:51 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        andrea.parri@...rulasolutions.com, peterz@...radead.org
Subject: Re: [PATCH 1/2] btrfs: Implement DRW lock

On Thu, Jun 06, 2019 at 04:52:18PM +0300, Nikolay Borisov wrote:
> A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
> to have multiple readers or multiple writers but not multiple readers
> and writers holding it concurrently. The code is factored out from
> the existing open-coded locking scheme used to exclude pending
> snapshots from nocow writers and vice-versa. Current implementation
> actually favors Readers (that is snapshot creaters) to writers (nocow
> writers of the filesystem).
> 
> Signed-off-by: Nikolay Borisov <nborisov@...e.com>

A preliminary question...

What prevents the following sequence of events from happening?

o	btrfs_drw_write_lock() invokes btrfs_drw_try_write_lock(),
	which sees that lock->readers is zero and thus executes
	percpu_counter_inc(&lock->writers).

o	btrfs_drw_read_lock() increments lock->readers, does the
	smp_mb__after_atomic(), and then does the wait_event().
	Because btrfs_drw_try_write_lock() incremented its CPU's
	lock->writers, the sum is the value one, so it blocks.

o	btrfs_drw_try_write_lock() checks lock->readers, sees that
	it is now nonzero, and thus invokes btrfs_drw_read_unlock()
	(which decrements the current CPU's counter, so that a future
	sum would get zero), and returns false.

o	btrfs_drw_write_lock() therefore does its wait_event().
	Because lock->readers is nonzero, it blocks.

o	Both tasks are now blocked.  In the absence of future calls
	to these functions (and perhaps even given such future calls),
	we have deadlock.

So what am I missing here?

							Thanx, Paul

> ---
>  fs/btrfs/Makefile   |  2 +-
>  fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/drw_lock.h | 23 +++++++++++++++
>  3 files changed, 95 insertions(+), 1 deletion(-)
>  create mode 100644 fs/btrfs/drw_lock.c
>  create mode 100644 fs/btrfs/drw_lock.h
> 
> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
> index ca693dd554e9..dc60127791e6 100644
> --- a/fs/btrfs/Makefile
> +++ b/fs/btrfs/Makefile
> @@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>  	   export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
>  	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>  	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
> -	   uuid-tree.o props.o free-space-tree.o tree-checker.o
> +	   uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o
>  
>  btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>  btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
> diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
> new file mode 100644
> index 000000000000..9681bf7544be
> --- /dev/null
> +++ b/fs/btrfs/drw_lock.c
> @@ -0,0 +1,71 @@
> +#include "drw_lock.h"
> +#include "ctree.h"
> +
> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
> +{
> +	atomic_set(&lock->readers, 0);
> +	percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
> +	init_waitqueue_head(&lock->pending_readers);
> +	init_waitqueue_head(&lock->pending_writers);
> +}
> +
> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
> +{
> +	percpu_counter_destroy(&lock->writers);
> +}
> +
> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
> +{
> +	if (atomic_read(&lock->readers))
> +		return false;
> +
> +	percpu_counter_inc(&lock->writers);
> +
> +	/*
> +	 * Ensure writers count is updated before we check for
> +	 * pending readers
> +	 */
> +	smp_mb();
> +	if (atomic_read(&lock->readers)) {
> +		btrfs_drw_read_unlock(lock);
> +		return false;
> +	}
> +
> +	return true;
> +}
> +
> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
> +{
> +	while(true) {
> +		if (btrfs_drw_try_write_lock(lock))
> +			return;
> +		wait_event(lock->pending_writers, !atomic_read(&lock->readers));
> +	}
> +}
> +
> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
> +{
> +	percpu_counter_dec(&lock->writers);
> +	cond_wake_up(&lock->pending_readers);
> +}
> +
> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
> +{
> +	atomic_inc(&lock->readers);
> +	smp_mb__after_atomic();
> +
> +	wait_event(lock->pending_readers,
> +		   percpu_counter_sum(&lock->writers) == 0);
> +}
> +
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
> +{
> +	/*
> +	 * Atomic RMW operations imply full barrier, so woken up writers
> +	 * are guaranteed to see the decrement
> +	 */
> +	if (atomic_dec_and_test(&lock->readers))
> +		wake_up(&lock->pending_writers);
> +}
> +
> +
> diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
> new file mode 100644
> index 000000000000..baff59561c06
> --- /dev/null
> +++ b/fs/btrfs/drw_lock.h
> @@ -0,0 +1,23 @@
> +#ifndef BTRFS_DRW_LOCK_H
> +#define BTRFS_DRW_LOCK_H
> +
> +#include <linux/atomic.h>
> +#include <linux/wait.h>
> +#include <linux/percpu_counter.h>
> +
> +struct btrfs_drw_lock {
> +	atomic_t readers;
> +	struct percpu_counter writers;
> +	wait_queue_head_t pending_writers;
> +	wait_queue_head_t pending_readers;
> +};
> +
> +void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
> +void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
> +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
> +bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
> +void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
> +
> +#endif
> -- 
> 2.17.1
> 

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