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From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-xfs@vger.kernel.org
Subject: Re: [PATCH 10/27] libxfs: add kernel-compatible completion API
Date: Thu, 15 Oct 2020 10:09:17 -0700
Message-ID: <20201015170917.GU9832@magnolia> (raw)
In-Reply-To: <20201015072155.1631135-11-david@fromorbit.com>

On Thu, Oct 15, 2020 at 06:21:38PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> This is needed for the kernel buffer cache conversion to be able
> to wait on IO synchrnously. It is implemented with pthread mutexes
> and conditional variables.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  include/Makefile     |  1 +
>  include/completion.h | 61 ++++++++++++++++++++++++++++++++++++++++++++
>  include/libxfs.h     |  1 +
>  libxfs/libxfs_priv.h |  1 +
>  4 files changed, 64 insertions(+)
>  create mode 100644 include/completion.h
> 
> diff --git a/include/Makefile b/include/Makefile
> index f7c40a5ce1a1..98031e70fa0d 100644
> --- a/include/Makefile
> +++ b/include/Makefile
> @@ -12,6 +12,7 @@ LIBHFILES = libxfs.h \
>  	atomic.h \
>  	bitops.h \
>  	cache.h \
> +	completion.h \
>  	hlist.h \
>  	kmem.h \
>  	list.h \
> diff --git a/include/completion.h b/include/completion.h
> new file mode 100644
> index 000000000000..92194c3f1484
> --- /dev/null
> +++ b/include/completion.h
> @@ -0,0 +1,61 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2019 RedHat, Inc.
> + * All Rights Reserved.
> + */
> +#ifndef __LIBXFS_COMPLETION_H__
> +#define __LIBXFS_COMPLETION_H__
> +
> +/*
> + * This implements kernel compatible completion semantics. This is slightly
> + * different to the way pthread conditional variables work in that completions
> + * can be signalled before the waiter tries to wait on the variable. In the
> + * pthread case, the completion is ignored and the waiter goes to sleep, whilst
> + * the kernel will see that the completion has already been completed and so
> + * will not block. This is handled through the addition of the the @signalled
> + * flag in the struct completion.

Hmm... do any of the existing pthread_cond_t users need these semantics?

I suspect the ones in scrub/vfs.c might actually be vulnerable to the
signal-before-wait race that this completion structure solves.

In any case, seeing as this primitive isn't inherent to the xfs disk
format, maybe these new concurrency management things belong in libfrog?

--D

> + */
> +struct completion {
> +	pthread_mutex_t		lock;
> +	pthread_cond_t		cond;
> +	bool			signalled; /* for kernel completion behaviour */
> +	int			waiters;
> +};
> +
> +static inline void
> +init_completion(struct completion *w)
> +{
> +	pthread_mutex_init(&w->lock, NULL);
> +	pthread_cond_init(&w->cond, NULL);
> +	w->signalled = false;
> +}
> +
> +static inline void
> +complete(struct completion *w)
> +{
> +	pthread_mutex_lock(&w->lock);
> +	w->signalled = true;
> +	pthread_cond_broadcast(&w->cond);
> +	pthread_mutex_unlock(&w->lock);
> +}
> +
> +/*
> + * Support for mulitple waiters requires that we count the number of waiters
> + * we have and only clear the signalled variable once all those waiters have
> + * been woken.
> + */
> +static inline void
> +wait_for_completion(struct completion *w)
> +{
> +	pthread_mutex_lock(&w->lock);
> +	if (!w->signalled) {
> +		w->waiters++;
> +		pthread_cond_wait(&w->cond, &w->lock);
> +		w->waiters--;
> +	}
> +	if (!w->waiters)
> +		w->signalled = false;
> +	pthread_mutex_unlock(&w->lock);
> +}
> +
> +#endif /* __LIBXFS_COMPLETION_H__ */
> diff --git a/include/libxfs.h b/include/libxfs.h
> index caf4a5139469..d03ec8aeaf5c 100644
> --- a/include/libxfs.h
> +++ b/include/libxfs.h
> @@ -19,6 +19,7 @@
>  #include "libfrog/radix-tree.h"
>  #include "atomic.h"
>  #include "spinlock.h"
> +#include "completion.h"
>  
>  #include "xfs_types.h"
>  #include "xfs_fs.h"
> diff --git a/libxfs/libxfs_priv.h b/libxfs/libxfs_priv.h
> index e134f65c5dd1..5cbc4fe69732 100644
> --- a/libxfs/libxfs_priv.h
> +++ b/libxfs/libxfs_priv.h
> @@ -49,6 +49,7 @@
>  #include "libfrog/radix-tree.h"
>  #include "atomic.h"
>  #include "spinlock.h"
> +#include "completion.h"
>  
>  #include "xfs_types.h"
>  #include "xfs_arch.h"
> -- 
> 2.28.0
> 

  reply index

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-15  7:21 [PATCH 00/27] [RFC, WIP] xfsprogs: xfs_buf unification and AIO Dave Chinner
2020-10-15  7:21 ` [PATCH 01/27] xfsprogs: remove unused buffer tracing code Dave Chinner
2020-10-15  7:21 ` [PATCH 02/27] xfsprogs: remove unused IO_DEBUG functionality Dave Chinner
2020-11-16  2:31   ` Eric Sandeen
2020-10-15  7:21 ` [PATCH 03/27] libxfs: get rid of b_bcount from xfs_buf Dave Chinner
2020-11-23 19:53   ` Eric Sandeen
2020-10-15  7:21 ` [PATCH 04/27] libxfs: rename buftarg->dev to btdev Dave Chinner
2020-11-16  2:33   ` Eric Sandeen
2020-10-15  7:21 ` [PATCH 05/27] xfsprogs: get rid of ancient btree tracing fragments Dave Chinner
2020-11-16  2:35   ` Eric Sandeen
2020-10-15  7:21 ` [PATCH 06/27] xfsprogs: remove xfs_buf_t typedef Dave Chinner
2020-10-15 15:22   ` Darrick J. Wong
2020-10-15 20:54     ` Dave Chinner
2020-10-15  7:21 ` [PATCH 07/27] xfsprogs: introduce liburcu support Dave Chinner
2020-10-15  7:21 ` [PATCH 08/27] libxfs: add spinlock_t wrapper Dave Chinner
2020-10-15  7:21 ` [PATCH 09/27] atomic: convert to uatomic Dave Chinner
2020-10-15  7:21 ` [PATCH 10/27] libxfs: add kernel-compatible completion API Dave Chinner
2020-10-15 17:09   ` Darrick J. Wong [this message]
2020-10-19 22:21     ` Dave Chinner
2020-10-15  7:21 ` [PATCH 11/27] libxfs: add wrappers for kernel semaphores Dave Chinner
2020-10-15  7:21 ` [PATCH 12/27] xfsprogs: convert use-once buffer reads to uncached IO Dave Chinner
2020-10-15 17:12   ` Darrick J. Wong
2020-10-19 22:36     ` Dave Chinner
2020-10-15  7:21 ` [PATCH 13/27] libxfs: introduce userspace buftarg infrastructure Dave Chinner
2020-10-15  7:21 ` [PATCH 14/27] xfs: rename libxfs_buftarg_init to libxfs_open_devices() Dave Chinner
2020-10-15  7:21 ` [PATCH 15/27] libxfs: introduce userspace buftarg infrastructure Dave Chinner
2020-10-15 17:16   ` Darrick J. Wong
2020-10-15  7:21 ` [PATCH 16/27] libxfs: add a synchronous IO engine to the buftarg Dave Chinner
2020-10-15  7:21 ` [PATCH 17/27] xfsprogs: convert libxfs_readbufr to libxfs_buf_read_uncached Dave Chinner
2020-10-15  7:21 ` [PATCH 18/27] libxfs: convert libxfs_bwrite to buftarg IO Dave Chinner
2020-10-15  7:21 ` [PATCH 19/27] libxfs: add cache infrastructure to buftarg Dave Chinner
2020-10-15  7:21 ` [PATCH 20/27] libxfs: add internal lru to btcache Dave Chinner
2020-10-15  7:21 ` [PATCH 21/27] libxfs: Add kernel list_lru wrapper Dave Chinner
2020-10-15  7:21 ` [PATCH 22/27] libxfs: introduce new buffer cache infrastructure Dave Chinner
2020-10-15 17:46   ` Darrick J. Wong
2020-10-15  7:21 ` [PATCH 23/27] libxfs: use PSI information to detect memory pressure Dave Chinner
2020-10-15 17:56   ` Darrick J. Wong
2020-10-15 21:20     ` Dave Chinner
2020-10-15  7:21 ` [PATCH 24/27] libxfs: add a buftarg cache shrinker implementation Dave Chinner
2020-10-15 18:01   ` Darrick J. Wong
2020-10-15 21:33     ` Dave Chinner
2020-10-15  7:21 ` [PATCH 25/27] libxfs: switch buffer cache implementations Dave Chinner
2020-10-15  7:21 ` [PATCH 26/27] build: set platform_defs.h.in dependency correctly Dave Chinner
2020-10-15  7:21 ` [PATCH 27/27] libxfs: convert sync IO buftarg engine to AIO Dave Chinner
2020-10-15 18:26   ` Darrick J. Wong
2020-10-15 21:42     ` Dave Chinner
2020-10-15  7:29 ` [PATCH 00/27] [RFC, WIP] xfsprogs: xfs_buf unification and AIO Dave Chinner
2020-10-15 18:37 ` Darrick J. Wong
2020-10-15 22:35   ` Dave Chinner

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