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From: Christoph Hellwig <hch@lst.de>
To: Waiman Long <longman@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-ext4@vger.kernel.org, cluster-devel@redhat.com,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 02/12] locking/rwsem: Exit early when held by an anonymous owner
Date: Tue, 14 Jan 2020 19:25:14 +0100	[thread overview]
Message-ID: <20200114182514.GA9949@lst.de> (raw)
In-Reply-To: <925d1343-670e-8f92-0e73-6e9cee0d3ffb@redhat.com>

On Tue, Jan 14, 2020 at 01:17:45PM -0500, Waiman Long wrote:
> The owner field is just a pointer to the task structure with the lower 3
> bits served as flag bits. Setting owner to RWSEM_OWNER_UNKNOWN (-2) will
> stop optimistic spinning. So under what condition did the crash happen?

When running xfstests with all patches in this series except for this
one, IIRC in generic/114.

> Anyway, PeterZ is working on revising the percpu-rwsem implementation to
> more gracefully handle the frozen case. At the end, there will not be a
> need for the RWSEM_OWNER_UNKNOWN magic and it can be removed.

Well, this series relies on that value.  And I think it fundamentally
is the right thing to do for AIO, and potentially other I/O related
locking where we take a lock to synchronize access to data, then
do I/O and then eventually get an I/O completion from an interrupt.
Even thinking from the PREEMP_RT context we want to boost the
initial thread as long as we can, then do nothing when it is off
to I/O hardware (except maybe providing good diagnostics that the cause
for the latency is I/O), and then boost the thread that is handling
the completion.  Things like the i_dio_count hack can't provide that.

WARNING: multiple messages have this Message-ID (diff)
From: Christoph Hellwig <hch@lst.de>
To: cluster-devel.redhat.com
Subject: [Cluster-devel] [PATCH 02/12] locking/rwsem: Exit early when held by an anonymous owner
Date: Tue, 14 Jan 2020 19:25:14 +0100	[thread overview]
Message-ID: <20200114182514.GA9949@lst.de> (raw)
In-Reply-To: <925d1343-670e-8f92-0e73-6e9cee0d3ffb@redhat.com>

On Tue, Jan 14, 2020 at 01:17:45PM -0500, Waiman Long wrote:
> The owner field is just a pointer to the task structure with the lower 3
> bits served as flag bits. Setting owner to RWSEM_OWNER_UNKNOWN (-2) will
> stop optimistic spinning. So under what condition did the crash happen?

When running xfstests with all patches in this series except for this
one, IIRC in generic/114.

> Anyway, PeterZ is working on revising the percpu-rwsem implementation to
> more gracefully handle the frozen case. At the end, there will not be a
> need for the RWSEM_OWNER_UNKNOWN magic and it can be removed.

Well, this series relies on that value.  And I think it fundamentally
is the right thing to do for AIO, and potentially other I/O related
locking where we take a lock to synchronize access to data, then
do I/O and then eventually get an I/O completion from an interrupt.
Even thinking from the PREEMP_RT context we want to boost the
initial thread as long as we can, then do nothing when it is off
to I/O hardware (except maybe providing good diagnostics that the cause
for the latency is I/O), and then boost the thread that is handling
the completion.  Things like the i_dio_count hack can't provide that.




  reply	other threads:[~2020-01-14 18:25 UTC|newest]

Thread overview: 80+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 16:12 RFC: hold i_rwsem until aio completes Christoph Hellwig
2020-01-14 16:12 ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 01/12] mm: fix a comment in sys_swapon Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-02-10 23:29   ` Andrew Morton
2020-02-10 23:29     ` [Cluster-devel] " Andrew Morton
2020-02-12  7:37     ` Christoph Hellwig
2020-02-12  7:37       ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 02/12] locking/rwsem: Exit early when held by an anonymous owner Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 18:17   ` Waiman Long
2020-01-14 18:17     ` [Cluster-devel] " Waiman Long
2020-01-14 18:25     ` Christoph Hellwig [this message]
2020-01-14 18:25       ` Christoph Hellwig
2020-01-14 18:33       ` Waiman Long
2020-01-14 18:33         ` [Cluster-devel] " Waiman Long
2020-01-14 18:55       ` Waiman Long
2020-01-14 18:55         ` [Cluster-devel] " Waiman Long
2020-01-14 16:12 ` [PATCH 03/12] xfs: fix IOCB_NOWAIT handling in xfs_file_dio_aio_read Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 04/12] gfs2: move setting current->backing_dev_info Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 05/12] gfs2: fix O_SYNC write handling Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-27  9:03   ` Christoph Hellwig
2020-01-28 16:57     ` Bob Peterson
2020-02-06 15:31   ` Andreas Gruenbacher
2020-02-06 15:31     ` Andreas Gruenbacher
2020-02-06 15:31     ` Andreas Gruenbacher
2020-01-14 16:12 ` [PATCH 06/12] iomap: pass a flags value to iomap_dio_rw Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 07/12] iomap: allow holding i_rwsem until aio completion Christoph Hellwig
2020-01-14 16:12 ` [PATCH 08/12] ext4: hold i_rwsem until AIO completes Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 21:50   ` Theodore Y. Ts'o
2020-01-14 21:50     ` [Cluster-devel] " Theodore Y. Ts'o
2020-01-15  6:48     ` Christoph Hellwig
2020-01-15  6:48       ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 09/12] gfs2: " Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 10/12] xfs: " Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 11/12] xfs: don't set IOMAP_DIO_SYNCHRONOUS for unaligned I/O Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 16:12 ` [PATCH 12/12] iomap: remove the inode_dio_begin/end calls Christoph Hellwig
2020-01-14 16:12   ` [Cluster-devel] " Christoph Hellwig
2020-01-14 18:47 ` RFC: hold i_rwsem until aio completes Matthew Wilcox
2020-01-14 18:47   ` [Cluster-devel] " Matthew Wilcox
2020-01-15  6:54   ` Christoph Hellwig
2020-01-15  6:54     ` [Cluster-devel] " Christoph Hellwig
2020-01-14 19:27 ` Jason Gunthorpe
2020-01-14 19:27   ` [Cluster-devel] " Jason Gunthorpe
2020-01-15  6:56   ` Christoph Hellwig
2020-01-15  6:56     ` [Cluster-devel] " Christoph Hellwig
2020-01-15 13:24     ` Jason Gunthorpe
2020-01-15 13:24       ` [Cluster-devel] " Jason Gunthorpe
2020-01-15 14:33       ` Peter Zijlstra
2020-01-15 14:33         ` [Cluster-devel] " Peter Zijlstra
2020-01-15 14:49         ` Jason Gunthorpe
2020-01-15 14:49           ` [Cluster-devel] " Jason Gunthorpe
2020-01-15 19:03           ` Waiman Long
2020-01-15 19:03             ` [Cluster-devel] " Waiman Long
2020-01-15 19:07             ` Christoph Hellwig
2020-01-15 19:07               ` [Cluster-devel] " Christoph Hellwig
2020-01-18 22:40         ` Matthew Wilcox
2020-01-18 22:40           ` [Cluster-devel] " Matthew Wilcox
2020-01-15 15:36       ` Christoph Hellwig
2020-01-15 15:36         ` [Cluster-devel] " Christoph Hellwig
2020-01-15 16:26         ` Jason Gunthorpe
2020-01-15 16:26           ` [Cluster-devel] " Jason Gunthorpe
2020-01-16 14:00 ` Jan Kara
2020-01-16 14:00   ` [Cluster-devel] " Jan Kara
2020-02-03 17:44   ` Christoph Hellwig
2020-02-03 17:44     ` [Cluster-devel] " Christoph Hellwig
2020-01-18  9:28 ` Dave Chinner
2020-01-18  9:28   ` [Cluster-devel] " Dave Chinner
2020-02-03 17:46   ` Christoph Hellwig
2020-02-03 17:46     ` [Cluster-devel] " Christoph Hellwig
2020-02-03 23:02     ` Dave Chinner
2020-02-03 23:02       ` [Cluster-devel] " Dave Chinner

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