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From: Dave Chinner <david@fromorbit.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>,
	linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-xfs@vger.kernel.org, Ted Tso <tytso@mit.edu>,
	Christoph Hellwig <hch@infradead.org>,
	Amir Goldstein <amir73il@gmail.com>
Subject: Re: [PATCH 2/7] mm: Protect operations adding pages to page cache with i_mapping_lock
Date: Thu, 15 Apr 2021 12:05:20 +1000
Message-ID: <20210415020520.GI63242@dread.disaster.area> (raw)
In-Reply-To: <20210414222531.GZ2531743@casper.infradead.org>

On Wed, Apr 14, 2021 at 11:25:31PM +0100, Matthew Wilcox wrote:
> On Wed, Apr 14, 2021 at 10:01:13AM +1000, Dave Chinner wrote:
> > > +	if (iocb->ki_flags & IOCB_NOWAIT) {
> > > +		if (!down_read_trylock(&mapping->host->i_mapping_sem))
> > > +			return -EAGAIN;
> > > +	} else {
> > > +		down_read(&mapping->host->i_mapping_sem);
> > > +	}
> > 
> > We really need a lock primitive for this. The number of times this
> > exact lock pattern is being replicated all through the IO path is
> > getting out of hand.
> > 
> > static inline bool
> > down_read_try_or_lock(struct rwsem *sem, bool try)
> > {
> > 	if (try) {
> > 		if (!down_read_trylock(sem))
> > 			return false;
> > 	} else {
> > 		down_read(&mapping->host->i_mapping_sem);
> > 	}
> > 	return true;
> > }
> > 
> > and the callers become:
> > 
> > 	if (!down_read_try_or_lock(sem, (iocb->ki_flags & IOCB_NOWAIT)))
> > 		return -EAGAIN;
> 
> I think that should be written:
> 
> 	if (!iocb_read_lock(iocb, &rwsem))
> 		return -EAGAIN;
> 
> and implemented as:
> 
> static inline int iocb_read_lock(struct kiocb *iocb, struct rwsem *sem)
> {
> 	if (iocb->ki_flags & IOCB_NOWAIT)
> 		return down_read_trylock(sem) ? 0 : -EAGAIN;
> 	return down_read_killable(sem);
> }

Yup, we already have done that with xfs_ilock_iocb(), but my point
is that this "non blocking try lock or lock" pattern is slowly being
used in more places than just IOCB_NOWAIT situations.  e.g. We use
if for IOMAP_NOWAIT locking in XFS, too, and ISTR other places where
optimisitic locking is used are replicating it, too.

Hence my suggestion that is moved up into the locking primitives,
not merely have context specific wrappers added...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-13 11:28 [PATCH 0/7 RFC v3] fs: Hole punch vs page cache filling races Jan Kara
2021-04-13 11:28 ` [PATCH 1/7] mm: Fix comments mentioning i_mutex Jan Kara
2021-04-13 12:38   ` Christoph Hellwig
2021-04-13 11:28 ` [PATCH 2/7] mm: Protect operations adding pages to page cache with i_mapping_lock Jan Kara
2021-04-13 12:57   ` Christoph Hellwig
2021-04-13 13:56     ` Jan Kara
2021-04-14  0:01   ` Dave Chinner
2021-04-14 12:23     ` Jan Kara
2021-04-14 21:57       ` Dave Chinner
2021-04-15 13:11         ` Jan Kara
2021-04-14 22:25     ` Matthew Wilcox
2021-04-15  2:05       ` Dave Chinner [this message]
2021-04-13 11:28 ` [PATCH 3/7] ext4: Convert to use inode->i_mapping_sem Jan Kara
2021-04-13 11:28 ` [PATCH 4/7] ext2: Convert to using i_mapping_sem Jan Kara
2021-04-13 11:28 ` [PATCH 5/7] xfs: Convert to use i_mapping_sem Jan Kara
2021-04-13 13:05   ` Christoph Hellwig
2021-04-13 13:42     ` Jan Kara
2021-04-13 11:28 ` [PATCH 6/7] zonefs: Convert to using i_mapping_sem Jan Kara
2021-04-13 11:28 ` [PATCH 7/7] fs: Remove i_mapping_sem protection from .page_mkwrite handlers Jan Kara
2021-04-13 13:09 ` [PATCH 0/7 RFC v3] fs: Hole punch vs page cache filling races Christoph Hellwig
2021-04-13 14:17   ` Jan Kara
2021-04-19 15:20 ` Matthew Wilcox
2021-04-19 16:25   ` Jan Kara
2021-04-20 22:12   ` Dave Chinner

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