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Date:   Wed, 14 Apr 2021 23:25:31 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Dave Chinner <david@fromorbit.com>
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
Message-ID: <20210414222531.GZ2531743@casper.infradead.org>
References: <20210413105205.3093-1-jack@suse.cz>
 <20210413112859.32249-2-jack@suse.cz>
 <20210414000113.GG63242@dread.disaster.area>
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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);
}