linux-kernel - Re: [PATCH 14/25] aio: Make aio_read

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Message-ID: <20121129003816.GJ18574@lenny.home.zabbo.net>
Date:	Wed, 28 Nov 2012 16:38:16 -0800
From:	Zach Brown <zab@...hat.com>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, bcrl@...ck.org, jmoyer@...hat.com,
	axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 14/25] aio: Make aio_read_evt() more efficient

> We can't use cmpxchg() on the ring buffer's head pointer directly, since
> it's modded to nr_events and would be susceptible to ABA. So instead we
> maintain a shadow head that uses the full 32 bits, and cmpxchg() that
> and then updated the real head pointer.

Time to update this comment to reflect the mutex instead of the shadow
head? :)

> +		ev = kmap(page);
> +		copy_ret = copy_to_user(event + ret, ev + pos, sizeof(*ev) * i);
> +		kunmap(page);

For lack of a better time: do we want to bring up the missing
flush_dcache_page() calls around the kernel mappings of ring pages that
are also mapped to user addresses?

>  		prepare_to_wait_exclusive(&ctx->wait, &wait,
>  					  TASK_INTERRUPTIBLE);
>  
> +		ret = aio_read_events(ctx, event + i, nr - i);
> +		if (ret < 0)
>  			break;

As mentioned offlist: we don't want to be blocking under
TASK_INTERRUPTIBLE.  Is the plan to do a non-blocking check and pop
outside the wait loop to do a blocking copy?

- z
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