linux-kernel - Re: [RESEND PATCH 1/2] aio: simplify fetching ioctx_table pointer from, mm

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Date:	Fri, 07 Mar 2014 18:41:32 +0800
From:	Gu Zheng <guz.fnst@...fujitsu.com>
To:	Benjamin <bcrl@...ck.org>
CC:	Kent <kmo@...erainc.com>, Jens <axboe@...nel.dk>,
	linux-aio@...ck.org, linux-kernel <linux-kernel@...r.kernel.org>,
	miaox@...fujitsu.com
Subject: Re: [RESEND PATCH 1/2] aio: simplify fetching ioctx_table pointer
 from, mm_struct

ping...

On 02/28/2014 06:27 PM, Gu Zheng wrote:

> Using rcu_dereference_protected() rather than the "rcu_read_lock-->
> rcu_dereference-->rcu_read_unlock" group to simplify fetching the
> ioctx_table pointer in ioctx_add_table and kill_ioctx, because it
> is protected by the ioctx_lock.
> And in the exit_aio(), there are no other users manipulating ioctx_table
> at this stage, so we can use rcu_access_pointer directly.
> 
> Signed-off-by: Gu Zheng <guz.fnst@...fujitsu.com>
> ---
>  fs/aio.c |   42 ++++++++++++++++--------------------------
>  1 files changed, 16 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 062a5f6..7eaa631 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -544,8 +544,8 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
>  	struct aio_ring *ring;
>  
>  	spin_lock(&mm->ioctx_lock);
> -	rcu_read_lock();
> -	table = rcu_dereference(mm->ioctx_table);
> +	table = rcu_dereference_protected(mm->ioctx_table,
> +					lockdep_is_held(&mm->ioctx_lock));
>  
>  	while (1) {
>  		if (table)
> @@ -553,7 +553,6 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
>  				if (!table->table[i]) {
>  					ctx->id = i;
>  					table->table[i] = ctx;
> -					rcu_read_unlock();
>  					spin_unlock(&mm->ioctx_lock);
>  
>  					ring = kmap_atomic(ctx->ring_pages[0]);
> @@ -564,7 +563,6 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
>  
>  		new_nr = (table ? table->nr : 1) * 4;
>  
> -		rcu_read_unlock();
>  		spin_unlock(&mm->ioctx_lock);
>  
>  		table = kzalloc(sizeof(*table) + sizeof(struct kioctx *) *
> @@ -575,8 +573,8 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
>  		table->nr = new_nr;
>  
>  		spin_lock(&mm->ioctx_lock);
> -		rcu_read_lock();
> -		old = rcu_dereference(mm->ioctx_table);
> +		old = rcu_dereference_protected(mm->ioctx_table,
> +					lockdep_is_held(&mm->ioctx_lock));
>  
>  		if (!old) {
>  			rcu_assign_pointer(mm->ioctx_table, table);
> @@ -711,12 +709,11 @@ static void kill_ioctx(struct mm_struct *mm, struct kioctx *ctx)
>  		struct kioctx_table *table;
>  
>  		spin_lock(&mm->ioctx_lock);
> -		rcu_read_lock();
> -		table = rcu_dereference(mm->ioctx_table);
> +		table = rcu_dereference_protected(mm->ioctx_table,
> +					lockdep_is_held(&mm->ioctx_lock));
>  
>  		WARN_ON(ctx != table->table[ctx->id]);
>  		table->table[ctx->id] = NULL;
> -		rcu_read_unlock();
>  		spin_unlock(&mm->ioctx_lock);
>  
>  		/* percpu_ref_kill() will do the necessary call_rcu() */
> @@ -765,27 +762,17 @@ EXPORT_SYMBOL(wait_on_sync_kiocb);
>  void exit_aio(struct mm_struct *mm)
>  {
>  	struct kioctx_table *table;
> -	struct kioctx *ctx;
>  	unsigned i = 0;
>  
> -	while (1) {
> -		rcu_read_lock();
> -		table = rcu_dereference(mm->ioctx_table);
> -
> -		do {
> -			if (!table || i >= table->nr) {
> -				rcu_read_unlock();
> -				rcu_assign_pointer(mm->ioctx_table, NULL);
> -				if (table)
> -					kfree(table);
> -				return;
> -			}
> -
> -			ctx = table->table[i++];
> -		} while (!ctx);
> +	table = rcu_access_pointer(mm->ioctx_table);
> +	if (!table)
> +		return;
>  
> -		rcu_read_unlock();
> +	while (i < table->nr) {
> +		struct kioctx *ctx = table->table[i++];
>  
> +		if (!ctx)
> +			continue;
>  		/*
>  		 * We don't need to bother with munmap() here -
>  		 * exit_mmap(mm) is coming and it'll unmap everything.
> @@ -798,6 +785,9 @@ void exit_aio(struct mm_struct *mm)
>  
>  		kill_ioctx(mm, ctx);
>  	}
> +
> +	rcu_assign_pointer(mm->ioctx_table, NULL);
> +	kfree(table);
>  }
>  
>  static void put_reqs_available(struct kioctx *ctx, unsigned nr)


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