linux-ext4 - Re: [PATCH] ext4: don't lock buffer in ext4_commit

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Message-ID: <20161124123850.GF24138@quack2.suse.cz>
Date:   Thu, 24 Nov 2016 13:38:50 +0100
From:   Jan Kara <jack@...e.cz>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] ext4: don't lock buffer in ext4_commit_super if holding
 spinlock

On Sun 13-11-16 17:32:56, Ted Tso wrote:
> If there is an error reported in mballoc via ext4_grp_locked_error(),
> the code is holding a spinlock, so ext4_commit_super() must not try to
> lock the buffer head, or else it will trigger a BUG:
> 
>   BUG: sleeping function called from invalid context at ./include/linux/buffer_head.h:358
>   in_atomic(): 1, irqs_disabled(): 0, pid: 993, name: mount
>   CPU: 0 PID: 993 Comm: mount Not tainted 4.9.0-rc1-clouder1 #62
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.1-0-g4adadbd-20150316_085822-nilsson.home.kraxel.org 04/01/2014
>    ffff880006423548 ffffffff81318c89 ffffffff819ecdd0 0000000000000166
>    ffff880006423558 ffffffff810810b0 ffff880006423580 ffffffff81081153
>    ffff880006e5a1a0 ffff88000690e400 0000000000000000 ffff8800064235c0
>   Call Trace:
>     [<ffffffff81318c89>] dump_stack+0x67/0x9e
>     [<ffffffff810810b0>] ___might_sleep+0xf0/0x140
>     [<ffffffff81081153>] __might_sleep+0x53/0xb0
>     [<ffffffff8126c1dc>] ext4_commit_super+0x19c/0x290
>     [<ffffffff8126e61a>] __ext4_grp_locked_error+0x14a/0x230
>     [<ffffffff81081153>] ? __might_sleep+0x53/0xb0
>     [<ffffffff812822be>] ext4_mb_generate_buddy+0x1de/0x320
> 
> Since ext4_grp_locked_error() calls ext4_commit_super with sync == 0
> (and it is the only caller which does so), avoid locking and unlocking
> the buffer in this case.
> 
> This can result in races with ext4_commit_super() if there are other
> problems (which is what commit 4743f83990614 was trying to address),
> but a Warning is better than BUG.
> 
> Fixes: 4743f83990614
> Cc: stable@...r.kernel.org
> Reported-by: Nikolay Borisov <kernel@...p.com>
> Signed-off-by: Theodore Ts'o <tytso@....edu>

Looks good. You can add:

Reviewed-by: Jan Kara <jack@...e.cz>

							Honza

> ---
>  fs/ext4/super.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index e4f61c39328a..ff6f3ab09c7e 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4537,7 +4537,8 @@ static int ext4_commit_super(struct super_block *sb, int sync)
>  				&EXT4_SB(sb)->s_freeinodes_counter));
>  	BUFFER_TRACE(sbh, "marking dirty");
>  	ext4_superblock_csum_set(sb);
> -	lock_buffer(sbh);
> +	if (sync)
> +		lock_buffer(sbh);
>  	if (buffer_write_io_error(sbh)) {
>  		/*
>  		 * Oh, dear.  A previous attempt to write the
> @@ -4553,8 +4554,8 @@ static int ext4_commit_super(struct super_block *sb, int sync)
>  		set_buffer_uptodate(sbh);
>  	}
>  	mark_buffer_dirty(sbh);
> -	unlock_buffer(sbh);
>  	if (sync) {
> +		unlock_buffer(sbh);
>  		error = __sync_dirty_buffer(sbh,
>  			test_opt(sb, BARRIER) ? WRITE_FUA : WRITE_SYNC);
>  		if (error)
> -- 
> 2.11.0.rc0.7.gbe5a750
> 
> --
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-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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