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Date: Wed, 16 Apr 2014 14:11:32 -0500
From: Eric Sandeen <sandeen@sandeen.net>
MIME-Version: 1.0
Subject: Re: [PATCH 2/2] xfs: Nuke XFS_ERROR macro
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To: Christoph Hellwig <hch@infradead.org>
Cc: xfs-oss <xfs@oss.sgi.com>

On 4/16/14, 12:55 PM, Eric Sandeen wrote:
> On 4/16/14, 12:51 PM, Christoph Hellwig wrote:
>> On Wed, Apr 16, 2014 at 12:48:50PM -0500, Eric Sandeen wrote:
>>> XFS_ERROR was designed long ago to trap return values,
>>> but it's not runtime configurable, it's not consistently used,
>>> and we can do the same thing today with systemtap, using
>>> something like:
>>>
>>> probe module("xfs").function("xfs_*").return { if (@defined($return) && $return == VALUE) { ... } }
>>
>> Gives me a version just using ftrace, or at least a kprobes based module
>> that we can merged in the kernel tree and this would be fine for me.
>>
>> Requiring a massive blob of questionable out of tree module code and a
>> compiler is an absolute no-go.
> 
> Ok, fair point.
>  
>> NAK for now.
> 
> Even if we don't have a replacement, I have yet to find anyone who has
> ever used it...
> 
> Anyway, I'll look into options besides systemtap.

Here's the "best" I've come up with so far...

# for FUNCTION in `grep "t xfs_" /proc/kallsyms | awk '{print $3}'`; do echo "r:ret_$FUNCTION $FUNCTION \$retval" >> /sys/kernel/debug/tracing/kprobe_events; done

# for ENABLE in /sys/kernel/debug/tracing/events/kprobes/ret_xfs_*/enable; do echo 1 > $ENABLE; done

run a test that fails:

# dd if=/dev/zero of=newfile bs=513 oflag=direct
dd: writing `newfile': Invalid argument

# for ENABLE in /sys/kernel/debug/tracing/events/kprobes/ret_xfs_*/enable; do echo 0 > $ENABLE; done

# cat /sys/kernel/debug/tracing/trace
<snip>
           <...>-63791 [000] d... 705435.568913: ret_xfs_vn_mknod: (xfs_vn_create+0x13/0x20 [xfs] <- xfs_vn_mknod) arg1=0
           <...>-63791 [000] d... 705435.568913: ret_xfs_vn_create: (vfs_create+0xdb/0x100 <- xfs_vn_create) arg1=0
           <...>-63791 [000] d... 705435.568918: ret_xfs_file_open: (do_dentry_open+0x24e/0x2e0 <- xfs_file_open) arg1=0
           <...>-63791 [000] d... 705435.568934: ret_xfs_file_dio_aio_write: (xfs_file_aio_write+0x147/0x150 [xfs] <- xfs_file_dio_aio_write) arg1=ffffffffffffffea

Hey look, it's "-22" in hex!  ;)

so it's possible, but bleah.

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