linux-kernel - Re: [osd-dev] [PATCH] scsi_lib: Bug in completion of bidi commands

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Message-ID: <4B4C5A13.6090709@panasas.com>
Date:	Tue, 12 Jan 2010 13:16:35 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	James Bottomley <James.Bottomley@...e.de>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	open-osd <osd-dev@...n-osd.org>,
	Benny Halevy <bhalevy@...asas.com>,
	Alan Stern <stern@...land.harvard.edu>
CC:	Stable Tree <stable@...nel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [osd-dev] [PATCH] scsi_lib: Bug in completion of bidi commands

On 12/15/2009 05:25 PM, Boaz Harrosh wrote:
> 
> Because of the terrible structuring of scsi-bidi-commands
> it breaks some of the life time rules of a scsi-command.
> It is now not allowed to free up the block-request before
> cleanup and partial deallocation of the scsi-command. (Which
> is not so for none bidi commands)
> 
> The right fix to this problem would be to make bidi command
> a first citizen by allocating a scsi_sdb pointer at scsi command
> just like cmd->prot_sdb. The bidi sdb should be allocated/deallocated
> as part of the get/put_command (Again like the prot_sdb) and the
> current decoupling of scsi_cmnd and blk-request should be kept.
> 
> For now make sure scsi_release_buffers() is called before the
> call to blk_end_request_all() which might cause the suicide of
> the block requests. At best the leak of bidi buffers, at worse
> a crash, as there is a race between the existence of the bidi_request
> and the free of the associated bidi_sdb.
> 
> The reason this was never hit before is because only OSD has the potential
> of doing asynchronous bidi commands. (So does bsg but it is never used)
> And OSD clients just happen to do all their bidi commands synchronously, up
> until recently.
> 
> CC: Stable Tree <stable@...nel.org>

James hi.

Have you had the chance on looking at this issue. It's a serious bug
dated back a long time.

Technically it is quite simple.

It used to be:
	blk_end_request_all(req, 0);
	scsi_release_buffers(cmd);

Now scsi_release_buffers tries to use cmd->req for inspecting the req->next
pointer, but req was just freed by blk_end_request_all()

Reversing the call to:
	scsi_release_buffers(cmd);
	blk_end_request_all(req, 0);

Is the right thing to do and does not have any side effects what's so ever.

Please put me out of my misery ;-)
Boaz

> Signed-off-by: Boaz Harrosh <bharrosh@...asas.com>
> ---
>  drivers/scsi/scsi_lib.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 5987da8..bc9a881 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -749,9 +749,9 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
>  			 */
>  			req->next_rq->resid_len = scsi_in(cmd)->resid;
>  
> +			scsi_release_buffers(cmd);
>  			blk_end_request_all(req, 0);
>  
> -			scsi_release_buffers(cmd);
>  			scsi_next_command(cmd);
>  			return;
>  		}

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