linux-kernel - Re: [PROBLEM] hard-lock with kmemtrace, relayfs, and splice

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Date:	Sat, 11 Oct 2008 21:17:29 +0300
From:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
To:	Tom Zanussi <zanussi@...cast.net>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PROBLEM] hard-lock with kmemtrace, relayfs, and splice

On Fri, Oct 10, 2008 at 11:58:51PM -0500, Tom Zanussi wrote:
> It worked for me, but I also had to apply the following patch to
> kmemtraced:
> 
> diff --git a/kmemtraced.c b/kmemtraced.c
> index 217478d..324ced9 100644
> --- a/kmemtraced.c
> +++ b/kmemtraced.c
> @@ -109,6 +109,8 @@ static void *reader_thread(void *data)
>  		if (retval < 0)
>  			panic("splice() (from) failed: %s\n",
>  			      strerror(errno));
> +		if (!retval)
> +			continue;
>  		retval = splice(pipe_fd[0], NULL, log_fd, NULL,
>  				128, SPLICE_F_MOVE);
>  		if (retval < 0)
> 
> Otherwise it would end up hanging kmemtraced in the second splice (pipe
> to log_fd) if the return from the first splice was 0 (i.e. there's no
> data available (and we can never know if there will ever be any
> more)).

Thanks, I'll apply it.

> I'm not sure why kmemtraced is only splicing 128 bytes at a time - it
> seems to defeat the purpose - or why it wouldn't be using poll to know
> when there's at least a whole sub-buffer to splice, but to each his own.
> Hopefully the kernel patch at least fixes the loop. 

Yeah, it was a misguided attempt to fix the strange behavior.

> Tom


	Cheers,
	Eduard

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