Re: [patch] disable READA

[Marcelo Tosatti]

Who is using READA this days ?

On Tue, 30 Jul 2002, Andrew Morton wrote:

>
> There's a bug in bread() which can cause it to misinterpret a
> failed READA request as an IO error on SMP.
>
> If a filesytem block is subject to READA and __make_request()
> decides that the request would block then __make_request()
> will (via end_buffer_io_sync) mark the buffer as not uptodate
> and will unlock it.
>
> But if another CPU attempts to bread() the same buffer at the same
> time, the following happens:
>
> 1: ll_rw_block() sees the buffer is locked and does nothing at all
>
> 2: bread() does a wait_on_buffer()
>
> 3: the other CPU (the one doing READA) unlocks the non uptodate buffer
>
> 4: bread() sees the buffer come unlocked.  It's not uptodate, so
>    bread() bogusly reports an IO error.
>
> I haven't seen it in the wild, because it is rare to get that
> much read I/O in flight.  reiserfs and ext3 (at least) use READA.
>
> An appropriate fix for 2.4 is to disable READA.
>
>
>  ll_rw_blk.c |    2 ++
>  1 files changed, 2 insertions
>
> --- 2.4.19-rc3/drivers/block/ll_rw_blk.c~no-readahead	Tue Jul 30 14:18:17 2002
> +++ 2.4.19-rc3-akpm/drivers/block/ll_rw_blk.c	Tue Jul 30 14:19:52 2002
> @@ -841,7 +841,9 @@ static int __make_request(request_queue_
>  	rw_ahead = 0;	/* normal case; gets changed below for READA */
>  	switch (rw) {
>  		case READA:
> +#if 0	/* bread() misinterprets failed READA attempts as IO errors on SMP */
>  			rw_ahead = 1;
> +#endif
>  			rw = READ;	/* drop into READ */
>  		case READ:
>  		case WRITE:
>
> .
> -
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