On Mon, 16 Jul 2012 15:11:29 +0800 majianpeng <majianpeng@xxxxxxxxx> wrote:
> On 2012-07-16 15:07 NeilBrown <neilb@xxxxxxx> Wrote:
> >On Mon, 16 Jul 2012 14:42:54 +0800 majianpeng <majianpeng@xxxxxxxxx> wrote:
> >
> >> On 2012-07-16 13:40 NeilBrown <neilb@xxxxxxx> Wrote:
> >> >On Mon, 16 Jul 2012 09:31:55 +0800 majianpeng <majianpeng@xxxxxxxxx> wrote:
> >> >
> [snip]
> >> > Normal 'sync' requests use WRITE_SYNC which includes "REQ_NOIDLE" which means
> >> > /* don't anticipate more IO after this one */
> >> > O_DIRECT request use WRITE_ODIRECT which does not include this flag.
> >> >
> >
> >> Using REQ_NOIDEL to difference odirect and sync.Why not using:
> >> + if (bi->bi_rw & WRITE_ODIRECT)
> >> + bi->bi_rw &= ~REQ_SYNC;
> >
> >Because that code is wrong. WRITE_ODIRECT is not one flag, it is two flags
> >'or'ed together. So this code does not do what you expect.
> >
> No, I used those code test and it's ok.
> The code used & not &&.
> Maybe I wrong?
Think about it...
#define REQ_WRITE (1 << __REQ_WRITE)
#define REQ_SYNC (1 << __REQ_SYNC)
#define RW_MASK REQ_WRITE
#define WRITE RW_MASK
#define WRITE_ODIRECT (WRITE | REQ_SYNC)
So
(bi->bi_rw & WRITE_ODIRECT)
will be true if either REQ_WRITE or REQ_SYNC are set in bi_rw
So whenever REQ_SYNC is set, your code clears the flag.
So your code is functionally identical to
bi->bi_rw &= ~REQ_SYNC;
NeilBrown
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