[PATCH 2.5.67 1/2] i_size atomic access

Daniel McNeil (daniel@osdl.org)
18 Apr 2003 11:55:26 -0700


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This adds a sequence counter only version of the reader/writer
consistent mechanism to seqlock.h This is used in the second
part of this patch give atomic access to i_size.

re-diff against 2.5.67.

The patch is also available for download from OSDL's patch lifecycle
manageer (PLM):

http://osdl.org/cgi-bin/plm?module=patch_info&patch_id=1778

-- 
Daniel McNeil <daniel@osdl.org>

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--- linux-2.5.67/include/linux/seqlock.h Mon Apr 7 10:31:14 2003 +++ linux-2.5.67-isize/include/linux/seqlock.h Wed Apr 16 13:54:49 2003 @@ -94,6 +94,57 @@ static inline int read_seqretry(const se return (iv & 1) | (sl->sequence ^ iv); } + +/* + * Version using sequence counter only. + * This can be used when code has its own mutex protecting the + * updating starting before the write_seqcntbeqin() and ending + * after the write_seqcntend(). + */ + +typedef struct seqcnt { + unsigned sequence; +} seqcnt_t; + +#define SEQCNT_ZERO { 0 } +#define seqcnt_init(x) do { *(x) = (seqcnt_t) SEQCNT_ZERO; } while (0) + +/* Start of read using pointer to a sequence counter only. */ +static inline unsigned read_seqcntbegin(const seqcnt_t *s) +{ + unsigned ret = s->sequence; + smp_rmb(); + return ret; +} + +/* Test if reader processed invalid data. + * Equivalent to: iv is odd or sequence number has changed. + * (iv & 1) || (*s != iv) + * Using xor saves one conditional branch. + */ +static inline int read_seqcntretry(const seqcnt_t *s, unsigned iv) +{ + smp_rmb(); + return (iv & 1) | (s->sequence ^ iv); +} + + +/* + * Sequence counter only version assumes that callers are using their + * own mutexing. + */ +static inline void write_seqcntbegin(seqcnt_t *s) +{ + s->sequence++; + smp_wmb(); +} + +static inline void write_seqcntend(seqcnt_t *s) +{ + smp_wmb(); + s->sequence++; +} + /* * Possible sw/hw IRQ protected versions of the interfaces. */

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