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Generation numbers to catch write back of stale copies

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Oops when a stale copy is written back, i.e. the processor was yielded
since the copy was made.  Such bugs are difficult to spot.  Sequence
numbers catch them when they do actual harm (they also catch different
bugs).  Generation numbers catch them even when they don't.

New ef_generation to count generations.  Call new ef_make_stale() to
increment it whenever the processor may be yielded.

New struct emptypedstr member generation.  To conserve space, make it
a bit-field of twelve bits, i.e. generations are only recorded modulo
2^12.  Make sure all members of unit empobj_storage share it.  It is
only used in copies; its value on disk and in the cache is
meaningless.  Copies with generation other than ef_generation are
stale.  Stale copies that are a multiple of 2^12 generations old can't
be detected, but that is sufficiently improbable.

Set generation to ef_generation by calling new ef_mark_fresh() when
making copies in ef_read() and ef_blank().  nav_ship() and
fltp_to_list() make copies without going through ef_read(), and
therefore need to call ef_mark_fresh() as well.  Also call it in
obj_changed() to make check_sect_ok() & friends freshen their argument
when it is unchanged.

New must_be_fresh() oopses when its argument is stale.  Call it in
ef_write() to catch write back of stale copies.
This commit is contained in:
Markus Armbruster 2009-12-31 13:14:36 +01:00
parent 358aee203e
commit 2fa5f65257
22 changed files with 93 additions and 3 deletions
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2
include/nat.h
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@ -72,6 +72,7 @@ struct realmstr {
/* initial part must match struct empobj */
signed ef_type: 8;
unsigned r_seqno: 12;
unsigned r_generation: 12;
int r_uid; /* realm table index */
time_t r_timestamp; /* Last time this realm was touched */
natid r_cnum; /* country number */
@ -85,6 +86,7 @@ struct natstr {
/* initial part must match struct empobj */
signed ef_type: 8;
unsigned nat_seqno: 12;
unsigned nat_generation: 12;
int nat_uid; /* equals nat_cnum */
time_t nat_timestamp;
natid nat_cnum; /* our country number */