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seqlock: livelock fix

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Thomas Gleixner debugged a particularly ugly seqlock related livelock:
do not process the seq-read section if we know it beforehand that the
test at the end of the section will fail ...

Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Ingo Molnar 2008-04-03 09:06:13 +02:00
parent b69d3987f4
commit 88a411c07b
1 changed files with 29 additions and 17 deletions
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46
include/linux/seqlock.h
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@ -85,23 +85,29 @@ static inline int write_tryseqlock(seqlock_t *sl)
/* Start of read calculation -- fetch last complete writer token */
static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
{
unsigned ret = sl->sequence;
unsigned ret;
repeat:
ret = sl->sequence;
smp_rmb();
if (unlikely(ret & 1)) {
cpu_relax();
goto repeat;
}
return ret;
}
/* Test if reader processed invalid data.
* If initial values is odd,
* then writer had already started when section was entered
* If sequence value changed
* then writer changed data while in section
*
* Using xor saves one conditional branch.
/*
* Test if reader processed invalid data.
*
* If sequence value changed then writer changed data while in section.
*/
static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
{
smp_rmb();
return (iv & 1) | (sl->sequence ^ iv);
return (sl->sequence != start);
}
@ -122,20 +128,26 @@ typedef struct seqcount {
/* Start of read using pointer to a sequence counter only. */
static inline unsigned read_seqcount_begin(const seqcount_t *s)
{
unsigned ret = s->sequence;
unsigned ret;
repeat:
ret = s->sequence;
smp_rmb();
if (unlikely(ret & 1)) {
cpu_relax();
goto repeat;
}
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.
/*
* Test if reader processed invalid data because sequence number has changed.
*/
static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
{
smp_rmb();
return (iv & 1) | (s->sequence ^ iv);
return s->sequence != start;
}