diff options
author | Ingo Molnar <mingo@elte.hu> | 2008-04-03 09:06:13 +0200 |
---|---|---|
committer | Ingo Molnar <mingo@elte.hu> | 2008-04-25 00:25:08 +0200 |
commit | 88a411c07b6fedcfc97b8dc51ae18540bd2beda0 (patch) | |
tree | 9aa732fdcaeadcde06c931672aa7da7b168bd82c /include | |
parent | b69d3987f4360a5e7e9e55465b3cdd0cc204b79e (diff) |
seqlock: livelock fix
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>
Diffstat (limited to 'include')
-rw-r--r-- | include/linux/seqlock.h | 46 |
1 files changed, 29 insertions, 17 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index 26e4925bc35b..632205ccc25d 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -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; } |