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authorIngo Molnar <mingo@elte.hu>2008-04-03 09:06:13 +0200
committerIngo Molnar <mingo@elte.hu>2008-04-25 00:25:08 +0200
commit88a411c07b6fedcfc97b8dc51ae18540bd2beda0 (patch)
tree9aa732fdcaeadcde06c931672aa7da7b168bd82c /include/linux/seqlock.h
parentb69d3987f4360a5e7e9e55465b3cdd0cc204b79e (diff)
downloadlinux-88a411c07b6fedcfc97b8dc51ae18540bd2beda0.tar.gz
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/linux/seqlock.h')
-rw-r--r--include/linux/seqlock.h46
1 files changed, 29 insertions, 17 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 26e4925bc35..632205ccc25 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;
}