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Commit 88a411c0 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

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: default avatarIngo Molnar <mingo@elte.hu>
parent b69d3987

include/linux/seqlock.h

+29 −17
Original line number Diff line number Diff line
@@ -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

/*

 * Test if reader processed invalid data.

 *

 * Using xor saves one conditional branch.

 * 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;

}