From: Mikael Pettersson <mikpe@csd.uu.se>

perfctr seqlocks 2/4: x86 changes
- use write_perfseq_begin/end in perfctr_cpu_suspend/resume/sample
  to indicate that the state has changed
- in mmap:ed state, redefine filler field as the sequence number

Signed-off-by: Mikael Pettersson <mikpe@csd.uu.se>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 drivers/perfctr/x86.c      |    7 ++++++-
 include/asm-i386/perfctr.h |    5 ++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff -puN drivers/perfctr/x86.c~perfctr-seqlocks-for-mmaped-state-x86 drivers/perfctr/x86.c
--- 25/drivers/perfctr/x86.c~perfctr-seqlocks-for-mmaped-state-x86	2005-05-26 22:57:39.000000000 -0700
+++ 25-akpm/drivers/perfctr/x86.c	2005-05-26 22:57:39.000000000 -0700
@@ -1162,6 +1162,7 @@ void perfctr_cpu_suspend(struct perfctr_
 	unsigned int i, cstatus, nractrs;
 	struct perfctr_low_ctrs now;
 
+	write_perfseq_begin(&state->user.sequence);
 	if (perfctr_cstatus_has_ictrs(state->user.cstatus))
 	    perfctr_cpu_isuspend(state);
 	perfctr_cpu_read_counters(state, &now);
@@ -1172,10 +1173,12 @@ void perfctr_cpu_suspend(struct perfctr_
 	for(i = 0; i < nractrs; ++i)
 		state->user.pmc[i].sum += now.pmc[i] - state->user.pmc[i].start;
 	/* perfctr_cpu_disable_rdpmc(); */	/* not for x86 */
+	write_perfseq_end(&state->user.sequence);
 }
 
 void perfctr_cpu_resume(struct perfctr_cpu_state *state)
 {
+	write_perfseq_begin(&state->user.sequence);
 	if (perfctr_cstatus_has_ictrs(state->user.cstatus))
 	    perfctr_cpu_iresume(state);
 	/* perfctr_cpu_enable_rdpmc(); */	/* not for x86 or global-mode */
@@ -1192,7 +1195,7 @@ void perfctr_cpu_resume(struct perfctr_c
 		for(i = 0; i < nrctrs; ++i)
 			state->user.pmc[i].start = now.pmc[i];
 	}
-	/* XXX: if (SMP && start.tsc == now.tsc) ++now.tsc; */
+	write_perfseq_end(&state->user.sequence);
 }
 
 void perfctr_cpu_sample(struct perfctr_cpu_state *state)
@@ -1200,6 +1203,7 @@ void perfctr_cpu_sample(struct perfctr_c
 	unsigned int i, cstatus, nractrs;
 	struct perfctr_low_ctrs now;
 
+	write_perfseq_begin(&state->user.sequence);
 	perfctr_cpu_read_counters(state, &now);
 	cstatus = state->user.cstatus;
 	if (perfctr_cstatus_has_tsc(cstatus)) {
@@ -1211,6 +1215,7 @@ void perfctr_cpu_sample(struct perfctr_c
 		state->user.pmc[i].sum += now.pmc[i] - state->user.pmc[i].start;
 		state->user.pmc[i].start = now.pmc[i];
 	}
+	write_perfseq_end(&state->user.sequence);
 }
 
 static void (*clear_counters)(void);
diff -puN include/asm-i386/perfctr.h~perfctr-seqlocks-for-mmaped-state-x86 include/asm-i386/perfctr.h
--- 25/include/asm-i386/perfctr.h~perfctr-seqlocks-for-mmaped-state-x86	2005-05-26 22:57:39.000000000 -0700
+++ 25-akpm/include/asm-i386/perfctr.h	2005-05-26 22:57:39.000000000 -0700
@@ -21,7 +21,10 @@ struct perfctr_cpu_control_header {
 
 struct perfctr_cpu_state_user {
 	__u32 cstatus;
-	__u32 _filler;
+	/* This is a sequence counter to ensure atomic reads by
+	 * userspace.  The mechanism is identical to that used for
+	 * seqcount_t in include/linux/seqlock.h. */
+	__u32 sequence;
 	__u64 tsc_start;
 	__u64 tsc_sum;
 	struct {
_