[PATCH] monotonic seqlock for HPET timer

[Stephen Hemminger]

Replace read/write lock used for HPET timer monotonic_lock with seqlock.
Similar to locking used on xtime and monotonic_lock in timers/timer_tsc.c

It builds and runs, but I don't have hardware with HPET support to test.
Not a big deal (yet) since only hangcheck timer uses monotonic clock so far.

diff -urN -X dontdiff linux-2.5/arch/i386/kernel/timers/timer_hpet.c linux-2.5-net/arch/i386/kernel/timers/timer_hpet.c
--- linux-2.5/arch/i386/kernel/timers/timer_hpet.c	2003-09-30 13:53:48.000000000 -0700
+++ linux-2.5-net/arch/i386/kernel/timers/timer_hpet.c	2003-09-16 12:54:26.000000000 -0700
@@ -24,7 +24,7 @@
 static unsigned long last_tsc_low;	/* lsb 32 bits of Time Stamp Counter */
 static unsigned long last_tsc_high; 	/* msb 32 bits of Time Stamp Counter */
 static unsigned long long monotonic_base;
-static rwlock_t monotonic_lock = RW_LOCK_UNLOCKED;
+static seqlock_t monotonic_lock = SEQLOCK_UNLOCKED;
 
 /* convert from cycles(64bits) => nanoseconds (64bits)
  *  basic equation:
@@ -57,12 +57,14 @@
 static unsigned long long monotonic_clock_hpet(void)
 {
 	unsigned long long last_offset, this_offset, base;
+	unsigned seq;
 
 	/* atomically read monotonic base & last_offset */
-	read_lock_irq(&monotonic_lock);
-	last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
-	base = monotonic_base;
-	read_unlock_irq(&monotonic_lock);
+	do {
+		seq = read_seqbegin(&monotonic_lock);
+		last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
+		base = monotonic_base;
+	} while (read_seqretry(&monotonic_lock, seq));
 
 	/* Read the Time Stamp Counter */
 	rdtscll(this_offset);
@@ -99,7 +101,7 @@
 	unsigned long long this_offset, last_offset;
 	unsigned long offset;
 
-	write_lock(&monotonic_lock);
+	write_seqlock(&monotonic_lock);
 	last_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
 	rdtsc(last_tsc_low, last_tsc_high);
 
@@ -113,7 +115,7 @@
 	/* update the monotonic base value */
 	this_offset = ((unsigned long long)last_tsc_high<<32)|last_tsc_low;
 	monotonic_base += cycles_2_ns(this_offset - last_offset);
-	write_unlock(&monotonic_lock);
+	write_sequnlock(&monotonic_lock);
 }
 
 void delay_hpet(unsigned long loops)
-
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