lkml.org 
[lkml]   [2023]   [Jan]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] x86/pvclock: improve atomic update of last_value in pvclock_clocksource_read
Date
Improve atomic update of last_value in pvclock_clocksource_read:

- Atomic update can be skipped if the "last_value" is already
equal to "ret".

- The detection of atomic update failure is not correct. The value,
returned by atomic64_cmpxchg should be compared to the old value
from the location to be updated. If these two are the same, then
atomic update succeeded and "last_value" location is updated to
"ret" in an atomic way. Otherwise, the atomic update failed and
it should be retried with the value from "last_value" - exactly
what atomic64_try_cmpxchg does in a correct and more optimal way.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
arch/x86/kernel/pvclock.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index eda37df016f0..5a2a517dd61b 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -102,10 +102,9 @@ u64 pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
*/
last = atomic64_read(&last_value);
do {
- if (ret < last)
+ if (ret <= last)
return last;
- last = atomic64_cmpxchg(&last_value, last, ret);
- } while (unlikely(last != ret));
+ } while (!atomic64_try_cmpxchg(&last_value, &last, ret));

return ret;
}
--
2.39.0
\
 
 \ /
  Last update: 2023-03-26 23:46    [W:0.059 / U:0.420 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site