[PATCH v2 3/3] xen: Use sync_try_cmpxchg instead of sync_cmpxchg

[Uros Bizjak <ubizjak@xxxxxxxxx>]

Use sync_try_cmpxchg instead of sync_cmpxchg (*ptr, old, new) == old
in clear_masked_cond(), clear_linked() and
gnttab_end_foreign_access_ref_v1(). x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg
(and related move instruction in front of cmpxchg), improving the
cmpxchg loop in gnttab_end_foreign_access_ref_v1 from:

     174:       eb 0e                   jmp    184 <...>
     176:       89 d0                   mov    %edx,%eax
     178:       f0 66 0f b1 31          lock cmpxchg %si,(%rcx)
     17d:       66 39 c2                cmp    %ax,%dx
     180:       74 11                   je     193 <...>
     182:       89 c2                   mov    %eax,%edx
     184:       89 d6                   mov    %edx,%esi
     186:       66 83 e6 18             and    $0x18,%si
     18a:       74 ea                   je     176 <...>

to:

     614:       89 c1                   mov    %eax,%ecx
     616:       66 83 e1 18             and    $0x18,%cx
     61a:       75 11                   jne    62d <...>
     61c:       f0 66 0f b1 0a          lock cmpxchg %cx,(%rdx)
     621:       75 f1                   jne    614 <...>

No functional change intended.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Acked-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
---
v2: Improve commit description.
---
 drivers/xen/events/events_fifo.c | 26 ++++++++++++--------------
 drivers/xen/grant-table.c        | 10 ++++------
 2 files changed, 16 insertions(+), 20 deletions(-)

diff --git a/drivers/xen/events/events_fifo.c b/drivers/xen/events/events_fifo.c
index ad9fe51d3fb3..655775db7caf 100644
--- a/drivers/xen/events/events_fifo.c
+++ b/drivers/xen/events/events_fifo.c
@@ -226,21 +226,20 @@ static bool evtchn_fifo_is_masked(evtchn_port_t port)
  */
 static bool clear_masked_cond(volatile event_word_t *word)
 {
-       event_word_t new, old, w;
+       event_word_t new, old;
 
-       w = *word;
+       old = *word;
 
        do {
-               if (!(w & (1 << EVTCHN_FIFO_MASKED)))
+               if (!(old & (1 << EVTCHN_FIFO_MASKED)))
                        return true;
 
-               if (w & (1 << EVTCHN_FIFO_PENDING))
+               if (old & (1 << EVTCHN_FIFO_PENDING))
                        return false;
 
-               old = w & ~(1 << EVTCHN_FIFO_BUSY);
+               old = old & ~(1 << EVTCHN_FIFO_BUSY);
                new = old & ~(1 << EVTCHN_FIFO_MASKED);
-               w = sync_cmpxchg(word, old, new);
-       } while (w != old);
+       } while (!sync_try_cmpxchg(word, &old, new));
 
        return true;
 }
@@ -259,17 +258,16 @@ static void evtchn_fifo_unmask(evtchn_port_t port)
 
 static uint32_t clear_linked(volatile event_word_t *word)
 {
-       event_word_t new, old, w;
+       event_word_t new, old;
 
-       w = *word;
+       old = *word;
 
        do {
-               old = w;
-               new = (w & ~((1 << EVTCHN_FIFO_LINKED)
-                            | EVTCHN_FIFO_LINK_MASK));
-       } while ((w = sync_cmpxchg(word, old, new)) != old);
+               new = (old & ~((1 << EVTCHN_FIFO_LINKED)
+                              | EVTCHN_FIFO_LINK_MASK));
+       } while (!sync_try_cmpxchg(word, &old, new));
 
-       return w & EVTCHN_FIFO_LINK_MASK;
+       return old & EVTCHN_FIFO_LINK_MASK;
 }
 
 static void consume_one_event(unsigned cpu, struct evtchn_loop_ctrl *ctrl,
diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 35659bf70746..04a6b470b15d 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -427,16 +427,14 @@ EXPORT_SYMBOL_GPL(gnttab_grant_foreign_access);
 
 static int gnttab_end_foreign_access_ref_v1(grant_ref_t ref)
 {
-       u16 flags, nflags;
-       u16 *pflags;
+       u16 *pflags = &gnttab_shared.v1[ref].flags;
+       u16 flags;
 
-       pflags = &gnttab_shared.v1[ref].flags;
-       nflags = *pflags;
+       flags = *pflags;
        do {
-               flags = nflags;
                if (flags & (GTF_reading|GTF_writing))
                        return 0;
-       } while ((nflags = sync_cmpxchg(pflags, flags, 0)) != flags);
+       } while (!sync_try_cmpxchg(pflags, &flags, 0));
 
        return 1;
 }
-- 
2.41.0