[Xen-devel] [PATCH 3/4] xen/watchdog: Drop all locked operations on the watchdog_inuse_map

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

All modifications to the watchdog_inuse_map happen with d->watchdog_lock held,
so there are no concurrency problems to deal with.

Furthermore, there is no need to use a loop to locate the next available
watchdog.  As the bitmap is currently 2 bits wide and is stored in a uint32_t,
the next available timer can be located in O(1) time using bit-scanning
instructions.

No change in behaviour, but should have less cache-coherency impact.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>
CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
CC: Edwin Török <edvin.torok@xxxxxxxxxx>
CC: Christian Lindig <christian.lindig@xxxxxxxxxx>
CC: Pau Ruiz Safont <pau.safont@xxxxxxxxxx>
---
 xen/common/schedule.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 89aba88..98c2c35 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -1068,17 +1068,15 @@ static long domain_watchdog(struct domain *d, uint32_t 
id, uint32_t timeout)
     }
     else /* Allocate the next available timer. */
     {
-        for ( id = 0; id < NR_DOMAIN_WATCHDOG_TIMERS; id++ )
-        {
-            if ( test_and_set_bit(id, &d->watchdog_inuse_map) )
-                continue;
-            break;
-        }
-        if ( id == NR_DOMAIN_WATCHDOG_TIMERS )
+        id = ffs(~d->watchdog_inuse_map) - 1;
+
+        if ( unlikely(id >= NR_DOMAIN_WATCHDOG_TIMERS) )
         {
             rc = -ENOSPC;
             goto unlock;
         }
+
+        __set_bit(id, &d->watchdog_inuse_map);
         rc = id + 1;
     }
 
@@ -1086,7 +1084,7 @@ static long domain_watchdog(struct domain *d, uint32_t 
id, uint32_t timeout)
     if ( unlikely(timeout == 0) )
     {
         stop_timer(&d->watchdog_timer[id]);
-        clear_bit(id, &d->watchdog_inuse_map);
+        __clear_bit(id, &d->watchdog_inuse_map);
     }
     else
         set_timer(&d->watchdog_timer[id], NOW() + SECONDS(timeout));
-- 
2.1.4


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