[xen staging] x86/memsharing: use an atomic add instead of a cmpxchg loop

[patchbot@xxxxxxx]

commit 6d05d529404a7fad1ed277865ceb31273272a23e
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Feb 29 08:19:22 2024 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Feb 29 08:19:22 2024 +0100

    x86/memsharing: use an atomic add instead of a cmpxchg loop
    
    The usage of a cmpxchg loop in get_next_handle() is unnecessary, as the same
    can be achieved with an atomic increment, which is both simpler to read, and
    avoid any need for a loop.
    
    The cmpxchg usage is likely a remnant of 32bit support, which didn't have an
    instruction to do an atomic 64bit add, and instead a cmpxchg had to be used.
    
    Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-of-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
    Acked-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
---
 xen/arch/x86/mm/mem_sharing.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
index 4f810706a3..fe299a2bf9 100644
--- a/xen/arch/x86/mm/mem_sharing.c
+++ b/xen/arch/x86/mm/mem_sharing.c
@@ -179,13 +179,7 @@ static void mem_sharing_page_unlock(struct page_info *pg)
 
 static shr_handle_t get_next_handle(void)
 {
-    /* Get the next handle get_page style */
-    uint64_t x, y = next_handle;
-    do {
-        x = y;
-    }
-    while ( (y = cmpxchg(&next_handle, x, x + 1)) != x );
-    return x + 1;
+    return arch_fetch_and_add(&next_handle, 1) + 1;
 }
 
 static atomic_t nr_saved_mfns   = ATOMIC_INIT(0);
--
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