tb-maint: do not use mb_read/mb_set

[PATCH] tb-maint: do not use mb_read/mb_set

Paolo Bonzini posted 1 patch 1 year ago

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Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/qemu tags/patchew/20230430112557.240261-1-pbonzini@redhat.com

Maintainers: Richard Henderson <richard.henderson@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>

accel/tcg/tb-maint.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

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[PATCH] tb-maint: do not use mb_read/mb_set

Posted by Paolo Bonzini 1 year ago

The load side can use a relaxed load, which will surely happen before
the work item is run by async_safe_run_on_cpu() or before double-checking
under mmap_lock.  The store side can use an atomic RMW operation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 accel/tcg/tb-maint.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/accel/tcg/tb-maint.c b/accel/tcg/tb-maint.c
index cb1f806f00d4..0dd173fbf048 100644
--- a/accel/tcg/tb-maint.c
+++ b/accel/tcg/tb-maint.c
@@ -746,7 +746,7 @@ static void do_tb_flush(CPUState *cpu, run_on_cpu_data tb_flush_count)
 
     tcg_region_reset_all();
     /* XXX: flush processor icache at this point if cache flush is expensive */
-    qatomic_mb_set(&tb_ctx.tb_flush_count, tb_ctx.tb_flush_count + 1);
+    qatomic_inc(&tb_ctx.tb_flush_count);
 
 done:
     mmap_unlock();
@@ -758,7 +758,7 @@ done:
 void tb_flush(CPUState *cpu)
 {
     if (tcg_enabled()) {
-        unsigned tb_flush_count = qatomic_mb_read(&tb_ctx.tb_flush_count);
+        unsigned tb_flush_count = qatomic_read(&tb_ctx.tb_flush_count);
 
         if (cpu_in_exclusive_context(cpu)) {
             do_tb_flush(cpu, RUN_ON_CPU_HOST_INT(tb_flush_count));
-- 
2.40.0

Re: [PATCH] tb-maint: do not use mb_read/mb_set

Posted by Richard Henderson 1 year ago

On 4/30/23 12:25, Paolo Bonzini wrote:
> The load side can use a relaxed load, which will surely happen before
> the work item is run by async_safe_run_on_cpu() or before double-checking
> under mmap_lock.  The store side can use an atomic RMW operation.
> 
> Signed-off-by: Paolo Bonzini<pbonzini@redhat.com>
> ---
>   accel/tcg/tb-maint.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~