Module Name: src
Committed By: pgoyette
Date: Fri May 12 22:12:23 UTC 2017
Modified Files:
src/sys/kern [prg-localcount2]: subr_localcount.c
Log Message:
When we're draining the localcount's references, transfer the local
CPU's reference count to the global total, and then zero it. Any
further calls to localcount_release() will adjust only the global
counter.
Avoids a race condition which depends on having localcount_release()
being aware of whether the local CPU's contribution has already been
accounted for. Thanks to Kengo NAKAHARA for bringing up the question,
and to riastradh@ for the solution.
To generate a diff of this commit:
cvs rdiff -u -r1.1.6.4 -r1.1.6.5 src/sys/kern/subr_localcount.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/kern/subr_localcount.c
diff -u src/sys/kern/subr_localcount.c:1.1.6.4 src/sys/kern/subr_localcount.c:1.1.6.5
--- src/sys/kern/subr_localcount.c:1.1.6.4 Thu May 11 21:31:12 2017
+++ src/sys/kern/subr_localcount.c Fri May 12 22:12:23 2017
@@ -1,4 +1,4 @@
-/* $NetBSD: subr_localcount.c,v 1.1.6.4 2017/05/11 21:31:12 pgoyette Exp $ */
+/* $NetBSD: subr_localcount.c,v 1.1.6.5 2017/05/12 22:12:23 pgoyette Exp $ */
/*-
* Copyright (c) 2016 The NetBSD Foundation, Inc.
@@ -44,7 +44,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: subr_localcount.c,v 1.1.6.4 2017/05/11 21:31:12 pgoyette Exp $");
+__KERNEL_RCSID(0, "$NetBSD: subr_localcount.c,v 1.1.6.5 2017/05/12 22:12:23 pgoyette Exp $");
#include <sys/param.h>
#include <sys/localcount.h>
@@ -148,6 +148,14 @@ localcount_fini(struct localcount *lc)
percpu_free(lc->lc_percpu, sizeof(uint64_t));
}
+/*
+ * localcount_xc(cookie0, cookie1)
+ *
+ * Accumulate and transfer the per-CPU reference counts to a
+ * global total, resetting the per-CPU counter to zero. Once
+ * localcount_drain() has started, we only maintain the total
+ * count in localcount_release().
+ */
static void
localcount_xc(void *cookie0, void *cookie1)
{
@@ -158,6 +166,7 @@ localcount_xc(void *cookie0, void *cooki
mutex_enter(interlock);
localp = percpu_getref(lc->lc_percpu);
*lc->lc_totalp += *localp;
+ *localp -= *localp; /* ie, *localp = 0; */
percpu_putref(lc->lc_percpu);
mutex_exit(interlock);
}
@@ -235,9 +244,7 @@ localcount_release(struct localcount *lc
* the last reference.
*/
mutex_enter(interlock);
- localcount_adjust(lc, -1);
- *lc->lc_totalp -= 1;
- if (*lc->lc_totalp == 0)
+ if (--*lc->lc_totalp == 0)
cv_broadcast(cv);
mutex_exit(interlock);
goto out;
