linux-kernel - Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20131030232605.GA28823@redhat.com>
Date:	Thu, 31 Oct 2013 01:26:05 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
	gleb@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH RFC] kvm: optimize out smp_mb using srcu_read_unlock

> > Paul, could you review this patch please?
> > Documentation/memory-barriers.txt says that unlock has a weaker
> > uni-directional barrier, but in practice srcu_read_unlock calls
> > smp_mb().
> > 
> > Is it OK to rely on this? If not, can I add
> > smp_mb__after_srcu_read_unlock (making it an empty macro for now)
> > so we can avoid an actual extra smp_mb()?
> 
> Please use smp_mb__after_srcu_read_unlock().  After all, it was not
> that long ago that srcu_read_unlock() contained no memory barriers,
> and perhaps some day it won't need to once again.
> 
> 							Thanx, Paul
>

Thanks!
Something like this will be enough?
 
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index c114614..9b058ee 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -237,4 +237,18 @@ static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
 	__srcu_read_unlock(sp, idx);
 }
 
+/**
+ * smp_mb__after_srcu_read_unlock - ensure full ordering after srcu_read_unlock
+ *
+ * Converts the preceding srcu_read_unlock into a two-way memory barrier.
+ *
+ * Call this after srcu_read_unlock, to guarantee that all memory operations
+ * that occur after smp_mb__after_srcu_read_unlock will appear to happen after
+ * the preceding srcu_read_unlock.
+ */
+static inline void smp_mb__after_srcu_read_unlock(void)
+{
+	/* __srcu_read_unlock has smp_mb() internally so nothing to do here. */
+}
+
 #endif
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/