From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60520) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YWKf6-0007YQ-8H for qemu-devel@nongnu.org; Fri, 13 Mar 2015 04:09:40 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YWKf5-0005A1-Cm for qemu-devel@nongnu.org; Fri, 13 Mar 2015 04:09:40 -0400 Received: from mail-we0-x236.google.com ([2a00:1450:400c:c03::236]:44052) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YWKf5-00059a-6k for qemu-devel@nongnu.org; Fri, 13 Mar 2015 04:09:39 -0400 Received: by wesp10 with SMTP id p10so21511028wes.11 for ; Fri, 13 Mar 2015 01:09:38 -0700 (PDT) Sender: Paolo Bonzini Message-ID: <55029B3E.4000501@redhat.com> Date: Fri, 13 Mar 2015 09:09:34 +0100 From: Paolo Bonzini MIME-Version: 1.0 References: <1426210723-16735-1-git-send-email-famz@redhat.com> <1426210723-16735-2-git-send-email-famz@redhat.com> In-Reply-To: <1426210723-16735-2-git-send-email-famz@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 1/4] exec: Atomic access to bounce buffer List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Fam Zheng , qemu-devel@nongnu.org On 13/03/2015 02:38, Fam Zheng wrote: > There could be a race condition when two processes call > address_space_map concurrently and both want to use the bounce buffer. > > Add an in_use flag in BounceBuffer to sync it. > > Signed-off-by: Fam Zheng > --- > exec.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/exec.c b/exec.c > index 60b9752..8d4e134 100644 > --- a/exec.c > +++ b/exec.c > @@ -2481,6 +2481,7 @@ typedef struct { > void *buffer; > hwaddr addr; > hwaddr len; > + bool in_use; > } BounceBuffer; > > static BounceBuffer bounce; > @@ -2569,9 +2570,10 @@ void *address_space_map(AddressSpace *as, > l = len; > mr = address_space_translate(as, addr, &xlat, &l, is_write); > if (!memory_access_is_direct(mr, is_write)) { > - if (bounce.buffer) { > + if (atomic_cmpxchg(&bounce.in_use, false, true)) { atomic_or is enough... > return NULL; > } > + smp_mb(); ... and it already includes a memory barrier. Paolo > /* Avoid unbounded allocations */ > l = MIN(l, TARGET_PAGE_SIZE); > bounce.buffer = qemu_memalign(TARGET_PAGE_SIZE, l); > @@ -2639,6 +2641,7 @@ void address_space_unmap(AddressSpace *as, void *buffer, hwaddr len, > qemu_vfree(bounce.buffer); > bounce.buffer = NULL; > memory_region_unref(bounce.mr); > + atomic_mb_set(&bounce.in_use, false); > cpu_notify_map_clients(); > } > >