From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41627) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YROEu-0000fu-RC for qemu-devel@nongnu.org; Fri, 27 Feb 2015 11:58:13 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YROEs-0003OK-4V for qemu-devel@nongnu.org; Fri, 27 Feb 2015 11:58:12 -0500 Message-ID: <54F0A212.3050807@redhat.com> Date: Fri, 27 Feb 2015 11:57:54 -0500 From: Max Reitz MIME-Version: 1.0 References: <1425045947-9271-1-git-send-email-mreitz@redhat.com> <20150227165723.GB32542@stefanha-thinkpad.redhat.com> In-Reply-To: <20150227165723.GB32542@stefanha-thinkpad.redhat.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2] block/vdi: Add locking for parallel requests List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Paolo Bonzini , qemu-devel@nongnu.org, qemu-block@nongnu.org, qemu-stable@nongnu.org On 2015-02-27 at 11:57, Stefan Hajnoczi wrote: > On Fri, Feb 27, 2015 at 09:05:47AM -0500, Max Reitz wrote: >> Concurrently modifying the bmap does not seem to be a good idea; this patch adds >> a lock for it. See https://bugs.launchpad.net/qemu/+bug/1422307 for what >> can go wrong without. >> >> Cc: qemu-stable >> Signed-off-by: Max Reitz >> --- >> v2: >> - Make the mutex cover vdi_co_write() completely [Kevin] >> - Add a TODO comment [Kevin] >> --- >> block/vdi.c | 11 +++++++++++ >> 1 file changed, 11 insertions(+) >> >> diff --git a/block/vdi.c b/block/vdi.c >> index 74030c6..f5f42ef 100644 >> --- a/block/vdi.c >> +++ b/block/vdi.c >> @@ -51,6 +51,7 @@ >> >> #include "qemu-common.h" >> #include "block/block_int.h" >> +#include "block/coroutine.h" >> #include "qemu/module.h" >> #include "migration/migration.h" >> >> @@ -196,6 +197,8 @@ typedef struct { >> /* VDI header (converted to host endianness). */ >> VdiHeader header; >> >> + CoMutex bmap_lock; >> + >> Error *migration_blocker; >> } BDRVVdiState; >> >> @@ -498,6 +501,8 @@ static int vdi_open(BlockDriverState *bs, QDict *options, int flags, >> goto fail_free_bmap; >> } >> >> + qemu_co_mutex_init(&s->bmap_lock); >> + >> /* Disable migration when vdi images are used */ >> error_set(&s->migration_blocker, >> QERR_BLOCK_FORMAT_FEATURE_NOT_SUPPORTED, >> @@ -607,6 +612,9 @@ static int vdi_co_write(BlockDriverState *bs, >> >> logout("\n"); >> >> + /* TODO: Figure out why this is necessary */ >> + qemu_co_mutex_lock(&s->bmap_lock); > If we don't know why bmap_lock works, it would be more approprate to > take the same approach as VMDK and VHDX where there is a simply s->lock > that protects all reads and writes. That way we know for sure there is > no parallel I/O going on. > > (Since the problem is not understood, maybe reads in parallel with > writes could also cause problems. Better to really do a coarse lock > instead of just bmap_lock in write.) OK, will do. Max