From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55846) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1edazZ-0004Lx-Va for qemu-devel@nongnu.org; Mon, 22 Jan 2018 07:14:26 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1edazW-00063Q-OQ for qemu-devel@nongnu.org; Mon, 22 Jan 2018 07:14:25 -0500 From: Vladimir Sementsov-Ogievskiy References: <20171220154945.88410-1-vsementsov@virtuozzo.com> <20171220154945.88410-4-vsementsov@virtuozzo.com> <20171228052418.GC9192@lemon> <20171229013140.GA13004@lemon> <9b13ad99-981c-f623-0a71-6d1aad73c159@redhat.com> <78110cb5-b69c-a35a-17ed-9bb1f121bc06@virtuozzo.com> Message-ID: <41e11a98-98e9-1766-17e9-8bb5182f4b91@virtuozzo.com> Date: Mon, 22 Jan 2018 15:14:12 +0300 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable Content-Language: en-US Subject: Re: [Qemu-devel] [PATCH v9 03/13] block/dirty-bitmap: add _locked version of bdrv_reclaim_dirty_bitmap List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini , Fam Zheng Cc: kwolf@redhat.com, peter.maydell@linaro.org, lirans@il.ibm.com, qemu-block@nongnu.org, quintela@redhat.com, jsnow@redhat.com, qemu-devel@nongnu.org, armbru@redhat.com, stefanha@redhat.com, den@openvz.org, amit.shah@redhat.com, mreitz@redhat.com, dgilbert@redhat.com 19.01.2018 17:12, Vladimir Sementsov-Ogievskiy wrote: > 18.01.2018 13:09, Paolo Bonzini wrote: >> On 18/01/2018 10:55, Vladimir Sementsov-Ogievskiy wrote: >>>> Most functions that looks at the list are "called with BQL taken". >>>> Functions that write to the list are "called with BQL taken" and call >>>> bdrv_dirty_bitmaps_lock/bdrv_dirty_bitmaps_unlock themselves. >>> Paolo, could you please explain about bitmap locking in more details? >>> Why do we need mutexes? >> We have three cases: >> >> 1) monitor creates and destroy bitmaps. >> >> 2) monitor also has to read the list.=C2=A0 We know it operates with BQL= . >> >> 3) users such as mirror.c create a dirty bitmap in the monitor command >> (under BQL), but they can operate without BQL in a separate iothread so >> we create a separate lock (bitmap->mutex). >> >> While in the second and third case, bitmaps cannot disappear. So in the >> first case you operate with BQL+dirty bitmap mutex.=C2=A0 The result is = that >> you lock out both the second and the third case while creating and >> destroying bitmaps. >> >>> Why do we do not need them >>> on read from the bitmap, only on write? >> Indeed, reading the bitmap also requires taking the lock.=C2=A0 So >> s/Modifying/Accessing/ in that comment. >> >> Paolo > > so, > > =C2=A0=C2=A0=C2=A0 /* Writing to the list requires the BQL_and_=C2=A0 the= dirty_bitmap_mutex. > =C2=A0=C2=A0=C2=A0=C2=A0 * Reading from the list can be done with either = the BQL or the > =C2=A0=C2=A0=C2=A0=C2=A0 * dirty_bitmap_mutex.=C2=A0 Accessing a bitmap o= nly requires > =C2=A0=C2=A0=C2=A0=C2=A0 * dirty_bitmap_mutex. */ > =C2=A0=C2=A0=C2=A0 QemuMutex dirty_bitmap_mutex; > > > > so, accessing the bitmap needs mutex lock? > > Then what do you mean under accessing the bitmap? Any touch of=20 > BdrvDirtyBitmap fields? Then "reading the list" will require bitmap=20 > mutex too. > Or accessing the bitmap is accessing any field except=20 > BdrvDirtyBitmap.list? Then in (2), what do you mean? For example=20 > query-block will go through > the list, but it touches other fields too, so it should lock mutex. > and one more question: What about qmp transactions? Should we lock mutex during the whole=20 transaction? --=20 Best regards, Vladimir