From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ville =?iso-8859-1?Q?Syrj=E4l=E4?= Date: Thu, 18 Mar 2010 15:26:04 +0000 Subject: Re: [PATCH v3 1/4] DSS2: OMAPFB: Add support for switching memory Message-Id: <20100318152604.GM18243@nokia.com> List-Id: References: <1267795582-21004-1-git-send-email-ville.syrjala@nokia.com> <20100317173407.GD30422@localhost> <20100317201425.GI18243@nokia.com> <20100318085239.GE30422@localhost> In-Reply-To: <20100318085239.GE30422@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable To: "Deak Imre (Nokia-D/Helsinki)" Cc: "Valkeinen Tomi (Nokia-D/Helsinki)" , "linux-fbdev@vger.kernel.org" , "linux-omap@vger.kernel.org" On Thu, Mar 18, 2010 at 09:52:39AM +0100, Deak Imre (Nokia-D/Helsinki) wrot= e: > On Wed, Mar 17, 2010 at 09:14:25PM +0100, Syrjala Ville (Nokia-D/Helsinki= ) wrote: > > On Wed, Mar 17, 2010 at 06:34:07PM +0100, Deak Imre (Nokia-D/Helsinki) = wrote: > > > Hi, > > >=20 > > > couple of minor comments inlined. > > >=20 > > > On Fri, Mar 05, 2010 at 02:26:19PM +0100, Syrjala Ville (Nokia-D/Hels= inki) wrote: > > > [...] > > > > @@ -115,30 +184,57 @@ static int omapfb_setup_mem(struct fb_info *f= bi, struct omapfb_mem_info *mi) > > > > struct omapfb_info *ofbi =3D FB2OFB(fbi); > > > > struct omapfb2_device *fbdev =3D ofbi->fbdev; > > > > struct omapfb2_mem_region *rg; > > > > - int r, i; > > > > + int r =3D 0; > > > > size_t size; > > > > + int i; > > > >=20 > > > > if (mi->type > OMAPFB_MEMTYPE_MAX) > > > > return -EINVAL; > > > >=20 > > > > size =3D PAGE_ALIGN(mi->size); > > > >=20 > > > > - rg =3D &ofbi->region; > > > > + rg =3D ofbi->region; > > > >=20 > > > > - for (i =3D 0; i < ofbi->num_overlays; i++) { > > > > - if (ofbi->overlays[i]->info.enabled) > > > > - return -EBUSY; > > > > + /* FIXME probably should be a rwsem ... */ > > > > + mutex_lock(&rg->mtx); > > > > + while (rg->ref) { > > > > + mutex_unlock(&rg->mtx); > > > > + schedule(); > > > > + mutex_lock(&rg->mtx); > > > > + } > > >=20 > > > Yes, rwsem would mean no unnecessary scheduling and also make things > > > clearer. > >=20 > > Just tried it and seems to be mostly OK. We get lockdep checking as a > > bonus. It didn't like setup_plane taking the same rwsem twice so I > > added a check to see if the old and new regions are the same and just > > lock once in that case. I thought rwsem was supposed to be OK with > > read recursion but perhaps I was mitaken, or perhaps it's just lockdep > > that's misbehaving. >=20 > Ah ok, so it's not so obvious change. Nested read locks could really lead > to a deadlock I think. A read lock will block if there is a write waiter > in the queue to avoid write starvation.. Yes but I think in out case it should be fine because if we hit this: t thread 1 thread 2 | | down_read(0) | down_write(1) v down_read(1) then thread 2 will eventually do a up_write() without taking any other region rwsem, and thread 1 can then continue. The other locks we have to worry about are the fb_info mutex which should always be taken before any region lock, and mmap_sem which in mmap() is taken before the region lock. I'm hoping there are no mmap_sem users lurking in the driver that already hold the region rwsem. --=20 Ville Syrj=E4l=E4