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McKenney" To: Eric Dumazet Cc: Dmitry Torokhov , LKML , Eric Dumazet , linux-input@vger.kernel.org Subject: Re: [PATCH] Input: mousedev - add a schedule point in mousedev_write() Reply-To: paulmck@linux.ibm.com References: <20181004154749.111595-1-edumazet@google.com> <20181004185949.GA233675@dtor-ws> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18100419-0052-0000-0000-0000033C8336 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009822; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000267; SDB=6.01097881; UDB=6.00567814; IPR=6.00877922; MB=3.00023616; MTD=3.00000008; XFM=3.00000015; UTC=2018-10-04 19:36:18 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18100419-0053-0000-0000-00005E4BD2EC Message-Id: <20181004193616.GL2674@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-10-04_08:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1810040175 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 04, 2018 at 12:28:56PM -0700, Eric Dumazet wrote: > On Thu, Oct 4, 2018 at 11:59 AM Dmitry Torokhov > wrote: > > > > Hi Eric, > > > > On Thu, Oct 04, 2018 at 08:47:49AM -0700, Eric Dumazet wrote: > > > syzbot was able to trigger rcu stalls by calling write() > > > with large number of bytes. > > > > > > Add a cond_resched() in the loop to avoid this. > > > > I think this simply masks a deeper issue. The code fetches characters > > from userspace in a loop, takes a lock, quickly places response in an > > output buffer, and releases interrupt. I do not see why this should > > cause stalls as we do not hold spinlock/interrupts off for extended > > period of time. > > > > Adding Paul so he can straighten me out... > > > > Well... > > write(fd, buffer, 0x7FFF0000); > > Takes between 20 seconds and 2 minutes depending on CONFIG options .... And two minutes would get you an RCU CPU stall warning, even on distro kernels that set the stall-warning time to a full minute (as opposed to 21 seconds in mainline). > So either apply my patch, or add a limit on the max count, and > possibly break legitimate user space ? > > I dunno... I vote for Eric's patch. In fact: Reviewed-by: Paul E. McKenney > > > Link: https://lkml.org/lkml/2018/8/23/1106 > > > Signed-off-by: Eric Dumazet > > > Reported-by: syzbot+9436b02171ac0894d33e@syzkaller.appspotmail.com > > > Cc: Dmitry Torokhov > > > Cc: linux-input@vger.kernel.org > > > --- > > > drivers/input/mousedev.c | 1 + > > > 1 file changed, 1 insertion(+) > > > > > > diff --git a/drivers/input/mousedev.c b/drivers/input/mousedev.c > > > index e08228061bcdd2f97aaadece31d6c83eb7539ae5..412fa71245afe26a7a8ad75705566f83633ba347 100644 > > > --- a/drivers/input/mousedev.c > > > +++ b/drivers/input/mousedev.c > > > @@ -707,6 +707,7 @@ static ssize_t mousedev_write(struct file *file, const char __user *buffer, > > > mousedev_generate_response(client, c); > > > > > > spin_unlock_irq(&client->packet_lock); > > > + cond_resched(); > > > } > > > > > > kill_fasync(&client->fasync, SIGIO, POLL_IN); > > > -- > > > 2.19.0.605.g01d371f741-goog > > > > > > > Thanks. > > > > -- > > Dmitry >