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Wed, 22 Nov 2023 14:04:51 GMT Received: from smtpav04.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5FD3E20043; Wed, 22 Nov 2023 14:04:51 +0000 (GMT) Received: from smtpav04.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6A52B20040; Wed, 22 Nov 2023 14:04:50 +0000 (GMT) Received: from tuxmaker.boeblingen.de.ibm.com (unknown [9.152.85.9]) by smtpav04.fra02v.mail.ibm.com (Postfix) with ESMTPS; Wed, 22 Nov 2023 14:04:50 +0000 (GMT) Date: Wed, 22 Nov 2023 15:04:49 +0100 From: Alexander Gordeev To: Jann Horn Cc: Suren Baghdasaryan , peterz@infradead.org, Ingo Molnar , Will Deacon , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to control it Message-ID: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-13-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-TM-AS-GCONF: 00 X-Proofpoint-GUID: 5MhcCP9ifUrUeiSZNpIhaNnBNovbexne X-Proofpoint-ORIG-GUID: zLZ94B08QXl1Fcylsg1aKWlm3qbMRA7p X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.987,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2023-11-22_09,2023-11-22_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 spamscore=0 suspectscore=0 priorityscore=1501 impostorscore=0 mlxlogscore=999 clxscore=1011 bulkscore=0 adultscore=0 lowpriorityscore=0 mlxscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311060000 definitions=main-2311220100 X-Rspamd-Queue-Id: 2B5D42009C X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: oi4abe7snz46nd6981en3bod57k4ciz4 X-HE-Tag: 1700661920-523135 X-HE-Meta: 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 5iS9l7Rz AYCnUkN25e8ggL+9hwQCrn9tdJOTNweHtQIdLiCkuO4xcLBhIqtL5HqCzTVb/mpj3BTyUnKHkFTGL3PWRVbbYWDIf9QOi540XZY69/E/9WX7kdhg5O+Mf1b1c91vcUrXteo0K5cGZ9/dBp2Bx2nEKHU+Y9IjQr1nxMcLD X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jan 17, 2023 at 10:45:25PM +0100, Jann Horn wrote: Hi Jann, > On Tue, Jan 17, 2023 at 10:28 PM Suren Baghdasaryan wrote: > > On Tue, Jan 17, 2023 at 10:03 AM Jann Horn wrote: > > > > > > +locking maintainers > > > > Thanks! I'll CC the locking maintainers in the next posting. > > > > > > > > On Mon, Jan 9, 2023 at 9:54 PM Suren Baghdasaryan wrote: > > > > Introduce a per-VMA rw_semaphore to be used during page fault handling > > > > instead of mmap_lock. Because there are cases when multiple VMAs need > > > > to be exclusively locked during VMA tree modifications, instead of the > > > > usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock > > > > exclusively and setting vma->lock_seq to the current mm->lock_seq. When > > > > mmap_write_lock holder is done with all modifications and drops mmap_lock, > > > > it will increment mm->lock_seq, effectively unlocking all VMAs marked as > > > > locked. > > > [...] > > > > +static inline void vma_read_unlock(struct vm_area_struct *vma) > > > > +{ > > > > + up_read(&vma->lock); > > > > +} > > > > > > One thing that might be gnarly here is that I think you might not be > > > allowed to use up_read() to fully release ownership of an object - > > > from what I remember, I think that up_read() (unlike something like > > > spin_unlock()) can access the lock object after it's already been > > > acquired by someone else. So if you want to protect against concurrent > > > deletion, this might have to be something like: > > > > > > rcu_read_lock(); /* keeps vma alive */ > > > up_read(&vma->lock); > > > rcu_read_unlock(); > > > > But for deleting VMA one would need to write-lock the vma->lock first, > > which I assume can't happen until this up_read() is complete. Is that > > assumption wrong? > > __up_read() does: > > rwsem_clear_reader_owned(sem); > tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count); > DEBUG_RWSEMS_WARN_ON(tmp < 0, sem); > if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) == > RWSEM_FLAG_WAITERS)) { > clear_nonspinnable(sem); > rwsem_wake(sem); > } This sequence is covered by preempt_disable()/preempt_enable(). Would not it preserve the RCU grace period until after __up_read() exited? > The atomic_long_add_return_release() is the point where we are doing > the main lock-releasing. > > So if a reader dropped the read-lock while someone else was waiting on > the lock (RWSEM_FLAG_WAITERS) and no other readers were holding the > lock together with it, the reader also does clear_nonspinnable() and > rwsem_wake() afterwards. > But in rwsem_down_write_slowpath(), after we've set > RWSEM_FLAG_WAITERS, we can return successfully immediately once > rwsem_try_write_lock() sees that there are no active readers or > writers anymore (if RWSEM_LOCK_MASK is unset and the cmpxchg > succeeds). We're not necessarily waiting for the "nonspinnable" bit or > the wake. > > So yeah, I think down_write() can return successfully before up_read() > is done with its memory accesses. Thanks!