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charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20250113014729.ms5sdfnhynlamgrk@master> User-Agent: NeoMutt/20170113 (1.7.2) X-Rspamd-Queue-Id: 36BD540006 X-Stat-Signature: we9phahx5ow5un95e8h55kqp3j8qqux9 X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1736735150-991715 X-HE-Meta: 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 hjjUgRsa 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 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 Mon, Jan 13, 2025 at 01:47:29AM +0000, Wei Yang wrote: >On Sat, Jan 11, 2025 at 12:14:47PM -0800, Suren Baghdasaryan wrote: >>On Sat, Jan 11, 2025 at 3:24 AM Mateusz Guzik wrote: >>> >>> On Fri, Jan 10, 2025 at 08:25:58PM -0800, Suren Baghdasaryan wrote: >>> >>> So there were quite a few iterations of the patch and I have not been >>> reading majority of the feedback, so it may be I missed something, >>> apologies upfront. :) >>> > >Hi, I am new to memory barriers. Hope not bothering. > >>> > /* >>> > * Try to read-lock a vma. The function is allowed to occasionally yield false >>> > * locked result to avoid performance overhead, in which case we fall back to >>> > @@ -710,6 +742,8 @@ static inline void vma_lock_init(struct vm_area_struct *vma) >>> > */ >>> > static inline bool vma_start_read(struct vm_area_struct *vma) >>> > { >>> > + int oldcnt; >>> > + >>> > /* >>> > * Check before locking. A race might cause false locked result. >>> > * We can use READ_ONCE() for the mm_lock_seq here, and don't need >>> > @@ -720,13 +754,19 @@ static inline bool vma_start_read(struct vm_area_struct *vma) >>> > if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence)) >>> > return false; >>> > >>> > - if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0)) >>> > + /* >>> > + * If VMA_LOCK_OFFSET is set, __refcount_inc_not_zero_limited() will fail >>> > + * because VMA_REF_LIMIT is less than VMA_LOCK_OFFSET. >>> > + */ >>> > + if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt, >>> > + VMA_REF_LIMIT))) >>> > return false; >>> > >>> >>> Replacing down_read_trylock() with the new routine loses an acquire >>> fence. That alone is not a problem, but see below. >> >>Hmm. I think this acquire fence is actually necessary. We don't want >>the later vm_lock_seq check to be reordered and happen before we take >>the refcount. Otherwise this might happen: >> >>reader writer >>if (vm_lock_seq == mm_lock_seq) // check got reordered >> return false; >> vm_refcnt += VMA_LOCK_OFFSET >> vm_lock_seq == mm_lock_seq >> vm_refcnt -= VMA_LOCK_OFFSET >>if (!__refcount_inc_not_zero_limited()) >> return false; >> >>Both reader's checks will pass and the reader would read-lock a vma >>that was write-locked. >> > >Here what we plan to do is define __refcount_inc_not_zero_limited() with >acquire fence, e.g. with atomic_try_cmpxchg_acquire(), right? > BTW, usually we pair acquire with release. The __vma_start_write() provide release fence when locked, so for this part we are ok, right? -- Wei Yang Help you, Help me