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Mon, 16 Jan 2023 03:14:55 -0800 (PST) Received: from localhost ([2400:8902::f03c:93ff:fe27:642a]) by smtp.gmail.com with ESMTPSA id 3-20020a621503000000b00581c741f95csm16176892pfv.46.2023.01.16.03.14.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Jan 2023 03:14:54 -0800 (PST) Date: Mon, 16 Jan 2023 11:14:38 +0000 From: Hyeonggon Yoo <42.hyeyoo@gmail.com> To: Suren Baghdasaryan Cc: 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, peterz@infradead.org, 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, jannh@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 41/41] mm: replace rw_semaphore with atomic_t in vma_lock Message-ID: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-42-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230109205336.3665937-42-surenb@google.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 09, 2023 at 12:53:36PM -0800, Suren Baghdasaryan wrote: > diff --git a/include/linux/mm.h b/include/linux/mm.h > index d40bf8a5e19e..294dd44b2198 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -627,12 +627,16 @@ static inline void vma_write_lock(struct vm_area_struct *vma) > * mm->mm_lock_seq can't be concurrently modified. > */ > mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq); > - if (vma->vm_lock_seq == mm_lock_seq) > + if (vma->vm_lock->lock_seq == mm_lock_seq) > return; > > - down_write(&vma->vm_lock->lock); > - vma->vm_lock_seq = mm_lock_seq; > - up_write(&vma->vm_lock->lock); > + if (atomic_cmpxchg(&vma->vm_lock->count, 0, -1)) > + wait_event(vma->vm_mm->vma_writer_wait, > + atomic_cmpxchg(&vma->vm_lock->count, 0, -1) == 0); > + vma->vm_lock->lock_seq = mm_lock_seq; > + /* Write barrier to ensure lock_seq change is visible before count */ > + smp_wmb(); > + atomic_set(&vma->vm_lock->count, 0); > } > > /* > @@ -643,20 +647,28 @@ static inline void vma_write_lock(struct vm_area_struct *vma) > static inline bool vma_read_trylock(struct vm_area_struct *vma) > { > /* Check before locking. A race might cause false locked result. */ > - if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > + if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > return false; > > - if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0)) > + if (unlikely(!atomic_inc_unless_negative(&vma->vm_lock->count))) > return false; > > + /* If atomic_t overflows, restore and fail to lock. */ > + if (unlikely(atomic_read(&vma->vm_lock->count) < 0)) { > + if (atomic_dec_and_test(&vma->vm_lock->count)) > + wake_up(&vma->vm_mm->vma_writer_wait); > + return false; > + } > + > /* > * Overflow might produce false locked result. > * False unlocked result is impossible because we modify and check > * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq > * modification invalidates all existing locks. > */ > - if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > - up_read(&vma->vm_lock->lock); > + if (unlikely(vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > + if (atomic_dec_and_test(&vma->vm_lock->count)) > + wake_up(&vma->vm_mm->vma_writer_wait); > return false; > } With this change readers can cause writers to starve. What about checking waitqueue_active() before or after increasing vma->vm_lock->count? -- Thanks, Hyeonggon