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charset="UTF-8" X-Rspamd-Queue-Id: 5E4A4A000C X-Stat-Signature: gefrr8xtsckj5m6aiy9mesg5xxhsfqhm X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1673991963-173644 X-HE-Meta: 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 OWl2wlV3 pzi81hUjjOiCgSmI= 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: 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); } 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. (Spinlocks are different - the kernel relies on being able to drop references via spin_unlock() in some places.)