On 19.01.22 16:08, Boqun Feng wrote: > Hi, > > On Wed, Jan 19, 2022 at 12:37:02PM +0100, David Hildenbrand wrote: >> Lockdep complains that we do during mmap of the vmcore: >> down_write(mmap_lock); >> down_read(vmcore_cb_rwsem); >> And during read of the vmcore: >> down_read(vmcore_cb_rwsem); >> down_read(mmap_lock); >> >> We cannot possibly deadlock when only taking vmcore_cb_rwsem in read >> mode, however, it's hard to teach that to lockdep. >> > > Lockdep warned about the above sequences because rw_semaphore is a fair > read-write lock, and the following can cause a deadlock: > > TASK 1 TASK 2 TASK 3 > ====== ====== ====== > down_write(mmap_lock); > down_read(vmcore_cb_rwsem) > down_write(vmcore_cb_rwsem); // blocked > down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness > down_read(mmap_lock); // blocked > > IOW, a reader can block another read if there is a writer queued by the > second reader and the lock is fair. > > So there is a deadlock possiblity. Task 3 will never take the mmap_lock before doing a down_write(vmcore_cb_rwsem). How would this happen? -- Thanks, David / dhildenb
