Re: [PATCH 4.19 02/80] shmem: fix possible deadlocks on shmlock_user_lock

Hi!

This may not risk an actual deadlock, since shmem inodes do not take part in writeback accounting, but there are several easy ways to avoid it.

...

Take info->lock out of the chain and the possibility of deadlock or lockdep warning goes away.

It is unclear to me if actual possibility of deadlock exists or not, but anyway:

int retval = -ENOMEM;

• spin_lock_irq(&info->lock);

• /*

• * What serializes the accesses to info->flags?
  

• * ipc_lock_object() when called from shmctl_do_lock(),
  

• * no serialization needed when called from shm_destroy().
  

• */
  

  if (lock && !(info->flags & VM_LOCKED)) { if (!user_shm_lock(inode->i_size, user)) goto out_nomem;

Should we have READ_ONCE() here? If it is okay, are concurency sanitizers smart enough to realize that it is okay? Replacing warning with different one would not be exactly a win...

Best regards,

Pavel