On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote: > +The simplest implementation just uses a mutex and an 'inited' flag. There's a perfectly good real word "initialised" / initialized. https://chambers.co.uk/search/?query=inited&title=21st > +For the single-pointer case, a further optimized implementation > +eliminates the mutex and instead uses compare-and-exchange: > + > + static struct foo *foo; > + > + int init_foo_if_needed(void) > + { > + struct foo *p; > + > + /* pairs with successful cmpxchg_release() below */ > + if (smp_load_acquire(&foo)) > + return 0; > + > + p = alloc_foo(); > + if (!p) > + return -ENOMEM; > + > + /* on success, pairs with smp_load_acquire() above and below */ > + if (cmpxchg_release(&foo, NULL, p) != NULL) { > + free_foo(p); > + /* pairs with successful cmpxchg_release() above */ > + smp_load_acquire(&foo); > + } > + return 0; > + } > + > +Note that when the cmpxchg_release() fails due to another task already > +having done it, a second smp_load_acquire() is required, since we still > +need to acquire the data that the other task released. You may be > +tempted to upgrade cmpxchg_release() to cmpxchg() with the goal of it > +acting as both ACQUIRE and RELEASE, but that doesn't work here because > +cmpxchg() only guarantees memory ordering if it succeeds. > + > +Because of the above subtlety, the version with the mutex instead of > +cmpxchg_release() should be preferred, except potentially in cases where > +it is difficult to provide anything other than a global mutex and where > +the one-time data is part of a frequently allocated structure. In that > +case, a global mutex might present scalability concerns. There are concerns other than scalability where we might want to eliminate the mutex. For example, if (likely) alloc_foo() needs to allocate memory and we would need foo to perform page writeback, then either we must allocate foo using GFP_NOFS or do without the mutex, lest we deadlock on this new mutex. You might think this would argue for just using GFP_NOFS always, but GFP_NOFS is a big hammer which forbids reclaiming from any filesystem, whereas we might only need this foo to reclaim from a particular filesystem.
