Messages in this thread | | | Date | Mon, 27 Oct 2014 09:06:54 +0100 | Subject | Re: [GIT PULL] overlay filesystem v25 | From | Miklos Szeredi <> |
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[Paul McKenney added to CC]
On Sat, Oct 25, 2014 at 7:06 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > On Sat, Oct 25, 2014 at 11:53:52AM +0200, Miklos Szeredi wrote: > >> Yes, but it's not about race with copy-up (which the ovl_path_upper() >> protects against), but race of two fsync calls with each other. If >> there's no synchronization between them, then that od->upperfile does >> indeed count as lockless access, no matter that the assignment was >> done under lock. > > p = global; > if (!p) { // outside of lock > p = alloc(); > grab lock > if (!global) { > global = p; > } else { > destroy(p); > p = global; > } > drop lock > } > is a very common pattern, especially if you look for cases when lock is > a spinlock and allocation is blocking (in those cases you'll often see > destroy() part done after dropping the lock; that's where what I fucked up in > what I'd originally pushed. And it wasn't even needed - fput() under > ->i_mutex is OK...)
Being a very common pattern does not automatically make it correct...
My understanding of these issues is very limited, but it's not clear to me what will order initialization of members of p with the storing of p into global. E.g. we start out with global == NULL and p->foo == 0.
CPU1: p->foo = 1 grab lock if (!global) global = p
CPU1: p = global if (p) q = p->foo
Is it guaranteed that the above sequence (as is, without any barriers or ACCESS_ONCE() other than the lock acquisition) will result in q == 1 if p != NULL?
Thanks, Miklos
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