On Mon, Oct 17, 2016 at 08:55:52PM +0200, Christoph Hellwig wrote: > On Mon, Oct 17, 2016 at 02:19:47PM -0400, Jeff Moyer wrote: > > This ends up being a call to __sb_end_write: > > > > void __sb_end_write(struct super_block *sb, int level) > > { > > percpu_up_read(sb->s_writers.rw_sem + level-1); > > } > > > > Nothing guarantees that submission and completion happen on the same > > CPU. Is this safe? > > Good point. From my reading of the percpu_rwsem implementation it > is not safe to release it from a different CPU. Which makes me > wonder how we can protect aio writes properly here.. percpu-rwsem has the same semantics as regular rwsems, so preemptible and 'owner' stuff. Therefore we must support doing up from a different cpu than we did down on; the owner could've been migrated while we held it. And while there's a metric ton of tricky in the implementation, this part is actually fairly straight forward. We only care about the direct sum of the per-cpu counter, see readers_active_check() -> per_cpu_sum(). So one cpu doing an inc and another doing a dec summed is still 0. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html