On Tue, Feb 25, 2014 at 02:03:36PM -0800, Eric W. Biederman wrote: > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> writes: > > > On Mon, Feb 24, 2014 at 04:01:29PM -0800, Eric W. Biederman wrote: > >> Miklos Szeredi <miklos@xxxxxxxxxx> writes: > >> > >> > > >> > You can optimize this by including the negative check within the above d_locked > >> > region and calling __d_drop() instead. > >> > >> For this patch just moving the code and not changing it is the corret > >> thing to do because it helps with review and understanding the code. > >> > >> There are two ways I could see going with optimizing the preamble. > >> Simply dropping the d_lock from around the d_unhashed test as a pointer > >> dereference should be atomic, and the test is racy against > >> d_materialise_unique. > > > > Could you explain? What's the race, and what are the consequences? Actually I was just confused as to whether the above was "is racy" was claiming the existance of some bug. I believe I should have read the above as more like "the test is already racy against d_materialise_unique, but it's a harmless race, and dropping the d_lock wouldn't make it any worse". > >> (We don't always hold the parent directories inode mutex when d_invalidate is called). > > d_unhashed is not a permanent condition because of d_materialise_unique, > and d_splice_alias. > > d_invalidate can be called on an unhashed dentry in one of two ways > (either d_revalidate dropped the dentry or another routine that drops > the dentry beat the current invocation of d_invalidate to the job). > > > There are 3 places d_revalidate is called. > > Once on the rcu path with with the appropriate flag set. > > Once without out the parent i_mutex held, just off of the rcu path, > on that path d_invalidate is when d_revalidate fails. > > Once during lookup with the parent directory i_mutex held. > > > Because the parent direcories i_mutex is not always held accross > d_revalidate and the following d_invalidate it happens that d_invalidate > is not always an atomic operation. > > > At worst the race results in a dentry that is dropped when it could be > hashed, Because somebody not holding the i_mutex calls d_invalidate based on old information and unhashes something that d_materialise_unique/d_splice_alias just hashed? > that we will resurrect next time someone attempts to look it > up and d_materialise_unique/d_splice_alias is called. OK. > None of that really matters for optimizing d_invalidate, but it is part > of the background in which d_invalidate lives. All that is significant > in d_invalidate is knowing that d_materialise_unique, and possibly > d_splice_alias may run concurrently with d_invalidate. It is unlikely > and essentially harmless. > > > After my patchset (because I removed all of the d_drop's from > .d_revalidate) the only race that should remain is between two parallel > calls of d_invalidate. Which probably means we can remove the test for > d_unhashed altogether. > > Right now I just want to make this first big step and make certain the > code is solid. After that optimization is easy. Thanks for the explanation! --b. -- 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
