Messages in this thread | | | Date | Mon, 2 Dec 2013 17:27:55 +0100 | From | Ingo Molnar <> | Subject | Re: Found it! (was Re: [3.10] Oopses in kmem_cache_allocate() via prepare_creds()) |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Sat, Nov 30, 2013 at 1:08 PM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > I still don't see what could be wrong with the pipe_inode_info thing, > > but the fact that it's been so consistent in your traces does make me > > suspect it really is *that* particular slab. > > I think I finally found it. > > I've spent waaayy too much time looking at and thinking about that > code without seeing anything wrong, but this morning I woke up and > thought to myself "What if.." > > And looking at the code again, I went "BINGO". > > All our reference counting etc seems right, but we have one very > subtle bug: on the freeing path, we have a pattern like this: > > spin_lock(&inode->i_lock); > if (!--pipe->files) { > inode->i_pipe = NULL; > kill = 1; > } > spin_unlock(&inode->i_lock); > __pipe_unlock(pipe); > if (kill) > free_pipe_info(pipe); > > which on the face of it is trying to be very careful in not accessing > the pipe-info after it is released by having that "kill" flag, and > doing the release last. > > And it's complete garbage. > > Why? > > Because the thread that decrements "pipe->files" *without* releasing > it, will very much access it after it has been dropped: that > "__pipe_unlock(pipe)" happens *after* we've decremented the pipe > reference count and dropped the inode lock. So another CPU can come > in and free the structure concurrently with that > __pipe_unlock(pipe). > > This happens in two places, and we don't actually need or want the > pipe lock for the pipe->files accesses (since pipe->files is > protected by inode->i_lock, not the pipe lock), so the solution is > to just do the __pipe_unlock() before the whole dance about the > pipe->files reference count. > > Patch appended. And no wonder nobody has ever seen it, because the > race is unlikely as hell to ever happen. Simon, I assume it will be > another few months before we can say "yeah, that fixed it", but I > really think this is it. It explains all the symptoms, including > "DEBUG_PAGEALLOC didn't catch it" (because the access happens just > as it is released, and DEBUG_PAGEALLOC takes too long to actually > free unmap the page etc). > > Linus > fs/pipe.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/fs/pipe.c b/fs/pipe.c > index d2c45e14e6d8..18f1a4b2dbbc 100644 > --- a/fs/pipe.c > +++ b/fs/pipe.c > @@ -743,13 +743,14 @@ pipe_release(struct inode *inode, struct file *file) > kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); > kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT); > } > + __pipe_unlock(pipe); > + > spin_lock(&inode->i_lock); > if (!--pipe->files) { > inode->i_pipe = NULL; > kill = 1; > } > spin_unlock(&inode->i_lock); > - __pipe_unlock(pipe);
I'm wondering why pipe-mutex was introduced. It was done fairly recently, with no justification given:
From 72b0d9aacb89f3759931ec440e1b535671145bb4 Mon Sep 17 00:00:00 2001 From: Al Viro <viro@zeniv.linux.org.uk> Date: Thu, 21 Mar 2013 02:32:24 -0400 Subject: [PATCH] pipe: don't use ->i_mutex
now it can be done - put mutex into pipe_inode_info, use it instead of ->i_mutex
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> --- fs/ocfs2/file.c | 6 ++---- fs/pipe.c | 5 +++-- include/linux/pipe_fs_i.h | 2 ++ 3 files changed, 7 insertions(+), 6 deletions(-)
It's not like there should be many (any?) VFS operations where a pipe is used via i_mutex and pipe->mutex in parallel, which would improve scalability - so I don't see the scalability advantage. (But I might be missing something)
Barring such kind of workload the extra mutex just adds extra micro-costs because now two locks have to be taken on creation/destruction, plus it adds extra complexity and races.
So unless I'm missing something obvious, another good fix would be to just revert pipe->mutex and rely on i_mutex as before?
Thanks,
Ingo
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