Messages in this thread | | | Date | Wed, 7 Feb 2018 17:58:02 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH RFC] ashmem: Fix lockdep RECLAIM_FS false positive |
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On Wed, Feb 07, 2018 at 08:09:36AM -0800, Joel Fernandes wrote: > Hi Peter, > > On Wed, Feb 7, 2018 at 12:07 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Feb 05, 2018 at 04:49:03PM -0800, Joel Fernandes wrote: > > > >> [ 2115.359650] -(1)[106:kswapd0]================================= > >> [ 2115.359665] -(1)[106:kswapd0][ INFO: inconsistent lock state ] > >> [ 2115.359684] -(1)[106:kswapd0]4.9.60+ #2 Tainted: G W O > >> [ 2115.359699] -(1)[106:kswapd0]--------------------------------- > >> [ 2115.359715] -(1)[106:kswapd0]inconsistent {RECLAIM_FS-ON-W} -> > >> {IN-RECLAIM_FS-W} usage. > > > > Please don't wrap log output, this is unreadable :/ > > Sorry about that, here's the unwrapped output, I'll fix the commit > message in next rev: https://pastebin.com/e0BNGkaN
So if you trim that leading garbage: "[ 2115.359650] -(1)[106:kswapd0]" you instantly have half you screen back.
> > Also, the output is from an ancient kernel and doesn't match the current > > code. > > Right, however the driver hasn't changed and I don't see immediately > how lockdep handles this differently upstream, so I thought of fixing > it upstream.
Well, the annotation got a complete rewrite. Granted, it _should_ be similar, but the output will be different.
> The bail out happens when GFP_FS is *not* set.
Argh, reading is hard.
> Lockdep reports this issue when GFP_FS is infact set, and we enter > this path and acquire the lock. So lockdep seems to be doing the right > thing however by design it is reporting a false-positive.
So I'm not seeing how its a false positive. fs/inode.c sets a different lock class per filesystem type. So recursing on an i_mutex within a filesystem does sound dodgy.
> The real issue is that the lock being acquired is of the same lock > class and a different lock instance is acquired under GFP_FS that > happens to be of the same class. > > So the issue seems to me to be: > Process A kswapd > --------- ------ > acquire i_mutex Enter RECLAIM_FS > > Enter RECLAIM_FS acquire different i_mutex
That's not a false positive, that's a 2 process way of writing i_mutex recursion.
What are the rules of acquiring two i_mutexes within a filesystem?
> Neil tried to fix this sometime back: > https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg623909.html > but it was kind of NAK'ed.
So that got nacked because Neil tried to fix it in the vfs core. Also not entirely sure that's the same problem.
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