On Tue, Apr 18, 2023 at 05:43:15PM +0200, Sebastian Andrzej Siewior wrote: > From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > > commit 1c0908d8e441631f5b8ba433523cf39339ee2ba0 upstream. > > Jan Kara reported the following bug triggering on 6.0.5-rt14 running dbench > on XFS on arm64. > > kernel BUG at fs/inode.c:625! > Internal error: Oops - BUG: 0 [#1] PREEMPT_RT SMP > CPU: 11 PID: 6611 Comm: dbench Tainted: G E 6.0.0-rt14-rt+ #1 > pc : clear_inode+0xa0/0xc0 > lr : clear_inode+0x38/0xc0 > Call trace: > clear_inode+0xa0/0xc0 > evict+0x160/0x180 > iput+0x154/0x240 > do_unlinkat+0x184/0x300 > __arm64_sys_unlinkat+0x48/0xc0 > el0_svc_common.constprop.4+0xe4/0x2c0 > do_el0_svc+0xac/0x100 > el0_svc+0x78/0x200 > el0t_64_sync_handler+0x9c/0xc0 > el0t_64_sync+0x19c/0x1a0 > > It also affects 6.1-rc7-rt5 and affects a preempt-rt fork of 5.14 so this > is likely a bug that existed forever and only became visible when ARM > support was added to preempt-rt. The same problem does not occur on x86-64 > and he also reported that converting sb->s_inode_wblist_lock to > raw_spinlock_t makes the problem disappear indicating that the RT spinlock > variant is the problem. > > Which in turn means that RT mutexes on ARM64 and any other weakly ordered > architecture are affected by this independent of RT. > > Will Deacon observed: > > "I'd be more inclined to be suspicious of the slowpath tbh, as we need to > make sure that we have acquire semantics on all paths where the lock can > be taken. Looking at the rtmutex code, this really isn't obvious to me > -- for example, try_to_take_rt_mutex() appears to be able to return via > the 'takeit' label without acquire semantics and it looks like we might > be relying on the caller's subsequent _unlock_ of the wait_lock for > ordering, but that will give us release semantics which aren't correct." > > Sebastian Andrzej Siewior prototyped a fix that does work based on that > comment but it was a little bit overkill and added some fences that should > not be necessary. > > The lock owner is updated with an IRQ-safe raw spinlock held, but the > spin_unlock does not provide acquire semantics which are needed when > acquiring a mutex. > > Adds the necessary acquire semantics for lock owner updates in the slow path > acquisition and the waiter bit logic. > > It successfully completed 10 iterations of the dbench workload while the > vanilla kernel fails on the first iteration. > > [ bigeasy@xxxxxxxxxxxxx: Initial prototype fix ] > > Fixes: 700318d1d7b38 ("locking/rtmutex: Use acquire/release semantics") > Fixes: 23f78d4a03c5 ("[PATCH] pi-futex: rt mutex core") > Reported-by: Jan Kara <jack@xxxxxxx> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx > Link: https://lore.kernel.org/r/20221202100223.6mevpbl7i6x5udfd@xxxxxxxxxxxxxxxxxxx > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > --- > > Could this be please backported to 5.15 and earlier? It is already part > of the 6.X kernels. I asked about this by the end of January and I'm > kindly asking again ;) I thought this was only an issues when using the out-of-tree RT patches with these kernels, right? Or is it relevant for 5.15.y from kernel.org without anything else? > This patch applies against v5.15. Should it not apply to earlier > versions, please let me know an I kindly provide a backport. How far back should it go? > I received reports that this fixes "mysterious" crashes and that is how > I noticed that it is not part of the earlier kernels. Again, isn't this only needed for -rt kernels? thanks, greg k-h