On Thu, Apr 30, 2020 at 05:43:56PM +0100, fdman...@kernel.org wrote: > From: Filipe Manana <fdman...@suse.com> > > Since 5.7-rc1, on btrfs we have a percpu counter initialization for which > we always pass a GFP_KERNEL gfp_t argument (this happens since commit > 2992df73268f78 ("btrfs: Implement DREW lock")). That is safe in some > contextes but not on others where allowing fs reclaim could lead to a > deadlock because we are either holding some btrfs lock needed for a > transaction commit or holding a btrfs transaction handle open. Because > of that we surround the call to the function that initializes the percpu > counter with a NOFS context using memalloc_nofs_save() (this is done at > btrfs_init_fs_root()). > > However it turns out that this is not enough to prevent a possible > deadlock because percpu_alloc() determines if it is in an atomic context > by looking exclusively at the gfp flags passed to it (GFP_KERNEL in this > case) and it is not aware that a NOFS context is set. Because it thinks > it is in a non atomic context it locks the pcpu_alloc_mutex, which can > result in a btrfs deadlock when pcpu_balance_workfn() is running, has > acquired that mutex and is waiting for reclaim, while the btrfs task that > called percpu_counter_init() (and therefore percpu_alloc()) is holding > either the btrfs commit_root semaphore or a transaction handle (done at > fs/btrfs/backref.c:iterate_extent_inodes()), which prevents reclaim from > finishing as an attempt to commit the current btrfs transaction will > deadlock. ... > This could be fixed by making btrfs pass GFP_NOFS instead of GFP_KERNEL to > percpu_counter_init() in contextes where it is not reclaim safe, however > that type of approach is discouraged since memalloc_[nofs|noio]_save() > were introduced. Therefore this change makes pcpu_alloc() look up into > an existing nofs/noio context before deciding whether it is in an atomic > context or not. > > Signed-off-by: Filipe Manana <fdman...@suse.com>
Acked-by: Tejun Heo <t...@kernel.org> Thanks. -- tejun