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authorVivek Goyal <vgoyal@redhat.com>2021-04-28 15:03:14 -0400
committerDan Williams <dan.j.williams@intel.com>2021-05-07 15:55:44 -0700
commit237388320deffde7c2d65ed8fc9eef670dc979b3 (patch)
treeef0c9e5adcbbbb37b4d8347cc63e0d216f2119ca /MAINTAINERS
parent4c3d043d271d4d629aa2328796cdfc96b37d3b3c (diff)
dax: Wake up all waiters after invalidating dax entry
I am seeing missed wakeups which ultimately lead to a deadlock when I am using virtiofs with DAX enabled and running "make -j". I had to mount virtiofs as rootfs and also reduce to dax window size to 256M to reproduce the problem consistently. So here is the problem. put_unlocked_entry() wakes up waiters only if entry is not null as well as !dax_is_conflict(entry). But if I call multiple instances of invalidate_inode_pages2() in parallel, then I can run into a situation where there are waiters on this index but nobody will wake these waiters. invalidate_inode_pages2() invalidate_inode_pages2_range() invalidate_exceptional_entry2() dax_invalidate_mapping_entry_sync() __dax_invalidate_entry() { xas_lock_irq(&xas); entry = get_unlocked_entry(&xas, 0); ... ... dax_disassociate_entry(entry, mapping, trunc); xas_store(&xas, NULL); ... ... put_unlocked_entry(&xas, entry); xas_unlock_irq(&xas); } Say a fault in in progress and it has locked entry at offset say "0x1c". Now say three instances of invalidate_inode_pages2() are in progress (A, B, C) and they all try to invalidate entry at offset "0x1c". Given dax entry is locked, all tree instances A, B, C will wait in wait queue. When dax fault finishes, say A is woken up. It will store NULL entry at index "0x1c" and wake up B. When B comes along it will find "entry=0" at page offset 0x1c and it will call put_unlocked_entry(&xas, 0). And this means put_unlocked_entry() will not wake up next waiter, given the current code. And that means C continues to wait and is not woken up. This patch fixes the issue by waking up all waiters when a dax entry has been invalidated. This seems to fix the deadlock I am facing and I can make forward progress. Reported-by: Sergio Lopez <slp@redhat.com> Fixes: ac401cc78242 ("dax: New fault locking") Reviewed-by: Jan Kara <jack@suse.cz> Suggested-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Link: https://lore.kernel.org/r/20210428190314.1865312-4-vgoyal@redhat.com Signed-off-by: Dan Williams <dan.j.williams@intel.com>
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