| Message ID | 20210419213636.1514816-4-vgoyal@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | dax: Fix missed wakeup in put_unlocked_entry() | expand |
On Mon 19-04-21 17:36:36, Vivek Goyal wrote: > 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 wait these. > > 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") > Suggested-by: Dan Williams <dan.j.williams@intel.com> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Looks good to me. Thanks for fixing this! Feel free to add: Reviewed-by: Jan Kara <jack@suse.cz> Honza > --- > fs/dax.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/dax.c b/fs/dax.c > index f19d76a6a493..cc497519be83 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -676,7 +676,7 @@ static int __dax_invalidate_entry(struct address_space *mapping, > mapping->nrexceptional--; > ret = 1; > out: > - put_unlocked_entry(&xas, entry, WAKE_NEXT); > + put_unlocked_entry(&xas, entry, WAKE_ALL); > xas_unlock_irq(&xas); > return ret; > } > -- > 2.25.4 >
diff --git a/fs/dax.c b/fs/dax.c index f19d76a6a493..cc497519be83 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -676,7 +676,7 @@ static int __dax_invalidate_entry(struct address_space *mapping, mapping->nrexceptional--; ret = 1; out: - put_unlocked_entry(&xas, entry, WAKE_NEXT); + put_unlocked_entry(&xas, entry, WAKE_ALL); xas_unlock_irq(&xas); return ret; }
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 wait these. 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") Suggested-by: Dan Williams <dan.j.williams@intel.com> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> --- fs/dax.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)