On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > This is V2 of the patch. Posted V1 here. > > https://lore.kernel.org/linux-fsdevel/20210416173524.GA1379987@xxxxxxxxxx/ > > Based on feedback from Dan and Jan, modified the patch to wake up > all waiters when dax entry is invalidated. This solves the issues > of missed wakeups. Care to send a formal patch with this commentary moved below the --- line? One style fixup below... > > 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@xxxxxxxxxx> > Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx> > --- > fs/dax.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > Index: redhat-linux/fs/dax.c > =================================================================== > --- redhat-linux.orig/fs/dax.c 2021-04-16 14:16:44.332140543 -0400 > +++ redhat-linux/fs/dax.c 2021-04-19 11:24:11.465213474 -0400 > @@ -264,11 +264,11 @@ static void wait_entry_unlocked(struct x > finish_wait(wq, &ewait.wait); > } > > -static void put_unlocked_entry(struct xa_state *xas, void *entry) > +static void put_unlocked_entry(struct xa_state *xas, void *entry, bool wake_all) > { > /* If we were the only waiter woken, wake the next one */ > if (entry && !dax_is_conflict(entry)) > - dax_wake_entry(xas, entry, false); > + dax_wake_entry(xas, entry, wake_all); > } > > /* > @@ -622,7 +622,7 @@ struct page *dax_layout_busy_page_range( > entry = get_unlocked_entry(&xas, 0); > if (entry) > page = dax_busy_page(entry); > - put_unlocked_entry(&xas, entry); > + put_unlocked_entry(&xas, entry, false); I'm not a fan of raw true/false arguments because if you read this line in isolation you need to go read put_unlocked_entry() to recall what that argument means. So lets add something like: /** * enum dax_entry_wake_mode: waitqueue wakeup toggle * @WAKE_NEXT: entry was not mutated * @WAKE_ALL: entry was invalidated, or resized */ enum dax_entry_wake_mode { WAKE_NEXT, WAKE_ALL, } ...and use that as the arg for dax_wake_entry(). So I'd expect this to be a 3 patch series, introduce dax_entry_wake_mode for dax_wake_entry(), introduce the argument for put_unlocked_entry() without changing the logic, and finally this bug fix. Feel free to add 'Fixes: ac401cc78242 ("dax: New fault locking")' in case you feel this needs to be backported.
