This is V2 of the patch. Posted V1 here. https://lore.kernel.org/linux-fsdevel/20210416173524.GA1379987@redhat.com/ 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.

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(a)redhat.com&gt; Signed-off-by: Vivek Goyal <vgoyal(a)redhat.com&gt; --- 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);

On Mon, Apr 19, 2021 at 12:48:58PM -0700, Dan Williams wrote: > On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <vgoyal(a)redhat.com&gt; wrote: > > > > This is V2 of the patch. Posted V1 here. > > > > https://lore.kernel.org/linux-fsdevel/20210416173524.GA1379987@redhat.com/ > > > > 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(a)redhat.com&gt; > > Signed-off-by: Vivek Goyal <vgoyal(a)redhat.com&gt; > > --- > > 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. Hi Dan, I will make changes as you suggested and post another version. I am wondering what to do with dax_wake_entry(). It also has a boolean parameter wake_all. Should that be converted as well to make use of enum dax_entry_wake_mode?