* [PATCH] dax: Use non-exclusive wait in wait_entry_unlocked()
@ 2018-12-21 19:38 Dan Williams
2019-01-02 15:35 ` Jan Kara
0 siblings, 1 reply; 2+ messages in thread
From: Dan Williams @ 2018-12-21 19:38 UTC (permalink / raw)
To: linux-fsdevel; +Cc: Linus Torvalds, Jan Kara, Matthew Wilcox, linux-nvdimm
get_unlocked_entry() uses an exclusive wait because it is guaranteed to
eventually obtain the lock and follow on with an unlock+wakeup cycle.
The wait_entry_unlocked() path does not have the same guarantee. Rather
than open-code an extra wakeup, just switch to a non-exclusive wait.
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
fs/dax.c | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 48132eca3761..042d3b31b413 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -246,18 +246,16 @@ static void wait_entry_unlocked(struct xa_state *xas, void *entry)
ewait.wait.func = wake_exceptional_entry_func;
wq = dax_entry_waitqueue(xas, entry, &ewait.key);
- prepare_to_wait_exclusive(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
+ /*
+ * Unlike get_unlocked_entry() there is no guarantee that this
+ * path ever successfully retrieves an unlocked entry before an
+ * inode dies. Perform a non-exclusive wait in case this path
+ * never successfully performs its own wake up.
+ */
+ prepare_to_wait(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
xas_unlock_irq(xas);
schedule();
finish_wait(wq, &ewait.wait);
-
- /*
- * Entry lock waits are exclusive. Wake up the next waiter since
- * we aren't sure we will acquire the entry lock and thus wake
- * the next waiter up on unlock.
- */
- if (waitqueue_active(wq))
- __wake_up(wq, TASK_NORMAL, 1, &ewait.key);
}
static void put_unlocked_entry(struct xa_state *xas, void *entry)
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^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] dax: Use non-exclusive wait in wait_entry_unlocked()
2018-12-21 19:38 [PATCH] dax: Use non-exclusive wait in wait_entry_unlocked() Dan Williams
@ 2019-01-02 15:35 ` Jan Kara
0 siblings, 0 replies; 2+ messages in thread
From: Jan Kara @ 2019-01-02 15:35 UTC (permalink / raw)
To: Dan Williams
Cc: linux-fsdevel, Linus Torvalds, Jan Kara, Matthew Wilcox, linux-nvdimm
On Fri 21-12-18 11:38:04, Dan Williams wrote:
> get_unlocked_entry() uses an exclusive wait because it is guaranteed to
> eventually obtain the lock and follow on with an unlock+wakeup cycle.
> The wait_entry_unlocked() path does not have the same guarantee. Rather
> than open-code an extra wakeup, just switch to a non-exclusive wait.
>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Matthew Wilcox <willy@infradead.org>
> Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> fs/dax.c | 16 +++++++---------
> 1 file changed, 7 insertions(+), 9 deletions(-)
Thanks for cleaning this up! The patch looks good to me. You can add:
Reviewed-by: Jan Kara <jack@suse.cz>
Honza
>
> diff --git a/fs/dax.c b/fs/dax.c
> index 48132eca3761..042d3b31b413 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -246,18 +246,16 @@ static void wait_entry_unlocked(struct xa_state *xas, void *entry)
> ewait.wait.func = wake_exceptional_entry_func;
>
> wq = dax_entry_waitqueue(xas, entry, &ewait.key);
> - prepare_to_wait_exclusive(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
> + /*
> + * Unlike get_unlocked_entry() there is no guarantee that this
> + * path ever successfully retrieves an unlocked entry before an
> + * inode dies. Perform a non-exclusive wait in case this path
> + * never successfully performs its own wake up.
> + */
> + prepare_to_wait(wq, &ewait.wait, TASK_UNINTERRUPTIBLE);
> xas_unlock_irq(xas);
> schedule();
> finish_wait(wq, &ewait.wait);
> -
> - /*
> - * Entry lock waits are exclusive. Wake up the next waiter since
> - * we aren't sure we will acquire the entry lock and thus wake
> - * the next waiter up on unlock.
> - */
> - if (waitqueue_active(wq))
> - __wake_up(wq, TASK_NORMAL, 1, &ewait.key);
> }
>
> static void put_unlocked_entry(struct xa_state *xas, void *entry)
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
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