On Thu, May 30, 2013 at 03:55:39PM -0400, Johannes Weiner wrote:
> On Thu, May 30, 2013 at 03:05:00PM -0300, Rafael Aquini wrote:
> > read_swap_cache_async() can race against get_swap_page(), and stumble across
> > a SWAP_HAS_CACHE entry in the swap map whose page wasn't brought into the
> > swapcache yet. This transient swap_map state is expected to be transitory,
> > but the actual placement of discard at scan_swap_map() inserts a wait for
> > I/O completion thus making the thread at read_swap_cache_async() to loop
> > around its -EEXIST case, while the other end at get_swap_page()
> > is scheduled away at scan_swap_map(). This can leave the system deadlocked
> > if the I/O completion happens to be waiting on the CPU workqueue where
>
> waitqueue?
>
Ugh! I will repost this to fix it and the "compeletion" typo at subject...
> > read_swap_cache_async() is busy looping and !CONFIG_PREEMPT.
> >
> > This patch introduces a cond_resched() call to make the aforementioned
> > read_swap_cache_async() busy loop condition to bail out when necessary,
> > thus avoiding the subtle race window.
> >
> > Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx>
> > ---
> > mm/swap_state.c | 14 +++++++++++++-
> > 1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index b3d40dc..9ad9e3b 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -336,8 +336,20 @@ struct page *read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > * Swap entry may have been freed since our caller observed it.
> > */
> > err = swapcache_prepare(entry);
> > - if (err == -EEXIST) { /* seems racy */
> > + if (err == -EEXIST) {
> > radix_tree_preload_end();
> > + /*
> > + * We might race against get_swap_page() and stumble
> > + * across a SWAP_HAS_CACHE swap_map entry whose page
> > + * has not been brought into the swapcache yet, while
> > + * the other end is scheduled away waiting on discard
> > + * I/O completion.
> > + * In order to avoid turning this transitory state
> > + * into a permanent loop around this -EEXIST case,
> > + * lets just conditionally invoke the scheduler,
> > + * if there are some more important tasks to run.
> > + */
> > + cond_resched();
>
> Might be worth mentioning the !CONFIG_PREEMPT deadlock scenario here,
> especially since under CONFIG_PREEMPT the radix_tree_preload_end() is
> already a scheduling point through the preempt_enable().
>
Nice suggestion, will do it. Thanks for reviewing this patch!
> Other than that, the patch looks good to me!
>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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