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
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();
continue;
}
if (err) { /* swp entry is obsolete ? */
--
1.8.1.4
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