On Thu, 30 May 2013, 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 > 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. Yes, I never realized this at the time I inserted discard there. As you know, Shaohua has a better swap discard implementation, which avoids the problem by using SWAP_MAP_BAD, but this cond_resched() is a good simple workaround for now - thanks. > > Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx > --- > 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 if (err) { /* swp entry is obsolete ?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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>