futex: Calculate the futex key based on a tail page for file-based futexes
Mike Galbraith reported that the LTP test case futex_wake04 was broken by commit 65d8fc77 ("futex: Remove requirement for lock_page() in get_futex_key()"). This test case uses futexes backed by hugetlbfs pages and so there is an associated inode with a futex stored on such pages. The problem is that the key is being calculated based on the head page index of the hugetlbfs page and not the tail page. Prior to the optimisation, the page lock was used to stabilise mappings and pin the inode is file-backed which is overkill. If the page was a compound page, the head page was automatically looked up as part of the page lock operation but the tail page index was used to calculate the futex key. After the optimisation, the compound head is looked up early and the page lock is only relied upon to identify truncated pages, special pages or a shmem page moving to swapcache. The head page is looked up because without the page lock, special care has to be taken to pin the inode correctly. However, the tail page is still required to calculate the futex key so this patch records the tail page. On vanilla 4.6, the output of the test case is; futex_wake04 0 TINFO : Hugepagesize 2097152 futex_wake04 1 TFAIL : futex_wake04.c:126: Bug: wait_thread2 did not wake after 30 secs. With the patch applied futex_wake04 0 TINFO : Hugepagesize 2097152 futex_wake04 1 TPASS : Hi hydra, thread2 awake! Fixes: 65d8fc77 "futex: Remove requirement for lock_page() in get_futex_key()" Reported-and-tested-by: NMike Galbraith <umgwanakikbuti@gmail.com> Signed-off-by: NMel Gorman <mgorman@techsingularity.net> Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: NDavidlohr Bueso <dave@stgolabs.net> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: stable@vger.kernel.org Link: http://lkml.kernel.org/r/20160608132522.GM2469@suse.deSigned-off-by: NThomas Gleixner <tglx@linutronix.de>
Showing
想要评论请 注册 或 登录