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    SubjectRe: [PATCH] sched: Avoid that __wait_on_bit_lock() hangs
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    On 08/13/2016 09:32 AM, Oleg Nesterov wrote:
    > On 08/12, Bart Van Assche wrote:
    >> before I started testing. It took some time
    >> before I could reproduce the hang in truncate_inode_pages_range().
    >
    > all I can say this contradicts with the previous testing results with
    > my previous patch or with your change in abort_exclusive_wait().

    Hello Oleg,

    My opinion is that all this means is that we do not yet have a full
    understanding of what is going on.

    BTW, I have improved my page lock owner instrumentation patch such that
    it prints a call stack of the lock owner if lock_page() takes too long.
    The following call stack was reported:

    __lock_page / pid 8549 / m 0x2: timeout - continuing to wait for 8549
    [<ffffffff8102b316>] save_stack_trace+0x26/0x50
    [<ffffffff81152bee>] add_to_page_cache_lru+0x7e/0x170
    [<ffffffff8121bfc5>] mpage_readpages+0xc5/0x170
    [<ffffffff81215548>] blkdev_readpages+0x18/0x20
    [<ffffffff81163a68>] __do_page_cache_readahead+0x268/0x310
    [<ffffffff811640a8>] force_page_cache_readahead+0xa8/0x100
    [<ffffffff81164139>] page_cache_sync_readahead+0x39/0x40
    [<ffffffff81153967>] generic_file_read_iter+0x707/0x920
    [<ffffffff81215920>] blkdev_read_iter+0x30/0x40
    [<ffffffff811d4b4b>] __vfs_read+0xbb/0x130
    [<ffffffff811d4f31>] vfs_read+0x91/0x130
    [<ffffffff811d62b4>] SyS_read+0x44/0xa0
    [<ffffffff816281e5>] entry_SYSCALL_64_fastpath+0x18/0xa8

    My understanding of mpage_readpages() is that the page unlock happens
    after readahead I/O completed (see also page_endio()). So this probably
    means that an I/O request submitted because of readahead code did not
    get completed. I will see whether I can find anything that's wrong in
    the block layer.

    Bart.

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