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fs/dcache.c: add cond_resched() in shrink_dentry_list()
As previously reported (https://patchwork.kernel.org/patch/8642031/) it's possible to call shrink_dentry_list with a large number of dentries (> 10000). This, in turn, could trigger the softlockup detector and possibly trigger a panic. In addition to the unmount path being vulnerable to this scenario, at SuSE we've observed similar situation happening during process exit on processes that touch a lot of dentries. Here is an excerpt from a crash dump. The number after the colon are the number of dentries on the list passed to shrink_dentry_list: PID 99760: 10722 PID 107530: 215 PID 108809: 24134 PID 108877: 21331 PID 141708: 16487 So we want to kill between 15k-25k dentries without yielding. And one possible call stack looks like: 4 [ffff8839ece41db0] _raw_spin_lock at ffffffff8152a5f8 5 [ffff8839ece41db0] evict at ffffffff811c3026 6 [ffff8839ece41dd0] __dentry_kill at ffffffff811bf258 7 [ffff8839ece41df0] shrink_dentry_list at ffffffff811bf593 8 [ffff8839ece41e18] shrink_dcache_parent at ffffffff811bf830 9 [ffff8839ece41e50] proc_flush_task at ffffffff8120dd61 10 [ffff8839ece41ec0] release_task at ffffffff81059ebd 11 [ffff8839ece41f08] do_exit at ffffffff8105b8ce 12 [ffff8839ece41f78] sys_exit at ffffffff8105bd53 13 [ffff8839ece41f80] system_call_fastpath at ffffffff81532909 While some of the callers of shrink_dentry_list do use cond_resched, this is not sufficient to prevent softlockups. So just move cond_resched into shrink_dentry_list from its callers. David said: I've found hundreds of occurrences of warnings that we emit when need_resched stays set for a prolonged period of time with the stack trace that is included in the change log. Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Nikolay Borisov <[email protected]> Reviewed-by: Andrew Morton <[email protected]> Acked-by: David Rientjes <[email protected]> Cc: Alexander Viro <[email protected]> Cc: Goldwyn Rodrigues <[email protected]> Cc: Jeff Mahoney <[email protected]> Cc: Davidlohr Bueso <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
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