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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/1521718946-31521-1-git-send-email-nborisov@suse.com Signed-off-by: Nikolay Borisov <nborisov@suse.com> Reviewed-by: Andrew Morton <akpm@linux-foundation.org> Acked-by: David Rientjes <rientjes@google.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Goldwyn Rodrigues <rgoldwyn@suse.de> Cc: Jeff Mahoney <jeffm@suse.com> Cc: Davidlohr Bueso <dave@stgolabs.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.