I received the following email and the Doc Text was updated but the X-Bugzilla-Reason header only shows CC.  So it ended up getting misfiled by my filtering rules.

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From: bugzilla
To: jvillalo
Subject: [Bug 977666] read_swap_cache_async() race to deadlock while waiting
 on discard I/O completion [rhel-6.2.z]
Date: Fri, 23 Aug 2013 10:11:33 +0000
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https://bugzilla.redhat.com/show_bug.cgi?id=977666

Miroslav Svoboda <msvoboda> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msvoboda


--- Doc Text *updated* ---
A race condition between the read_swap_cache_async() and get_swap_page() functions in the Memory management (mm) code could lead to a deadlock situation. The deadlock could occur only on systems that deployed swap partitions on devices supporting block DISCARD and TRIM operations if kernel preemption was disabled (the !CONFIG_PREEMPT parameter). If the read_swap_cache_async() function was given a SWAP_HAS_CACHE entry that did not have a page in the swap cache yet , a DISCARD operation was performed in the scan_swap_map() function. Consequently , completion of an I/O operation was scheduled on the same CPU's working queue the read_swap_cache_async() was running on. This caused the thread in read_swap_cache_async() to loop indefinitely around its "-EEXIST" case , rendering the system unresponsive. The problem has been fixed by adding an explicit cond_resched() call to read_swap_cache_async(), which allows other tasks to run on the affected CPU, and thus avoiding the deadlock.


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