Messages in this thread |  | | Date | Thu, 5 Apr 2018 19:07:06 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 02/10] locking/qspinlock: Remove unbounded cmpxchg loop from locking slowpath |
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On Thu, Apr 05, 2018 at 05:58:59PM +0100, Will Deacon wrote: > The qspinlock locking slowpath utilises a "pending" bit as a simple form > of an embedded test-and-set lock that can avoid the overhead of explicit > queuing in cases where the lock is held but uncontended. This bit is > managed using a cmpxchg loop which tries to transition the uncontended > lock word from (0,0,0) -> (0,0,1) or (0,0,1) -> (0,1,1). > > Unfortunately, the cmpxchg loop is unbounded and lockers can be starved > indefinitely if the lock word is seen to oscillate between unlocked > (0,0,0) and locked (0,0,1). This could happen if concurrent lockers are > able to take the lock in the cmpxchg loop without queuing and pass it > around amongst themselves. > > This patch fixes the problem by unconditionally setting _Q_PENDING_VAL > using atomic_fetch_or,
Of course, LL/SC or cmpxchg implementations of fetch_or do not in fact get anything from this ;-)
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