Hi!
From: Nicholas Piggin npiggin@gmail.com
[ Upstream commit bafb056ce27940c9994ea905336aa8f27b4f7275 ]
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io_uring 2b188cc1bb857 ("Add io_uring IO interface") added code which does a kthread_use_mm() from a mmget_not_zero() refcount.
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The basic fix for sparc64 is to remove its mm_cpumask clearing code. The optimisation could be effectively restored by sending IPIs to mm_cpumask members and having them remove themselves from mm_cpumask. This is more tricky so I leave it as an exercise for someone with a sparc64 SMP. powerpc has a (currently similarly broken) example.
So this removes optimalization from Sparc, because it clashes with 2b188cc1bb857 ("Add io_uring IO interface"). But that commit is not present in 4.19... so this probably is not good idea.
Best regards, Pavel