powerpc/mm: Fix some SMP issues with MMU context handling (3035c863) · Commits · 戴 / test

This patch fixes a couple of issues that can happen as a result of steal_context() dropping the context_lock when all possible PIDs are ineligible for stealing (hopefully an extremely hard to hit occurence). This case exposes the possibility of a stale context_mm[] entry to be seen since destroy_context() doesn't clear it and the free map isn't re-tested. It also means steal_context() will not notice a context freed while the lock was help, thus possibly trying to steal a context when a free one was available. This fixes it by always returning to the caller from steal_context when it dropped the lock with a return value that causes the caller to re-samble the number of free contexts, along with properly clearing the context_mm[] array for destroyed contexts. Signed-off-by:

Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/mmu_context_nohash.c

+8 −4

Original line number	Diff line number	Diff line
		@@ -73,7 +73,6 @@ static unsigned int steal_context_smp(unsigned int id)
		struct mm_struct *mm;
		unsigned int cpu, max;

		again:
		max = last_context - first_context;

		/* Attempt to free next_context first and then loop until we manage */
		@@ -108,7 +107,9 @@ static unsigned int steal_context_smp(unsigned int id)
		spin_unlock(&context_lock);
		cpu_relax();
		spin_lock(&context_lock);
		goto again;

		/* This will cause the caller to try again */
		return MMU_NO_CONTEXT;
		}
		#endif /* CONFIG_SMP */

		@@ -194,6 +195,8 @@ void switch_mmu_context(struct mm_struct prev, struct mm_struct next)
		WARN_ON(prev->context.active < 1);
		prev->context.active--;
		}

		again:
		#endif /* CONFIG_SMP */

		/* If we already have a valid assigned context, skip all that */
		@@ -212,7 +215,8 @@ void switch_mmu_context(struct mm_struct prev, struct mm_struct next)
		#ifdef CONFIG_SMP
		if (num_online_cpus() > 1) {
		id = steal_context_smp(id);
		goto stolen;
		if (id == MMU_NO_CONTEXT)
		goto again;
		}
		#endif /* CONFIG_SMP */
		id = steal_context_up(id);
		@@ -286,8 +290,8 @@ void destroy_context(struct mm_struct *mm)
		mm->context.id = MMU_NO_CONTEXT;
		#ifdef DEBUG_MAP_CONSISTENCY
		mm->context.active = 0;
		context_mm[id] = NULL;
		#endif
		context_mm[id] = NULL;
		nr_free_contexts++;
		}
		spin_unlock(&context_lock);

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