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powerpc/mm: Fix some SMP issues with MMU context handling 

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>
hifive-unleashed-5.1
Benjamin Herrenschmidt 2009-05-19 16:56:42 +00:00
parent ec097c84df
commit 3035c8634f
1 changed files with 8 additions and 4 deletions
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12
arch/powerpc/mm/mmu_context_nohash.c
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@ -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);