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Commit d835502f authored by Shaohua Li's avatar Shaohua Li Committed by Jens Axboe
Browse files

percpu_ida: fix a live lock 



steal_tags only happens when free tags is more than half of the total
tags.  This is too strict and can cause live lock. I found that if one
cpu has free tags, but other cpu can't steal (thread is bound to
specific cpus), threads which want to allocate tags are always
sleeping. I found this when I run next patch, but this could happen
without it I think.

I did performance test too with null_blk. Two cases (each cpu has enough
percpu tags, or total tags are limited), no performance changes were
observed.

Signed-off-by: default avatarShaohua Li <shli@fusionio.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 53d8ab29

lib/percpu_ida.c

+2 −5
Original line number Diff line number Diff line
@@ -54,9 +54,7 @@ static inline void move_tags(unsigned *dst, unsigned *dst_nr, 
/*

 * Try to steal tags from a remote cpu's percpu freelist.

 *

 * We first check how many percpu freelists have tags - we don't steal tags

 * unless enough percpu freelists have tags on them that it's possible more than

 * half the total tags could be stuck on remote percpu freelists.

 * We first check how many percpu freelists have tags

 *

 * Then we iterate through the cpus until we find some tags - we don't attempt

 * to find the "best" cpu to steal from, to keep cacheline bouncing to a
@@ -69,8 +67,7 @@ static inline void steal_tags(struct percpu_ida *pool, 
	struct percpu_ida_cpu *remote;



	for (cpus_have_tags = cpumask_weight(&pool->cpus_have_tags);

	     cpus_have_tags * pool->percpu_max_size > pool->nr_tags / 2;

	     cpus_have_tags--) {

	     cpus_have_tags; cpus_have_tags--) {

		cpu = cpumask_next(cpu, &pool->cpus_have_tags);



		if (cpu >= nr_cpu_ids) {