From c3df5fb57fe8756d67fd56ed29da65cdfde839f9 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Tue, 27 Jul 2021 13:12:20 -1000 Subject: cgroup: rstat: fix A-A deadlock on 32bit around u64_stats_sync 0fa294fb1985 ("cgroup: Replace cgroup_rstat_mutex with a spinlock") added cgroup_rstat_flush_irqsafe() allowing flushing to happen from the irq context. However, rstat paths use u64_stats_sync to synchronize access to 64bit stat counters on 32bit machines. u64_stats_sync is implemented using seq_lock and trying to read from an irq context can lead to A-A deadlock if the irq happens to interrupt the stat update. Fix it by using the irqsafe variants - u64_stats_update_begin_irqsave() and u64_stats_update_end_irqrestore() - in the update paths. Note that none of this matters on 64bit machines. All these are just for 32bit SMP setups. Note that the interface was introduced way back, its first and currently only use was recently added by 2d146aa3aa84 ("mm: memcontrol: switch to rstat"). Stable tagging targets this commit. Signed-off-by: Tejun Heo Reported-by: Rik van Riel Fixes: 2d146aa3aa84 ("mm: memcontrol: switch to rstat") Cc: stable@vger.kernel.org # v5.13+ --- block/blk-cgroup.c | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-) (limited to 'block/blk-cgroup.c') diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c index 575d7a2e7203..31fe9be179d9 100644 --- a/block/blk-cgroup.c +++ b/block/blk-cgroup.c @@ -790,6 +790,7 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) struct blkcg_gq *parent = blkg->parent; struct blkg_iostat_set *bisc = per_cpu_ptr(blkg->iostat_cpu, cpu); struct blkg_iostat cur, delta; + unsigned long flags; unsigned int seq; /* fetch the current per-cpu values */ @@ -799,21 +800,21 @@ static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu) } while (u64_stats_fetch_retry(&bisc->sync, seq)); /* propagate percpu delta to global */ - u64_stats_update_begin(&blkg->iostat.sync); + flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync); blkg_iostat_set(&delta, &cur); blkg_iostat_sub(&delta, &bisc->last); blkg_iostat_add(&blkg->iostat.cur, &delta); blkg_iostat_add(&bisc->last, &delta); - u64_stats_update_end(&blkg->iostat.sync); + u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags); /* propagate global delta to parent (unless that's root) */ if (parent && parent->parent) { - u64_stats_update_begin(&parent->iostat.sync); + flags = u64_stats_update_begin_irqsave(&parent->iostat.sync); blkg_iostat_set(&delta, &blkg->iostat.cur); blkg_iostat_sub(&delta, &blkg->iostat.last); blkg_iostat_add(&parent->iostat.cur, &delta); blkg_iostat_add(&blkg->iostat.last, &delta); - u64_stats_update_end(&parent->iostat.sync); + u64_stats_update_end_irqrestore(&parent->iostat.sync, flags); } } @@ -848,6 +849,7 @@ static void blkcg_fill_root_iostats(void) memset(&tmp, 0, sizeof(tmp)); for_each_possible_cpu(cpu) { struct disk_stats *cpu_dkstats; + unsigned long flags; cpu_dkstats = per_cpu_ptr(bdev->bd_stats, cpu); tmp.ios[BLKG_IOSTAT_READ] += @@ -864,9 +866,9 @@ static void blkcg_fill_root_iostats(void) tmp.bytes[BLKG_IOSTAT_DISCARD] += cpu_dkstats->sectors[STAT_DISCARD] << 9; - u64_stats_update_begin(&blkg->iostat.sync); + flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync); blkg_iostat_set(&blkg->iostat.cur, &tmp); - u64_stats_update_end(&blkg->iostat.sync); + u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags); } } } -- cgit v1.2.3-59-g8ed1b