Messages in this thread | | | Subject | Re: [PATCH] mm/vmalloc: Fix unlock order in s_stop() | From | Waiman Long <> | Date | Sun, 13 Dec 2020 14:42:51 -0500 |
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On 12/13/20 1:39 PM, Uladzislau Rezki wrote: > On Sun, Dec 13, 2020 at 01:08:43PM -0500, Waiman Long wrote: >> When multiple locks are acquired, they should be released in reverse >> order. For s_start() and s_stop() in mm/vmalloc.c, that is not the >> case. >> >> s_start: mutex_lock(&vmap_purge_lock); spin_lock(&vmap_area_lock); >> s_stop : mutex_unlock(&vmap_purge_lock); spin_unlock(&vmap_area_lock); >> >> This unlock sequence, though allowed, is not optimal. If a waiter is >> present, mutex_unlock() will need to go through the slowpath of waking >> up the waiter with preemption disabled. Fix that by releasing the >> spinlock first before the mutex. >> >> Fixes: e36176be1c39 ("mm/vmalloc: rework vmap_area_lock") >> Signed-off-by: Waiman Long <longman@redhat.com> >> --- >> mm/vmalloc.c | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >> index 6ae491a8b210..75913f685c71 100644 >> --- a/mm/vmalloc.c >> +++ b/mm/vmalloc.c >> @@ -3448,11 +3448,11 @@ static void *s_next(struct seq_file *m, void *p, loff_t *pos) >> } >> >> static void s_stop(struct seq_file *m, void *p) >> - __releases(&vmap_purge_lock) >> __releases(&vmap_area_lock) >> + __releases(&vmap_purge_lock) >> { >> - mutex_unlock(&vmap_purge_lock); >> spin_unlock(&vmap_area_lock); >> + mutex_unlock(&vmap_purge_lock); >> } >> >> static void show_numa_info(struct seq_file *m, struct vm_struct *v) > BTW, if navigation over both list is an issue, for example when there > are multiple heavy readers of /proc/vmallocinfo, i think, it make sense > to implement RCU safe lists iteration and get rid of both locks.
Making it lockless is certainly better, but doing lockless the right way is tricky. I will probably keep it as it unless there is a significant advantage of doing so.
Cheers, Longman
> > As for the patch: Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com> > > Thanks! > > -- > Vlad Rezki >
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