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Date: Fri, 8 Apr 2022 11:50:05 +0800 From: Qi Zheng <zhengqi.arch@...edance.com> To: Muchun Song <songmuchun@...edance.com> Cc: dennis@...nel.org, tj@...nel.org, cl@...ux.com, akpm@...ux-foundation.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, zhouchengming@...edance.com Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put() completes On 2022/4/8 10:54 AM, Muchun Song wrote: > On Thu, Apr 07, 2022 at 06:33:35PM +0800, Qi Zheng wrote: >> In the percpu_ref_call_confirm_rcu(), we call the wake_up_all() >> before calling percpu_ref_put(), which will cause the value of >> percpu_ref to be unstable when percpu_ref_switch_to_atomic_sync() >> returns. >> >> CPU0 CPU1 >> >> percpu_ref_switch_to_atomic_sync(&ref) >> --> percpu_ref_switch_to_atomic(&ref) >> --> percpu_ref_get(ref); /* put after confirmation */ >> call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu); >> >> percpu_ref_switch_to_atomic_rcu >> --> percpu_ref_call_confirm_rcu >> --> data->confirm_switch = NULL; >> wake_up_all(&percpu_ref_switch_waitq); >> >> /* here waiting to wake up */ >> wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch); >> (A)percpu_ref_put(ref); >> /* The value of &ref is unstable! */ >> percpu_ref_is_zero(&ref) >> (B)percpu_ref_put(ref); >> >> As shown above, assuming that the counts on each cpu add up to 0 before >> calling percpu_ref_switch_to_atomic_sync(), we expect that after switching >> to atomic mode, percpu_ref_is_zero() can return true. But actually it will >> return different values in the two cases of A and B, which is not what >> we expected. >> >> Maybe the original purpose of percpu_ref_switch_to_atomic_sync() is >> just to ensure that the conversion to atomic mode is completed, but it >> should not return with an extra reference count. >> >> Calling wake_up_all() after percpu_ref_put() ensures that the value of >> percpu_ref is stable after percpu_ref_switch_to_atomic_sync() returns. >> So just do it. >> >> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com> > > Are any users affected by this? If so, I think a Fixes tag > is necessary. Looks all current users(blk_pre_runtime_suspend() and set_in_sync()) are affected by this. I see that this patch has been merged into the mm tree, can Andrew help me add the following Fixes tag? Fixes: 490c79a65708 ("percpu_ref: decouple switching to atomic mode and killing") Thanks, Qi > > The fix LGTM. > > Reviewed-by: Muchun Song <songmuchun@...edance.com> > > Thanks. > -- Thanks, Qi
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