linux-kernel - Re: [PATCH v2] net/9p/client.c: fix misuse of spin_lock_irqsave for p9

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Date:   Tue, 17 Jul 2018 13:56:39 +0200
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     piaojun <piaojun@...wei.com>
Cc:     Eric Van Hensbergen <ericvh@...il.com>,
        Ron Minnich <rminnich@...dia.gov>,
        Latchesar Ionkov <lucho@...kov.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        v9fs-developer@...ts.sourceforge.net
Subject: Re: [PATCH v2] net/9p/client.c: fix misuse of spin_lock_irqsave for
 p9_client lock

piaojun wrote on Tue, Jul 17, 2018:
> In p9_read_work(), we use spin_lock for client->lock, but misuse
> spin_lock_irqsave for it in p9_fid_create(). As p9_client lock won't be
> locked in irq context, so spin_lock is enough. And that will improve the
> performance.

(I didn't say in v1, but the commit message sounds a bit odd to me, if
there is any other change to the patch could you please rephrase it a
bit?)

> Rebased on 9p-next.

(Likewise, this should probably go below the '---' mark to not be in the
final commit message, 9p-next does not make sense to linux as a whole)

> 
> Signed-off-by: Jun Piao <piaojun@...wei.com>

Nitpicks aside this still looks good to me, I'm not expert in locking
but I believe the conn_cancel stuff in trans_fd is called either from
user context or workqueue which also is process context, while the rest
comes from user context only.

There *are* common functions called from interrupt context (I see:
p9_tag_lookup, p9_client_cb, p9_parse_header) but luckily enough
none of these take the lock.

I'll give this one a little bit of time for others to review before
picking it up, just in case, but will do if no-one complains.

> ---
>  net/9p/client.c   | 18 ++++++++----------
>  net/9p/trans_fd.c |  7 +++----
>  2 files changed, 11 insertions(+), 14 deletions(-)
> 
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 30f9aee..f6897ee 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -275,14 +275,14 @@ static struct p9_fcall *p9_fcall_alloc(int alloc_msize)
>  	INIT_LIST_HEAD(&req->req_list);
> 
>  	idr_preload(GFP_NOFS);
> -	spin_lock_irq(&c->lock);
> +	spin_lock(&c->lock);
>  	if (type == P9_TVERSION)
>  		tag = idr_alloc(&c->reqs, req, P9_NOTAG, P9_NOTAG + 1,
>  				GFP_NOWAIT);
>  	else
>  		tag = idr_alloc(&c->reqs, req, 0, P9_NOTAG, GFP_NOWAIT);
>  	req->tc->tag = tag;
> -	spin_unlock_irq(&c->lock);
> +	spin_unlock(&c->lock);
>  	idr_preload_end();
>  	if (tag < 0)
>  		goto free;
> @@ -328,13 +328,12 @@ struct p9_req_t *p9_tag_lookup(struct p9_client *c, u16 tag)
>   */
>  static void p9_free_req(struct p9_client *c, struct p9_req_t *r)
>  {
> -	unsigned long flags;
>  	u16 tag = r->tc->tag;
> 
>  	p9_debug(P9_DEBUG_MUX, "clnt %p req %p tag: %d\n", c, r, tag);
> -	spin_lock_irqsave(&c->lock, flags);
> +	spin_lock(&c->lock);
>  	idr_remove(&c->reqs, tag);
> -	spin_unlock_irqrestore(&c->lock, flags);
> +	spin_unlock(&c->lock);
>  	kfree(r->tc);
>  	kfree(r->rc);
>  	kmem_cache_free(p9_req_cache, r);
> @@ -846,10 +845,10 @@ static struct p9_fid *p9_fid_create(struct p9_client *clnt)
>  	fid->fid = 0;
> 
>  	idr_preload(GFP_KERNEL);
> -	spin_lock_irq(&clnt->lock);
> +	spin_lock(&clnt->lock);
>  	ret = idr_alloc_u32(&clnt->fids, fid, &fid->fid, P9_NOFID - 1,
>  			    GFP_NOWAIT);
> -	spin_unlock_irq(&clnt->lock);
> +	spin_unlock(&clnt->lock);
>  	idr_preload_end();
> 
>  	if (!ret)
> @@ -862,13 +861,12 @@ static struct p9_fid *p9_fid_create(struct p9_client *clnt)
>  static void p9_fid_destroy(struct p9_fid *fid)
>  {
>  	struct p9_client *clnt;
> -	unsigned long flags;
> 
>  	p9_debug(P9_DEBUG_FID, "fid %d\n", fid->fid);
>  	clnt = fid->clnt;
> -	spin_lock_irqsave(&clnt->lock, flags);
> +	spin_lock(&clnt->lock);
>  	idr_remove(&clnt->fids, fid->fid);
> -	spin_unlock_irqrestore(&clnt->lock, flags);
> +	spin_unlock(&clnt->lock);
>  	kfree(fid->rdir);
>  	kfree(fid);
>  }
> diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
> index bf459ee..94ef685 100644
> --- a/net/9p/trans_fd.c
> +++ b/net/9p/trans_fd.c
> @@ -197,15 +197,14 @@ static void p9_mux_poll_stop(struct p9_conn *m)
>  static void p9_conn_cancel(struct p9_conn *m, int err)
>  {
>  	struct p9_req_t *req, *rtmp;
> -	unsigned long flags;
>  	LIST_HEAD(cancel_list);
> 
>  	p9_debug(P9_DEBUG_ERROR, "mux %p err %d\n", m, err);
> 
> -	spin_lock_irqsave(&m->client->lock, flags);
> +	spin_lock(&m->client->lock);
> 
>  	if (m->err) {
> -		spin_unlock_irqrestore(&m->client->lock, flags);
> +		spin_unlock(&m->client->lock);
>  		return;
>  	}
> 
> @@ -217,7 +216,7 @@ static void p9_conn_cancel(struct p9_conn *m, int err)
>  	list_for_each_entry_safe(req, rtmp, &m->unsent_req_list, req_list) {
>  		list_move(&req->req_list, &cancel_list);
>  	}
> -	spin_unlock_irqrestore(&m->client->lock, flags);
> +	spin_unlock(&m->client->lock);
> 
>  	list_for_each_entry_safe(req, rtmp, &cancel_list, req_list) {
>  		p9_debug(P9_DEBUG_ERROR, "call back req %p\n", req);
> -- 

-- 
Dominique Martinet